**7. Respiratory function exploration**

#### **7.1. Children under 2 years of age**

could have a strong prognostic value. However, Pohunek et al. [23] found no differences in ECP levels in children under 3, between both diagnoses when children were asymptomatic, as opposite to when they were suffering a crisis. From his work, it can be deducted the inter‐ est to assess the enzyme during a bronchobstructive crisis, in yet not diagnosed children, es‐ pecially during the first episode, for the possibility of establishing prophylactic measures to

**+1 SD +2 SD**

prevent or delay the establishment of asthma.

84 Asthma - From Childhood Asthma to ACOS Phenotypes

**Age Geometric mean**

**Table 2.** Normal serum IgE levels (Kjellman and Johanson, 1976).

**Table 3.** Non-allergic process more frequents in small children that occur with high IgE.

**Parasitic infections** *Ascaris lumbricoides Toxocara canis*

*Entamoeba haemolitica*

**Immunodeficiency disorders**

Wiskott-Aldrich S. Di George S. Hiper IgE S.

Selective IgA deficiency

*Echinococcus Trichinella spiralis*

*Filaria* Others **kU/l**

Umbilical cord 0.3 – – 6 weeks 0.7 2.1 6.1 3 months 0.8 1.8 3.8 6 months 2.7 6.6 16.3 9 months 2.4 4.2 7.3 12 months 7 21 58 2 years 11 26 61 3 years 11 21 40 4 years 20 37 70 7 years 26 75 221

In the diagnosis of asthma or of wheezy bronchitis, exploration in the infant has a relative utility. However, the greatest value of early exploration is focussed on distinguishing the processes that occur with breathing difficulties of various kinds, some of bronchopulmonary origin (cystic fibrosis, bronchopulmonary dysplasia, primary ciliary dyskinesia, bronchitis obliterans, malformations) or extrapulmonary (vascular rings, ductus persistence, other heart disease), which can raise the differential diagnosis with severe asthma or resistant to habitual treatment. Exploration in infants should be restricted due to the risk involved in the need to sedate the child, requiring hospital environment, which should be restricted to the following indications, in order to diagnose other possible diseases with apparent symptoms of asthma, although symptoms can have a different origin:


#### *Methodology*

The most common method is the rapid thoracoabdominal compression at ordinary volume. As the child breathes normally and at the end of an inspiration, an insufflation of the jacket is provoked to induce maximal forced expiration, thus obtaining the parameter of the maximum flow at functional residual capacity (VmaxFRC). Both the figure obtained and the appreciation of the layout of the flow-volume curve will provide information on the permeability of the airways and the degree of obstruction, if any.

Less suitable is the one based on thoracoabdominal compression provided by a pneumatic jacket, a kind of inflatable vest connected to an air compressor through a non-distensible thick tube. The exhaled airflow is measured with a pneumotachograph of appropriate size that adapts to the mouth through a mask. The biggest drawback is the need to sedate the child with chloral hydrate orally, which is not without risks.

As forced expiration is provoked during a normal respiratory movement, that is ordinary volume, this technique does not provide the value of the forced vital capacity (FVC) obtained with a spirometry, and therefore, no FEV1 or the FEV1/FVC ratio and FEV1% can be obtained. To acquire these values, we must perform the technique known as rapid thoracoabdominal compression with previous insufflation, which is artificially forcing a deep breath, for which, by a compressor connected to an oral–nasal mask, air is blown into child up to the maximum vital capacity and requiring a pressure of 2–3 kPa (20–30 cm H2O). Afterwards, a rapid inflation of the jacket is performed to provoke the forced expiration. This way, valuable data comparable to those provided by spirometry, can be obtained.

#### **7.2. Children over 2 years of age**

Although the following techniques are used in older children who do not cooperate during the exploration, some of them are also used in infants with devices adapted to their size (plethysmograph), requiring sedation of the child. With them, precise information on airway resistance (AWR) and compliance can be obtained.

The AWR is defined by the difference of pressure between both ends of the airways, that is the mouth and alveoli when an air flow of 1 l/s occurs. The circumstantial narrowing of the bronchi increases resistance, deducing the intensity of the obstruction by the degree of the increase in the value of the AWR.

Compliance refers to the intensity of the rigidity of the respiratory system, defined by the increase of the volume produced divided by the increase of the pressure unit, so that the less pressure necessary to produce an equal increase in volume, the higher the compliance.

The resistance can be measured through well-known techniques, such as interruption of flow (Rint), forced oscillation and its oscillometric impulse variation and plethysmography, a technique which also allows the compliance to be known.

#### *Flow interruption (Rint)*

The interruption of the airflow during normal breathing causes rapid and complete occlusion of the airways, enabling the measurement of the pressure in the oral cavity immediately before the interruption and during the normal respiratory cycle using a pneumotachograph. The measurement of the alveolar pressure is deduced from the pressure value established in the mouth when the momentary interruption of flow occurs. If this is suddenly interrupted, for a split second, an immediate rise in pressure occurs in the mouth. The pressure measured this way is very similar to that found in the alveoli, allowing the measurement of *resistance*, comparing this pressure with that prevailing in the mouth just before shutting off the flow. The Unit used to measure the resistance is Pascal in litres per second. In short, Rint is the most simple and affordable method to learn the AWR in young children, with the only requirement being that they breathe normally through the tube that is fitted into the mouth, preventing nasal breathing by pinching the nose.

#### *Forced oscillation: impulse oscillometry*

*Forced oscillometry* is based on the application of oscillatory pressure changes and therefore airflow, measuring the resistance from the relationship between the two. Easy to perform, it only requires that the child breathe calmly through a tube—not longer than 1 m—attached to the mouth and a volume of 100 ml, having inserted a pneumotachograph in between. The patient's air flow is superimposed with a oscillanting flow of 2 ml at a frequency of 10 Hz coming from a sinusoidal generator, thereby leading to pressure variations which are not perceived by the child and are proportional to the 'total' patient breathing resistance.

Best results are obtained with a newer variation of this process, the *impulse oscillometry* (IOS). A controlled deviation of the membrane of a loudspeaker, which is adapted to a nozzle, leads to the excitation of the airflow generating pressure impulses, from which ratio, the value of the central airways resistance and the lung elasticity is obtained; and therefore, the values result from the relationship between the exerted pressure and the airflow [4].

#### *Plethysmography*

tube. The exhaled airflow is measured with a pneumotachograph of appropriate size that adapts to the mouth through a mask. The biggest drawback is the need to sedate the child with

As forced expiration is provoked during a normal respiratory movement, that is ordinary volume, this technique does not provide the value of the forced vital capacity (FVC) obtained with a spirometry, and therefore, no FEV1 or the FEV1/FVC ratio and FEV1% can be obtained. To acquire these values, we must perform the technique known as rapid thoracoabdominal compression with previous insufflation, which is artificially forcing a deep breath, for which, by a compressor connected to an oral–nasal mask, air is blown into child up to the maximum vital capacity and requiring a pressure of 2–3 kPa (20–30 cm H2O). Afterwards, a rapid inflation of the jacket is performed to provoke the forced expiration. This way, valuable data comparable

Although the following techniques are used in older children who do not cooperate during the exploration, some of them are also used in infants with devices adapted to their size (plethysmograph), requiring sedation of the child. With them, precise information on airway

The AWR is defined by the difference of pressure between both ends of the airways, that is the mouth and alveoli when an air flow of 1 l/s occurs. The circumstantial narrowing of the bronchi increases resistance, deducing the intensity of the obstruction by the degree of the increase in

Compliance refers to the intensity of the rigidity of the respiratory system, defined by the increase of the volume produced divided by the increase of the pressure unit, so that the less pressure necessary to produce an equal increase in volume, the higher the compliance.

The resistance can be measured through well-known techniques, such as interruption of flow (Rint), forced oscillation and its oscillometric impulse variation and plethysmography, a

The interruption of the airflow during normal breathing causes rapid and complete occlusion of the airways, enabling the measurement of the pressure in the oral cavity immediately before the interruption and during the normal respiratory cycle using a pneumotachograph. The measurement of the alveolar pressure is deduced from the pressure value established in the mouth when the momentary interruption of flow occurs. If this is suddenly interrupted, for a split second, an immediate rise in pressure occurs in the mouth. The pressure measured this way is very similar to that found in the alveoli, allowing the measurement of *resistance*, comparing this pressure with that prevailing in the mouth just before shutting off the flow. The Unit used to measure the resistance is Pascal in litres per second. In short, Rint is the most simple and affordable method to learn the AWR in young children, with the only requirement

chloral hydrate orally, which is not without risks.

86 Asthma - From Childhood Asthma to ACOS Phenotypes

to those provided by spirometry, can be obtained.

resistance (AWR) and compliance can be obtained.

technique which also allows the compliance to be known.

**7.2. Children over 2 years of age**

the value of the AWR.

*Flow interruption (Rint)*

A plethysmograph measures the changes in volume of the thorax. Its principle is the law of Boyle and Mariotte, whereby the volume occupied by a mass of gas, at constant temperature, is inversely proportional to the pressure. By plethysmography, intrathoracic gas volume (TGV) through the volumetric variations of the thorax is measured simultaneously with the AWR, and from these values, conductance can be calculated.

#### *Bronchodynamic tests*

The above techniques report on the current state of the permeability of the lower airways but do not provide information on the level of BHR, the increase of which is significant in the diagnosis of asthma. Hence, the need to carry out tests to make it evident, that is bronchocon‐ strictor (methacholine, histamine) and bronchodilator (β-mimetic) tests. The three technical variations mentioned can be used for this purpose
