**Radio-Telemetry in Biomedical Research - Radio-Telemetry Blood Pressure Measurements in Animal Models of Hypertension, How It Revolutionized Hypertension Research**

Pierre Dumas, Dan Chiche, Johanne Tremblay, Ondřej Šeda, Junzheng Peng and Pavel Hamet *Centre for Ecogenomic Models of Human Diseases/CRCHUM, Technopôle Angus Canada* 

#### **1. Introduction**

Radiotelemetry is employed in several fields to circumvent several issues: areas difficult or dangerous to access, monitoring of dangerous processes, need for secret monitoring. In biological sciences, telemetry is mainly useful because it decreases the observer bias and interference. In the field of medicine, the current research is mostly aimed at findings the cause and appropriate cures to common diseases. Common diseases are widely prevalent diseases for which we know only partially the causes and for which, as a consequence, we only propose treatments to alleviate the symptoms or their impacts on target organs. The common examples of such diseases to name a few are: diabetes, cancer(s), obesity, multiple sclerosis and hypertension, not to mention most of the psychiatric illnesses. They are characterized by a strong genetic component and a strong environmental influence since their prevalence is markedly influenced by age, diet, exercise or other environmental stressors. This important environmental modulation makes them more difficult to study. Therefore, our goal here will be to illustrate the challenge of studying environmentmodulated traits. With hypertension as an example, we will describe the use and benefits of employing radiotelemetry in hypertension research in order to be able to subtract the role of the environment or, conversely to quantify its impact on blood pressure. In the current postgenome era, with enough financial support and colleagues from around the world, it has never been easier to design and perform huge genome-wide association studies to try to unveil the genetic determinants of common diseases. Each month, hundreds of loci are reported that are associated with a higher prevalence of diseases and single nucleotide polymorphisms covering the entire genome are proposed to be in linkage with disease genes. We also know that very few of these proposed loci end up being truly associated with diseases in replication studies and we will present the current arguments pro and against this approach in the field of hypertension. This, we hope, will illustrate the point that we want to make in this chapter: in order to perform valid genome-wide association studies in human or genetic studies in animal models to uncover the genetic determinants of common diseases, it is essential to clearly define the studied phenotype(s) and to ensure that their measurements are performed accurately with the least amount of confounders or artefacts.

Radio-Telemetry in Biomedical Research - Radio-Telemetry Blood Pressure

13.7% who were unaware of their hypertension).

of other countries.

Measurements in Animal Models of Hypertension, How It Revolutionized Hypertension Research 119

hypertensive subjects aware but not treated and not controlled was 22% while 21% percent were treated and not controlled, and only 13% were treated and controlled. At that time, although the prevalence of hypertension was similar in Canada and the United States, levels of awareness, treatment and control were higher in the United States. Leenen et al. (2008) have conducted a survey to evaluate the current prevalence and management of hypertension among adults in the province of Ontario. Blood pressures, measured with an automated device, were obtained for 2,551 of the respondents (age 20–79 years). Hypertension, defined as systolic blood pressure of 140 mm Hg or more, diastolic blood pressure of 90 mm Hg or more, or treatment with an antihypertensive medication, was identified in 21.3% of the population overall (23.8% of men and 19.0% of women). Prevalence increased with age, from 3.4% among participants 20–39 years of age to 51.6% among those 60–79 years of age. Hypertension was more prevalent in black people and people originating from South Asia than among white people; hypertension was also associated with higher body mass index. Among hypertensive participants, 65.7% were undergoing treatment with control of hypertension, 14.7% were undergoing treatment but the hypertension was not controlled, and 19.5% were not receiving any treatment (including

The latest Canadian report was published in 2010 by Wilkins et al (2010) with cycle 1 of the Canadian Health Measures Survey, conducted from March 2007 through February 2009 in 15 sites across Canada. This survey comprised a population-based sample and included direct BP measures using an automated device. During an interview, 3,514 subjects were asked two questions about BP: whether they had high BP diagnosed by a health professional and whether they had taken "medicine for high blood pressure" in the past month. BP measures were obtained at a mobile examination centre a few days after the initial interview. BP values were calculated by taking the average of the last five of six measures (taken one minute apart) of valid BP measurements. Results among adults aged 20 to 79 years showed that hypertension (systolic BP higher than or equal to 140 or diastolic BP higher than or equal to 90 mmHg, or self-reported recent medication use for high BP) was present in 19%. Of those with hypertension, 83% were aware, 80% were taking antihypertensive drugs, and 66% were controlled. With regards to the significant progress observed compared to Joffres et al report (1992), the authors conclude that their results are consistent with the large improvements in diagnosis, treatment and drug prescriptions for hypertension that have recently occurred in Canada (Campbell et al., 2009; Hemmelgarn et al., 2008). In the late 1990s, extensive efforts have been underway in Canada to improve physician and public awareness of the importance of diagnosis, treatment and control of hypertension, including the Canadian Hypertension Education Program (http://www.hypertension.ca/chep), yet the author cautioned against a potential overestimation of this change, particularity as it makes Canada being out of the range

Table 1 reports the main results from these 4 studies. When measured, the prevalence of hypertension is quite stable between 19 and 21.3%. The rate of hypertensive patients unaware of their condition has dramatically decreased from 43% to 16.6%. In addition, the rate of treated and controlled hypertensive patients among hypertensive population seems to have dramatically increased as well from 13% to 65.9%. This trend was still observed in treated hypertensive population taking into consideration the differences in the number of hypertensive patients aware of their condition and treated. Nevertheless, this comparison must be interpreted with caution since there are many differences in methodological approaches, mainly the use of an automated device known to report lower BP values.

Our contribution to this book entitled "Telemetry" will not be of a technical nature nor will it systematically review in all details the merit of telemetry in our field of research because it would go beyond the scope of this book and would not be suited for a broad audience. We direct the interested readers to the review of Kurtz and colleagues (2005) for that purpose. We are presenting here an aspect of what we think is the most important contribution of telemetry in our field, its use to monitor blood pressure free of stress artefacts or on the contrary, its use to evaluate the contribution of this environmental "stress".

### **2. Blood pressure and hypertension**

High blood pressure is a major risk factor for cardiovascular diseases. Because of its impact on cardiovascular diseases, their complications and the cost associated, it is important to evaluate its prevalence accurately. This essential data can then help design public health policies aimed at controlling high blood pressure in the population. With the example of recent Canadian epidemiological studies, we will try to demonstrate how the methods used to record blood pressure can affect the relative prevalence in the study population with direct consequences on public health policies. Our aim is to illustrate the importance of good phenotyping, a major challenge in hypertension research.

#### **2.1 Definition and prevalence of hypertension**

Blood pressure is a continuous quantitative trait genetically determined but under the strong influence of the environment. It is a consistent and independent risk factor for cardiovascular and renal diseases. At least one third of adult United States population have hypertension defined as systolic blood pressure ≥ 140 mm Hg, diastolic blood pressure ≥ 90 mm Hg and/or current use of antihypertensive medication (Brown et al., 2001). It is a major risk factor for premature cardiovascular morbidity and mortality (Lawes et al., 2008). Epidemiologic studies have indicated that, for people 40–69 years of age, each increase of 20 mm Hg in usual systolic blood pressure is associated with a doubling of mortality rates for stroke and ischemic heart disease (Lewington et al., 2002). Hypertension is considered by the World Health Organization to be the number one risk factor for death in the world in both developed and developing countries, responsible for an estimated 7.5 million deaths per year (12.8% of all deaths).

#### **2.2 Situation in Canada**

The prevalence of hypertension in Canada was recently assessed in four population-based studies between 1986 and 2009 (Table 1). Blood pressure measurement was performed in all but one study (Canadian Community Health Survey). An automated device, BpTRU was used for the two most recent studies (Leenen et al., 2008; Wilkins et al., 2010), whereas blood pressure was measured by a trained nurse in the other one (Joffres et al., 2001). The Canada Heart Health Study (Joffres et al., 2001) has been conducted from 1986 to 1992 as a population-based cross-sectional study to estimate the prevalence and distribution of elevated blood pressure among Canadian adults. A probability sample of 23,111 men and women aged 18 to 74 years were selected from the health insurance registers in each province. Mean of all available blood pressure measurements from four measurements was used. The rate of subjects with hypertension, defined by systolic BP at or above 140 mmHg or diastolic BP at or above 90 mmHg or treatment for hypertension was 21.1%. The proportion of hypertensive subjects unaware of their hypertension was 43%. The level of

Our contribution to this book entitled "Telemetry" will not be of a technical nature nor will it systematically review in all details the merit of telemetry in our field of research because it would go beyond the scope of this book and would not be suited for a broad audience. We direct the interested readers to the review of Kurtz and colleagues (2005) for that purpose. We are presenting here an aspect of what we think is the most important contribution of telemetry in our field, its use to monitor blood pressure free of stress artefacts or on the

High blood pressure is a major risk factor for cardiovascular diseases. Because of its impact on cardiovascular diseases, their complications and the cost associated, it is important to evaluate its prevalence accurately. This essential data can then help design public health policies aimed at controlling high blood pressure in the population. With the example of recent Canadian epidemiological studies, we will try to demonstrate how the methods used to record blood pressure can affect the relative prevalence in the study population with direct consequences on public health policies. Our aim is to illustrate the importance of good

Blood pressure is a continuous quantitative trait genetically determined but under the strong influence of the environment. It is a consistent and independent risk factor for cardiovascular and renal diseases. At least one third of adult United States population have hypertension defined as systolic blood pressure ≥ 140 mm Hg, diastolic blood pressure ≥ 90 mm Hg and/or current use of antihypertensive medication (Brown et al., 2001). It is a major risk factor for premature cardiovascular morbidity and mortality (Lawes et al., 2008). Epidemiologic studies have indicated that, for people 40–69 years of age, each increase of 20 mm Hg in usual systolic blood pressure is associated with a doubling of mortality rates for stroke and ischemic heart disease (Lewington et al., 2002). Hypertension is considered by the World Health Organization to be the number one risk factor for death in the world in both developed and developing countries, responsible for an estimated 7.5 million deaths

The prevalence of hypertension in Canada was recently assessed in four population-based studies between 1986 and 2009 (Table 1). Blood pressure measurement was performed in all but one study (Canadian Community Health Survey). An automated device, BpTRU was used for the two most recent studies (Leenen et al., 2008; Wilkins et al., 2010), whereas blood pressure was measured by a trained nurse in the other one (Joffres et al., 2001). The Canada Heart Health Study (Joffres et al., 2001) has been conducted from 1986 to 1992 as a population-based cross-sectional study to estimate the prevalence and distribution of elevated blood pressure among Canadian adults. A probability sample of 23,111 men and women aged 18 to 74 years were selected from the health insurance registers in each province. Mean of all available blood pressure measurements from four measurements was used. The rate of subjects with hypertension, defined by systolic BP at or above 140 mmHg or diastolic BP at or above 90 mmHg or treatment for hypertension was 21.1%. The proportion of hypertensive subjects unaware of their hypertension was 43%. The level of

contrary, its use to evaluate the contribution of this environmental "stress".

**2. Blood pressure and hypertension** 

phenotyping, a major challenge in hypertension research.

**2.1 Definition and prevalence of hypertension** 

per year (12.8% of all deaths).

**2.2 Situation in Canada** 

hypertensive subjects aware but not treated and not controlled was 22% while 21% percent were treated and not controlled, and only 13% were treated and controlled. At that time, although the prevalence of hypertension was similar in Canada and the United States, levels of awareness, treatment and control were higher in the United States. Leenen et al. (2008) have conducted a survey to evaluate the current prevalence and management of hypertension among adults in the province of Ontario. Blood pressures, measured with an automated device, were obtained for 2,551 of the respondents (age 20–79 years). Hypertension, defined as systolic blood pressure of 140 mm Hg or more, diastolic blood pressure of 90 mm Hg or more, or treatment with an antihypertensive medication, was identified in 21.3% of the population overall (23.8% of men and 19.0% of women). Prevalence increased with age, from 3.4% among participants 20–39 years of age to 51.6% among those 60–79 years of age. Hypertension was more prevalent in black people and people originating from South Asia than among white people; hypertension was also associated with higher body mass index. Among hypertensive participants, 65.7% were undergoing treatment with control of hypertension, 14.7% were undergoing treatment but the hypertension was not controlled, and 19.5% were not receiving any treatment (including 13.7% who were unaware of their hypertension).

The latest Canadian report was published in 2010 by Wilkins et al (2010) with cycle 1 of the Canadian Health Measures Survey, conducted from March 2007 through February 2009 in 15 sites across Canada. This survey comprised a population-based sample and included direct BP measures using an automated device. During an interview, 3,514 subjects were asked two questions about BP: whether they had high BP diagnosed by a health professional and whether they had taken "medicine for high blood pressure" in the past month. BP measures were obtained at a mobile examination centre a few days after the initial interview. BP values were calculated by taking the average of the last five of six measures (taken one minute apart) of valid BP measurements. Results among adults aged 20 to 79 years showed that hypertension (systolic BP higher than or equal to 140 or diastolic BP higher than or equal to 90 mmHg, or self-reported recent medication use for high BP) was present in 19%. Of those with hypertension, 83% were aware, 80% were taking antihypertensive drugs, and 66% were controlled. With regards to the significant progress observed compared to Joffres et al report (1992), the authors conclude that their results are consistent with the large improvements in diagnosis, treatment and drug prescriptions for hypertension that have recently occurred in Canada (Campbell et al., 2009; Hemmelgarn et al., 2008). In the late 1990s, extensive efforts have been underway in Canada to improve physician and public awareness of the importance of diagnosis, treatment and control of hypertension, including the Canadian Hypertension Education Program (http://www.hypertension.ca/chep), yet the author cautioned against a potential overestimation of this change, particularity as it makes Canada being out of the range of other countries.

Table 1 reports the main results from these 4 studies. When measured, the prevalence of hypertension is quite stable between 19 and 21.3%. The rate of hypertensive patients unaware of their condition has dramatically decreased from 43% to 16.6%. In addition, the rate of treated and controlled hypertensive patients among hypertensive population seems to have dramatically increased as well from 13% to 65.9%. This trend was still observed in treated hypertensive population taking into consideration the differences in the number of hypertensive patients aware of their condition and treated. Nevertheless, this comparison must be interpreted with caution since there are many differences in methodological approaches, mainly the use of an automated device known to report lower BP values.

Radio-Telemetry in Biomedical Research - Radio-Telemetry Blood Pressure

automated devices for BP measurements in a controlled environment.

and the effect of antihypertensive medications would indeed be welcomed.

information about blood pressure pattern during sleep.

**determination in humans** 

**2.4 Portapres, the only system that comes close to telemetry for blood pressure** 

Protocols and recommendations for blood pressure measurement in humans are available and they provide information for procedures in the clinical settings or at home and insist on the training of the observer (Pickering et al., 2005). Among the methods described, they cite the 100-year old auscultatory method employed with a stethoscope and a sphygmomanometer. It is based on the Korotkoff technique and uses a cuff placed around the upper arm and inflated above systolic pressure to occlude the brachial artery. The stethoscope is used to listen to the sounds of the pulsatile blood flow while the cuff is slowly deflated. Despite its accuracy, the classic mercury sphygmomanometer tends to disappear because of the hazard from mercury and is replaced by aneroid or hybrid sphygmomanometer. Other technologies and automated devices are also available, and the interested reader is directed to the review by Pickering et al. (2005) for details. Of note, the Korotkoff technique tends to underestimate the systolic blood pressure and overestimate the diastolic blood pressure when compared to intra-arterial pressure (Holland & Humerfelt, 1964). With technology advances, battery-powered automated devices are now available to measure and record ambulatory blood pressure. They employ the same upper arm cuff and, because of the time required for one measurement they cannot record rapid changes in blood pressure. Furthermore, a maximum of four measurements per hour for 24 hours are usually obtained because of the annoyance of having the device squeeze the arm at regular interval. Nevertheless, ambulatory blood pressure monitoring can be very useful for diagnostic to ascertain blood pressure level outside the clinic. It also provides good

There is, however, a method for blood pressure determination in humans that comes close to a telemetric method: the finger cuff method of Peñaz (1973). It is available commercially

Measurements in Animal Models of Hypertension, How It Revolutionized Hypertension Research 121

should be considered with caution, given the facts that the BP measurement protocols were different, with the most recent publications reporting lower BP values. An important advantage of an automated device is that it enables BP to be measured in the absence of another person. Its use, therefore, eliminates observer errors such as digit bias, zero preference and incorrect deflation rates, and also reduces "white coat hypertension". However, this specific device discard the first two readings, leading to elimination of highest pressures and these numbers were never validated for outcomes. According to the 2010 CHEP Recommendations for the Management of Hypertension (Hackam et al., 2010), home SBP values >135 mmHg or DBP values >85 mmHg using an automated device should be considered elevated and associated with an increased overall mortality risk analogous to office SBP readings of >140 mmHg or DBP >90 mmHg. Hence, maybe less emphasis should be put on the 140/90 mm Hg threshold with the more and more systematic use of

Thus, one should asks if these measurements are really reflecting the 'usual' blood pressure of an individual, knowing that the protocol demands that the first two values (always higher) be discarded and that the measurement be performed in a silent closed room without an observer. Does it really reflect the 'true' blood pressure and its minute-to-minute variability? Therefore, there is clearly a need to assess precisely blood pressure in humans since its 'usual value' as so much to do with the risk of cardiovascular events. Telemetric determination of blood pressure in human to assess its 'true values', its variations over time


1Among hypertensive population; 2Among treated for hypertension population.

Table 1. Recent Canadian population-based studies estimating the prevalence of hypertension.

#### **2.3 Measurement bias**

It is well recognized that the method used for BP measurement has a very significant impact on the results. As an example, we compared Joffre et al. (2001) results to that of Wilkins et al. (2010) in the younger group age (respectively 18 to 34, and 20 to 39) where the prevalence of hypertension is very rare (below 5%) with limited impact on mean systolic and diastolic BP. Mean SBP are significantly higher in male and female in Joffres et al. publication when compared to Wilkins et al.: in the range of 120/110 (male/female) as compared to 110/101, respectively. Similarly, DBP are 75/70 versus 72/67, respectively. Therefore, the remarkable improvement in the proportion of hypertensive subjects that are treated and controlled

hypertension

Mean SBP/DBP of 140/90 mm Hg or medication

Population aged 12 and over who report that they have been diagnosed by a health professional

Mean SBP/DBP of 140/90 mm Hg or treatment with antihyperten sive drugs

Mean SBP/DBP of 140/90 mm Hg or selfreported recent medication use for high BP.

Study Period

2001, 2003, 2005, 2007, (yearly)

Published

16.4%

Prevalence Hypertensive

1986-1992 21.1% 43% 13% 38%

(2008) NA NA NA

in 2008 21.3% 13.7% 65.7% 77%

2007-2009 19% 16.6% 65.9% 82%

unaware 1

Treated and

controlled 1

Treated and

controlled 2

Blood pressure measurement Definition of

Two BP measurements: one at the beginning of the interview and one at the end, twice, 2 weeks apart. The BP was measured by a trained nurse. Standardization for identification of the Korotkoff sounds. Correctly sized cuffs were used. Quiet for 5 min, sitting position, Right arm (if possible) The mean BP (four measurements for most participants) was used.

None

Arm circumference measured for selection of cuff. Cuff placed on left arm. Seated and quiet for 5 minutes. 6 measures and 5 records using BpTRU

BpTRU - Average of the last five measures of a set of six taken one minute apart.

1Among hypertensive population; 2Among treated for hypertension population.

Table 1. Recent Canadian population-based studies estimating the prevalence of

It is well recognized that the method used for BP measurement has a very significant impact on the results. As an example, we compared Joffre et al. (2001) results to that of Wilkins et al. (2010) in the younger group age (respectively 18 to 34, and 20 to 39) where the prevalence of hypertension is very rare (below 5%) with limited impact on mean systolic and diastolic BP. Mean SBP are significantly higher in male and female in Joffres et al. publication when compared to Wilkins et al.: in the range of 120/110 (male/female) as compared to 110/101, respectively. Similarly, DBP are 75/70 versus 72/67, respectively. Therefore, the remarkable improvement in the proportion of hypertensive subjects that are treated and controlled

Name, author

Canadian Heart Health Surveys, Joffres et al., 1992

Canadian Community Health Survey Campbell et al., 2008

Ontario Survey on the Prevention and Control of Hypertension 2006, Leenen et al., 2008

Canadian Health Measures Survey, Wilkins et al., 2010

hypertension.

**2.3 Measurement bias** 

Data collection and

sample

Population based, 18-74 year, Interview and clinic visit

Canadians aged 12 and over. 65,000 representative of 121 Health Regions Computer-Assisted Personal Interview or Telephone Interview

Random dwellings, 2551 participants, 20- 79 years, Ontario

Population based, Interview, home and mobile center visits, 20-79 years

representation

should be considered with caution, given the facts that the BP measurement protocols were different, with the most recent publications reporting lower BP values. An important advantage of an automated device is that it enables BP to be measured in the absence of another person. Its use, therefore, eliminates observer errors such as digit bias, zero preference and incorrect deflation rates, and also reduces "white coat hypertension". However, this specific device discard the first two readings, leading to elimination of highest pressures and these numbers were never validated for outcomes. According to the 2010 CHEP Recommendations for the Management of Hypertension (Hackam et al., 2010), home SBP values >135 mmHg or DBP values >85 mmHg using an automated device should be considered elevated and associated with an increased overall mortality risk analogous to office SBP readings of >140 mmHg or DBP >90 mmHg. Hence, maybe less emphasis should be put on the 140/90 mm Hg threshold with the more and more systematic use of automated devices for BP measurements in a controlled environment.

Thus, one should asks if these measurements are really reflecting the 'usual' blood pressure of an individual, knowing that the protocol demands that the first two values (always higher) be discarded and that the measurement be performed in a silent closed room without an observer. Does it really reflect the 'true' blood pressure and its minute-to-minute variability? Therefore, there is clearly a need to assess precisely blood pressure in humans since its 'usual value' as so much to do with the risk of cardiovascular events. Telemetric determination of blood pressure in human to assess its 'true values', its variations over time and the effect of antihypertensive medications would indeed be welcomed.

#### **2.4 Portapres, the only system that comes close to telemetry for blood pressure determination in humans**

Protocols and recommendations for blood pressure measurement in humans are available and they provide information for procedures in the clinical settings or at home and insist on the training of the observer (Pickering et al., 2005). Among the methods described, they cite the 100-year old auscultatory method employed with a stethoscope and a sphygmomanometer. It is based on the Korotkoff technique and uses a cuff placed around the upper arm and inflated above systolic pressure to occlude the brachial artery. The stethoscope is used to listen to the sounds of the pulsatile blood flow while the cuff is slowly deflated. Despite its accuracy, the classic mercury sphygmomanometer tends to disappear because of the hazard from mercury and is replaced by aneroid or hybrid sphygmomanometer. Other technologies and automated devices are also available, and the interested reader is directed to the review by Pickering et al. (2005) for details. Of note, the Korotkoff technique tends to underestimate the systolic blood pressure and overestimate the diastolic blood pressure when compared to intra-arterial pressure (Holland & Humerfelt, 1964). With technology advances, battery-powered automated devices are now available to measure and record ambulatory blood pressure. They employ the same upper arm cuff and, because of the time required for one measurement they cannot record rapid changes in blood pressure. Furthermore, a maximum of four measurements per hour for 24 hours are usually obtained because of the annoyance of having the device squeeze the arm at regular interval. Nevertheless, ambulatory blood pressure monitoring can be very useful for diagnostic to ascertain blood pressure level outside the clinic. It also provides good information about blood pressure pattern during sleep.

There is, however, a method for blood pressure determination in humans that comes close to a telemetric method: the finger cuff method of Peñaz (1973). It is available commercially

Radio-Telemetry in Biomedical Research - Radio-Telemetry Blood Pressure

the development of new drugs or the understanding of the disease.

Measurements in Animal Models of Hypertension, How It Revolutionized Hypertension Research 123

pathophysiology of the disease are important whereas the opponents argue that none of the current high-blood pressure drug-target were unveiled with GWAS and that finding thousands of common genetic variants with minute effects on blood pressure is of no use for

While recognizing the power of GWAS and the fine-mapping capabilities of the current genomic technologies, we think that the accumulation of data from large studies is of no use if this data is not accurate and reliable. Currently, significant efforts are spent to standardize large population studies (Knoppers et al., 2008). On the long term, this would permit the merging of several studies with similar design in order to pool the data and increase the statistical power. In addition to standardization, we think also that the definition of the traits to be phenotyped is cardinal to any study. Only when that is done precisely and agreed on can the methods for measuring them be standardized. As we have seen from the Canadian experience, in an effort to standardize blood pressure measurements in the clinic as well as for the multicentre GWAS, more and more automated devices were employed in rigorous conditions with rejections of the first, higher, blood pressure values. As a result, the prevalence of hypertension has dramatically decreased in the last ten years. On the optimistic side, it may indicate an impact of the public health campaign and compliance to drug regimen by the patients. On the other hand, it may just reflect 'wrong' phenotyping. As we will demonstrate in the remaining of this chapter, because up to 70% of the blood pressure variance can be of environmental origin, it is essential to take the impact of the environment into account. If it is somewhat subtracted by 'overstandardization', blood pressure values may underestimate the 'true' blood pressure, and we may end up never finding high blood pressure genes because very few people will ever display high blood pressure in those settings. We should also mention the fact that studies rarely try to discriminate between supine, standing or seated blood pressure and that the blood pressure medication is usually not withdrawn prior to measurements because of the risks (ethical concerns) or the cost of monitoring the patients during that time. For instance, we have shown that the impact of the *fto* gene on blood pressure is dependent on the body position (sympathetic modulation) and responsive to stress (Pausova et al., 2009). As we will try to demonstrate in animal models of hypertension, we think that it is important to understand what we are measuring. We will now present a section explaining how to study genes that are sensitive to the environment and how to measure the traits that are modulated by the environment. We will then show how telemetry is suitable to achieve this very specific

endeavour in research with animal models with some examples from our group.

**3. Genes X environment interactions, impact on blood pressure and** 

possible contributions of genes and environment to hypertension.

Up to 70% of blood pressure variance is attributed to the environment. The impact of the environment can be seen as additive or in interaction with genes. Figure 1 summarizes the

In the first case (a), the genes and the environment exert their influence on blood pressure independently. Thus, a disease gene (Gd) and a deleterious environment (Ed) would act separately to cause hypertension in an additive manner. In the second case (b), the environment is not "deleterious" *per se*. Its influence will depend on environment-sensitive genes (Gs). These are called susceptibility genes. They are not disease genes by themselves, they only permit the environment to reveal or amplify its impact. This is the G × E

**development of hypertension** 

under the name Finapres or Portapres (Wesseling et al., 1995; Finapres Medical Systems BV, Amsterdam, The Netherlands). Despite being cumbersome, the Portapres enables reading to be recorded over 24 hours while the subject is ambulatory (Omboni et al., 1995; Parati et al., 1996). A photoplethysmograph under a finger cuff detects the pulsation of the blood flow. The inflation is continuously adjusted by a servo-loop according to the output of the plethysmograh to keep the output constant. As a result, the artery is kept in a partially open state. The pressure oscillations of the cuff were found to resemble to the intra-arterial pressure curve. Hence, this method gives an accurate estimate of the rapid changes in blood pressure although it usually underestimates 'true' systolic and diastolic pressures. Monitoring of blood pressure variations may prove to be of high clinical significance as several reports indicate a striking relationship between blood pressure variability and stroke (Rothwell, 2010; Rothwell et al., 2010).

In conclusion, the techniques for blood pressure determination in humans provide relatively accurate values of systolic and diastolic blood pressure. But, for most of them, quality measurements and reproducibility are greatly dependent on good calibration of the equipment, the environment where the measurements are taking place and training of the personnel performing them. Finally, only the finger-cuff method provides high accuracy of blood pressure changes with a high, beat-to-beat, measurement frequency. Since it is not invasive and enables recording in ambulatory subjects in their real life activities, this is the closest to a telemetric method when it comes to blood pressure determination in humans. In addition we should not hide that its cost and the technical expertise it requires are also something it shares with telemetry…

#### **2.5 Methods to unveil the genetic basis of hypertension - Need to define high blood pressure**

With what we have presented in mind, we will now conclude this section by presenting the challenges that our field of research is facing. Hypertension stems from a combination of genetic and environmental factors. The blood pressure variance commonly attributed to the genetic component is estimated between 30% and 50% (Havlik et al., 1979). Moreover, from twin or family aggregation studies, the sibling recurrence risk of hypertension is estimated at 2,5 to 3,5 (relative risk of developing hypertension if a sibling is affected) (Tobin et al., 2007). This clearly demonstrates the important genetic basis of the disease. With the sequencing of the human genome in 2000 and the progress of the HapMap project, we entered the «genomic era». Nowadays, there are about one million single nucleotide polymorphisms available without counting other genetic polymorphisms like copy number variants. These can be used for genome-wide association studies (GWAS) to characterize thousands of individuals from cases and control population in order to localize the genetic differences associated with the disease or related quantitative traits. In the field of hypertension, several large GWAS and many meta-analyses of GWAS were performed. Many loci on several chromosomes associated with hypertension were unveiled (for review, see Dominiczak & Munroe, 2010). Despite these successes, critics from the community state that only a very small amount of the blood pressure variance is explained by each of these variants (about 1 mm Hg of systolic and 0,5 mm Hg of diastolic pressure). They rightfully ask: can such small-effect alleles have such an important population-wide role to play in the aetiology of the disease? And, with regard to the very high cost of these GWAS, is it worth continuing in the same direction with bigger and bigger study populations (Kurtz, 2010)? The proponents of the GWAS insist that the progress made in understanding the

under the name Finapres or Portapres (Wesseling et al., 1995; Finapres Medical Systems BV, Amsterdam, The Netherlands). Despite being cumbersome, the Portapres enables reading to be recorded over 24 hours while the subject is ambulatory (Omboni et al., 1995; Parati et al., 1996). A photoplethysmograph under a finger cuff detects the pulsation of the blood flow. The inflation is continuously adjusted by a servo-loop according to the output of the plethysmograh to keep the output constant. As a result, the artery is kept in a partially open state. The pressure oscillations of the cuff were found to resemble to the intra-arterial pressure curve. Hence, this method gives an accurate estimate of the rapid changes in blood pressure although it usually underestimates 'true' systolic and diastolic pressures. Monitoring of blood pressure variations may prove to be of high clinical significance as several reports indicate a striking relationship between blood pressure variability and stroke

In conclusion, the techniques for blood pressure determination in humans provide relatively accurate values of systolic and diastolic blood pressure. But, for most of them, quality measurements and reproducibility are greatly dependent on good calibration of the equipment, the environment where the measurements are taking place and training of the personnel performing them. Finally, only the finger-cuff method provides high accuracy of blood pressure changes with a high, beat-to-beat, measurement frequency. Since it is not invasive and enables recording in ambulatory subjects in their real life activities, this is the closest to a telemetric method when it comes to blood pressure determination in humans. In addition we should not hide that its cost and the technical expertise it requires are also

**2.5 Methods to unveil the genetic basis of hypertension - Need to define high blood** 

With what we have presented in mind, we will now conclude this section by presenting the challenges that our field of research is facing. Hypertension stems from a combination of genetic and environmental factors. The blood pressure variance commonly attributed to the genetic component is estimated between 30% and 50% (Havlik et al., 1979). Moreover, from twin or family aggregation studies, the sibling recurrence risk of hypertension is estimated at 2,5 to 3,5 (relative risk of developing hypertension if a sibling is affected) (Tobin et al., 2007). This clearly demonstrates the important genetic basis of the disease. With the sequencing of the human genome in 2000 and the progress of the HapMap project, we entered the «genomic era». Nowadays, there are about one million single nucleotide polymorphisms available without counting other genetic polymorphisms like copy number variants. These can be used for genome-wide association studies (GWAS) to characterize thousands of individuals from cases and control population in order to localize the genetic differences associated with the disease or related quantitative traits. In the field of hypertension, several large GWAS and many meta-analyses of GWAS were performed. Many loci on several chromosomes associated with hypertension were unveiled (for review, see Dominiczak & Munroe, 2010). Despite these successes, critics from the community state that only a very small amount of the blood pressure variance is explained by each of these variants (about 1 mm Hg of systolic and 0,5 mm Hg of diastolic pressure). They rightfully ask: can such small-effect alleles have such an important population-wide role to play in the aetiology of the disease? And, with regard to the very high cost of these GWAS, is it worth continuing in the same direction with bigger and bigger study populations (Kurtz, 2010)? The proponents of the GWAS insist that the progress made in understanding the

(Rothwell, 2010; Rothwell et al., 2010).

something it shares with telemetry…

**pressure** 

pathophysiology of the disease are important whereas the opponents argue that none of the current high-blood pressure drug-target were unveiled with GWAS and that finding thousands of common genetic variants with minute effects on blood pressure is of no use for the development of new drugs or the understanding of the disease.

While recognizing the power of GWAS and the fine-mapping capabilities of the current genomic technologies, we think that the accumulation of data from large studies is of no use if this data is not accurate and reliable. Currently, significant efforts are spent to standardize large population studies (Knoppers et al., 2008). On the long term, this would permit the merging of several studies with similar design in order to pool the data and increase the statistical power. In addition to standardization, we think also that the definition of the traits to be phenotyped is cardinal to any study. Only when that is done precisely and agreed on can the methods for measuring them be standardized. As we have seen from the Canadian experience, in an effort to standardize blood pressure measurements in the clinic as well as for the multicentre GWAS, more and more automated devices were employed in rigorous conditions with rejections of the first, higher, blood pressure values. As a result, the prevalence of hypertension has dramatically decreased in the last ten years. On the optimistic side, it may indicate an impact of the public health campaign and compliance to drug regimen by the patients. On the other hand, it may just reflect 'wrong' phenotyping. As we will demonstrate in the remaining of this chapter, because up to 70% of the blood pressure variance can be of environmental origin, it is essential to take the impact of the environment into account. If it is somewhat subtracted by 'overstandardization', blood pressure values may underestimate the 'true' blood pressure, and we may end up never finding high blood pressure genes because very few people will ever display high blood pressure in those settings. We should also mention the fact that studies rarely try to discriminate between supine, standing or seated blood pressure and that the blood pressure medication is usually not withdrawn prior to measurements because of the risks (ethical concerns) or the cost of monitoring the patients during that time. For instance, we have shown that the impact of the *fto* gene on blood pressure is dependent on the body position (sympathetic modulation) and responsive to stress (Pausova et al., 2009). As we will try to demonstrate in animal models of hypertension, we think that it is important to understand what we are measuring. We will now present a section explaining how to study genes that are sensitive to the environment and how to measure the traits that are modulated by the environment. We will then show how telemetry is suitable to achieve this very specific endeavour in research with animal models with some examples from our group.
