Mass spectrometry for monitoring respiratory and anesthetic gas waveforms in rats

A method is presented for real-time monitoring of airway gas concentration waveforms in rats and other small animals. Gas is drawn from the tracheal tube, analyzed by a mass spectrometer, and presented as concentration vs. time waveforms simultaneously for CO2, halothane, and other respiratory gases and anesthetics. By use of a respiratory simulation device, the accuracy of mass spectrometric end-tidal CO2 analysis was compared with both the actual gas composition and infrared spectrophotometry. The effects of various ventilator rates and inspiration-to-expiration ratios on sampling accuracy were also examined. The technique was validated in male Sprague-Dawley rats being ventilated mechanically. The difference between the arterial PCO2 (PaCO2) and the end-tidal PCO2 (PETCO2) was not significantly different from zero, and the correlation between PETCO2 and PaCO2 was strong (r = 0.97, P less than 0.0001). Continuous gas sampling for periods up to 5 min did not affect PaCO2, PETCO2, or airway pressures. By use of this new method for measuring end-tidal halothane concentrations in rats approximately 6.5 mo of age, the minimum alveolar concentration of halothane that prevented reflex movement in response to tail clamping was 0.97 +/- 0.04% atmospheric (n = 14). This mass spectrometric technique can be used in small laboratory animals, such as rats, weighing as little as 250 g. Gas monitoring did not distort either PETCO2 or PaCO2. Under the defined conditions of this study, accurate and simultaneous measurements of phasic respiratory concentrations of anesthetic and respiratory gases can be achieved.     

Transvalvular left ventricular pressure measurement in the isolated working rat heart.

A method of continuously measuring left ventricular (LV) pressure in an isolated buffer-perfused working rat heart is described. Transvalvular placement of a micromanometer through the aorta is the unique feature of this procedure. Advantages include catheter stability and lack of myocardial trauma. Changes in cardiac function were quantified by exposing hearts to either isoproterenol (10(-9) M) or halothane (1.5% vol/vol). To examine if any obstruction to LV outflow was caused by the micromanometer, cardiac performance was assessed during pullback from the ventricle to the aorta. Complications such as aortic insufficiency and ventricular arrhythmias were also studied. The results indicate that the transvalvular placement of a micromanometer can provide continuous, high-fidelity reproduction of LV pressure in this small-organ preparation. The presence of the micromanometer did not significantly alter cardiac performance, and proper catheter placement was achieved easily in a high percentage (> 90%) of cases.     

O2 content of blood sampled from different venous compartments of the rat

Male Sprague-Dawley rats (n = 18) weighing 548 +/- 30 g were anesthetized with pentobarbital sodium (40-65 mg/kg body wt ip), intubated via tracheotomy, and mechanically ventilated. After exposure of the great vessels in the thorax, blood was withdrawn from the pulmonary artery (PA), right ventricle (RV), right atrium (RA), inferior vena cava (IVC), and ascending aorta. The O2 content of these blood samples was determined by direct measurements and/or was calculated from the measured hemoglobin concentration, percent of O2 saturation, and PO2. Ventilatory rates and the inspired fraction of O2 were manipulated to vary the mixed venous O2 content (CvO2) of blood withdrawn from the PA from 1.4 to 12.9 ml O2/dl blood (vol%). Our results demonstrate that O2 contents of blood withdrawn from the PA, RV, and RA are not significantly different from one another (CPAO2 - CrvO2 = -0.02 +/- 0.25 and CPAO2 - CRAO2 = -0.07 +/- 0.41 vol%, n = 28, P greater than 0.05); however, the O2 content of blood withdrawn from the IVC is significantly lower than that withdrawn from the PA (CPAO2 - CIVCO2 = 2.11 +/- 0.34 vol%, P less than 0.001). In addition, the directly measured O2 contents were equivalent to those that were calculated. These results suggest that the O2 content of blood found in the RA and RV of the rat are indicative of the O2 content of blood found in the PA. Thus blood sampled from these areas can be used to estimate mixed venous oxygenation.     

Anesthetic effects on liver and muscle glycogen concentrations: rest and postexercise

We compared the effects of three different anesthetics (halothane, ketamine-xylazine, and diethyl ether) on arterial blood gases, acid-base status, and tissue glycogen concentrations in rats subjected to 20 min of rest or treadmill exercise (10% grade, 28 m/min). Results demonstrated that exercise produced significant increases in arterial lactate concentrations along with reductions in arterial Pco2 (PaCO2) and bicarbonate concentrations in all rats compared with resting values. Furthermore, exercise produced significant reductions in the glycogen concentrations in the liver and soleus and plantaris muscles, whereas the glycogen concentrations found in the diaphragm and white gastrocnemius muscles were similar to those found at rest. Rats that received halothane and ketamine-xylazine anesthesia demonstrated an increase in Paco2 and a respiratory acidosis compared with rats that received either anesthesia. These differences in arterial blood gases and acid-base status did not appear to have any effect on tissue glycogen concentrations, because the glycogen contents found in liver and different skeletal muscles were similar to one another cross all three anesthetic groups. These data suggest that even though halothane and ketamine-xylazine anesthesia will produce a significant amount of ventilatory depression in the rat, both anesthetics may be used in studies where changes in tissue glycogen concentrations are being measured and where adequate general anesthesia is required.