Normative data for arterial blood gas and electrolytes in anesthetized rats

Sir,

Rats are useful models to study acid/base and respiratory parameters in preclinical trials and toxicological studies. Normative data for arterial blood gases (ABG) and electrolytes would help identify healthy animals for the experiments. Studies report ABG values in awake rats.[1–7] In awake rats, mean pH has been found to be more than 7.4 and mean P_aCO₂ less than 41 mmHg. In anesthetized rats, there is considerable respiratory depression resulting in lower pH and higher P_aCO₂.[6,8] We report values obtained in rats anesthetized with intraperitoneal ketamine. The values are from the first samples obtained in anesthetized rats used in various toxicological experiments.

Values of ABG and serum electrolytes in the first arterial blood sample obtained from 106 rats used for various toxicological studies have been collated in this study to provide descriptive statistics. Wistar rats (150-300 gm) were used for the experiments after obtaining clearance from the Institutional Animal Ethics Committee. The rats were bred and housed at the institutional animal housing facility with adequate lighting and ventilation. Food and water were provided ad libitum. Animals were anesthetized with Ketamine (50-100mg/kgb.w., i.p.). 0.25 ml of 1000 IU/ml of Heparin was injected into the animal at the time of cannulation. Immediately after cannulation 0.25 ml of blood was withdrawn from the carotid artery and kept aside for reinfusion later. Another 0.25 ml was withdrawn and injected into the cartridge (iSTAT, EG7 cartridge) without any delay for ABG and electrolyte analysis; Care was taken to avoid air bubbles. Blood pH, PCO₂, PO₂, bicarbonate, sodium, potassium were taken for the analysis. Box plots and frequency distribution plots were plotted using SPSS software.

The ABG values in anesthetized rats are presented in Table 1. Frequency distribution plots [Figure 1] show the distribution of data around the mean. The plots for blood pH, PCO₂, PO₂, sodium and potassium almost follow a normal Gaussian curve. Tests of Normality for the above parameters show that there is very little deviation from normality. The p values for the Normality tests (Shapiro-Wilk's test)are as follows: blood pH - 0.474, PCO₂ - 0.386, PO₂ - 0.12, bicarbonate - 0.656, sodium - 0.124, potassium - 0.078. The observed frequency distributions are not significantly different from the expected normal distribution pattern. The box plots show the interquartile range and outliers for the above parameters [Figure 2].

Table 1.

Arterial blood gas and electrolyte values in 106 rats

Figure 1.

Histogram showing the frequency distribution of values. This shows that the distribution of values almost follows a normal Gaussian curve. A) Blood pH; B) P_aCO₂; C) P_aO₂; D) Sodium; F) Potassium

Figure 2.

Box plot of normal blood gas values. This shows the distribution of values around the median. The outliers are shown as circles. A) Blood pH; B) P_aCO₂; C) P_aO₂; D) Bicarbonate; E) Sodium; F) Potassium

Availability of normative arterial blood gas and electrolyte data in anesthetized animals can help in selection of healthy animals for experimentation. There are studies which report ABG values in anesthetized rats but considerable respiratory depression is reported in those rats.[6,8] Here we report normative arterial blood gas data in rats anesthetized with ketamine IP. The normal mean blood pH, PCO₂ and PO₂ values in awake un-anesthetized rats reported earlier are 7.47±0.02, 34.5±3 mmHg, 90 ±5.5 mmHg and 25.5±1.5mmol/l.[6] The same study compares the values in awake un-anesthetized rats and anesthetized rats and concludes that anesthesia causes respiratory depression in the rats and the values are significantly different from the un-anesthetized rats. The blood pH, PCO₂, PO₂ and bicarbonate values in rats anesthetized with Pentobarbitol and In actin were 7.365±0.1, 42.5±3.5 mmHg, 69.5±5 mmHg and 26.5±1.9mmol/l respectively.[6] The PO₂ and bicarbonate values of the present study are comparable to the values in awake un-anesthetized rats[6] whereas the PCO₂ and blood pH values are different. The reported hypoxia in anesthetized rats is not seen in the present study. In addition, acidosis is milder in our study as compared to previous studies.[6] This difference could be because of the anesthetic used or due to the procedure per se. Rats without intervention in this study, survive for more than eight hours with frequent top-up doses of ketamine. Cannulation in rats does not affect the acid base status according to a previous study.[7] Therefore the ABG values of the present study can serve as normative control data for toxicological experiments.

For choice of healthy animals for experimentation it is suggested that their values lie within two SDs (or one SD, if less biological variability is required) above or below the mean for each value shown in Table 1. Alternatively the 25^th to 75^th interquartile range may be used as inclusion criteria [Table 1].