**4.1 PR (PQ ) interval**

The PR (PQ ) interval is measured from the beginning of the P wave to the beginning of the QRS complex. This interval reflects the time that the electrical impulse passes from the SA node through the AV node. The PR interval provides information about the time required for the transmission of the electrical impulse from the atria through the AV node, His bundle, Tawar's branches, and Purkinje fibers to the start of ventricular muscle depolarization [107–109].

A prolonged PQ interval reflects a longer time of transmission of the impulse from the atrium to the ventricles in disorders of the conductive system of the AV node [110, 111]. A shortened PQ interval means that the impulse was transmitted to the ventricular conductive system earlier than normal; thus, it is likely that it passes around the AV node through abnormal connections of the conductive system [111–113]. The duration of the PR interval is a crucial marker in the diagnosis of atrioventricular blocks. However, it appears that the PR interval in rats also appears to be dependent on the type of anesthesia, and we have practically no information about sex differences and changes dependent on the LD cycle.

Although mean values of the duration of the PR (PQ ) interval were comparable among the different types of anesthesia and did not exhibit significant differences (**Table 2**, **Figure 2**), the shortest duration was found with nembutal anesthesia [114]. With this type of anesthesia, there is a problem with the validity of this value because it is from only one study. The situation is similar with desflurane [72], ketamine/



*Data are presented as the average value of PR (PQ ) interval duration (ms) (range); (n, number of baseline or control values from which heart rate was evaluated). Not specified - the methodology did not specify the lighted period when the experiments were performed.*

#### **Table 2.**

*Duration of PR (PQ ) interval duration under individual types of anesthesia according to sex and light cycle (light [inactive]) versus dark [active]).*

medetomidine [96], ketamine/diazepam [96], ketamine/midazolam, [97], anesthesia in isolated hearts [105, 115], and in tribromethal anesthesia [106, 116].

Duration of the PR (PQ ) interval from telemetry studies [21, 23–25, 30, 117–119], inhalation (isoflurane) [72, 74, 75, 120, 121] pentobarbital [32, 34, 36, 37, 40, 43–47, 49, 122–126], thiopental [63–65, 68, 71], urethane [45, 52, 53, 56, 60, 128], and ether anesthesia [99–101, 104] did not differ significantly from one another. The shortened duration of the PR (PQ ) interval was under ketamine/xylazine anesthesia [45, 78, 79, 84, 85, 89, 91, 92, 127–129]. The duration of the PQ (PR) interval in isolated hearts [105, 115] did not differ significantly from the duration with other types of anesthesia. *Rat Electrocardiography and General Anesthesia DOI: http://dx.doi.org/10.5772/intechopen.104928*

#### **Figure 2.**

*Distribution of average values and ranges of PR (PQ ) interval duration from telemetry studies and under different types of general anesthesia in male rats without taking into account the light periods of the rat regimen day when the experiments were performed. Only PR (PQ ) interval ranges from at least three studies where PR (PQ ) interval was evaluated and is shown in the figure. Telemetry studies (n = 10), pentobarbital anesthesia (n = 18), thiopental anesthesia (n = 6), ketamine/xylazine anesthesia (n = 13), isoflurane anesthesia (n = 6), ether anesthesia (n = 4), urethane anesthesia (n = 9). n, number of baseline or control values from which duration of PR (PQ ) interval was evaluated.*

For a given ECG parameter, it was difficult to determine sex differences, as well as differences dependent on the LD cycle because there was only one study (**Table 2**).

The P wave represents the depolarization of the atria. Atrial depolarization spreads from the SA node toward the AV node, and the right to the left atrium. In humans, but also in rats, the physiological sinus rhythm is characterized by the same P wave orientation as the R wave and its occurrence before each QRS complex in all cardiac cycles. P wave duration has been evaluated in Wistar rats, for which prolongation after myocardial infarction may be associated with increased sensitivity to supraventricular arrhythmias [130].

Other parameters of atrial complex evaluation include amplitude and polarity (either negative or positive, although it can also be so flat that it is indistinguishable from the isoelectric line). If the P wave is unusually high, it may reflect enlargement of the atria. Typically, an enlarged right atrium exhibits a high, spiked P wave, while an enlarged left atrium is reflected by a bifidic P wave on ECG. The absence of a P wave or its altered shape is present in various cardiac arrhythmias, the most common of which is atrial [131, 132]. Although the analysis of P wave duration and shape in humans provides clinically important information, there is a lack of experimental data from rats to draw definitive conclusions about sex-related changes and circadian rhythm in P wave amplitude and duration [45].

#### **4.2 P wave duration and amplitude**

The duration and amplitude of the P wave, despite their important prognostic significance, have only been sporadically evaluated in *in vivo* experiments involving rats. The average amplitudes of the P wave were essentially the same at all types of anesthesia (i.e., in studies where the given parameter was evaluated). Only one telemetry study [118] evaluated P wave duration, and if it is considered as a reference value, only in males, prolonged duration was under ketamine/xylazine anesthesia [84, 89]


*Data presented as average (range); (n, number of baseline or control values in which the amplitude and duration of the P wave were evaluated).*

#### **Table 3.**

*P wave amplitude and duration, regardless of synchronization of the male rats to the light and dark cycle under individual types of anesthesia.*

and ketamine/medetomidine [96]. Shorter durations were under pentobarbital [46, 126] and thiopental [64] anesthesia. Approximately the same duration of the P wave was under the other types of anesthesia (**Table 3**). The amplitude of the P wave was the smallest in all combinations with ketamine (ketamine/xylazine) [82, 84, 89], ketamine/medetomidine [96], ketamine/diazepam [96, 98], ketamine/midazolam [97] and urethane [133], and isolated hearts [115].

The extent to which these values are valid cannot yet be assessed because there are an insufficient number of studies; this problem also affects sex and the LD effect on the amplitude and duration of the P wave. There is an indication, however, that there may be sex differences in the duration of the P wave under ketamine/xylazine anesthesia (21.99 ms [range 17.38 ms–26.62 ms]) for females and 20.37 ms (range 18.84 ms–26.49 ms) in males [19]. However, to date, this is not statistically demonstrable for other types of anesthesia.
