Systemic arterial blood pressure is regulated mainly by the high-pressure baroreceptor reflex. This reflex is responsible for maintaining systemic arterial pressure despite diseases and pharmacological perturbations. An example of disease would congestive heart failure, in which the increase in sympathetic activity is responsible for reduction in baroreflex sensitivity. The clinical relevance of the baroreflex dysfunction lies in the fact that studies have shown that physical training and beta-adrenergic blockage improves baroreceptor sensitivity, as well as the patient’s prognosis. To investigate the baroreceptors in dogs, we hypothesized that baroreceptor function, as assessed by gain and time-constant, does not change with repeated exposures to morphine-chloralose anesthesia, which is a very common anesthestetic protocol used in laboratories of veterinary research. To study baroreflex, the most used tests are the oxford, neck suction, and tilt. The latter, is discussed in more detail here, since it was the method used to evaluate the baroreceptor response to morphine-chloralose anesthesia. The purpose of this study was to determine the effects of repeated doses of morphine- chloralose anesthesia on baroreceptors function, by tilting the dogs, and histological integrity, by blood profile and blood cell evaluations, as well as histopathology of the important organs. To accomplish these goals, 7 healthy hound-type dogs (20-25 kg) and 3 healthy beagle-hounds (9.5 - 12 kg), males, between 1 and 3 years of age, and healthy were studied. All dogs were given, IV, morphine sulfate (1.5 mg/kg) as a pre-anesthetic. They were then given, IV as a bolus, alpha chloralose (100 mg/kg), after which they received a continuous infusion of alpha chloralose (30 to 40 mg/kg/hour) to sustain anesthesia. Animals were ventilated with room air at a rate (12/minute) and a tidal volume (12.5 ml/kg) to sustain systemic arterial PaCO2 of approximately 40mmHg.
To access pressure within the carotid sinus, a fluid-filled catheter, attached to a pressure transducer, was advanced retrogradely into the region of the carotid sinus for 4 dogs. To measure aortic arch sinus arterial pressure a fluid-filled catheter attached to a pressure transducer was advanced through a femoral artery into the region of the junction of the ascending aorta with the arch for 4 dogs. Electrodes forming ECG leads I and II were attached to all. Six dogs were studied 3 times with 48 hours between studies. Each time, the dogs were studied 3 times with 30 minutes in between. After baseline measurements of pressures (4 dogs) and ECG’s (6 dogs) were made for 30 seconds, dogs were tilted within <1second to a head-up-position. This head-up tilt was maintained for 20 seconds, after which the dogs were returned to the horizontal. Measurements of pressures (4 dogs) and ECG’s (6 dogs) were made the entire time. The results show that in regards to the baseline heart rate did decrease consistently with days. Neither time (P=0.50) nor day (P=0.92) was significantly associated with the maximum absolute increase in heart rate. As the experiment advances from day 1 to day 2 to day 3, the absolute decrease in heart rate becomes larger. During the initial 5 seconds of the head-up tilt the heart rate increased approximately 12 beats/minute, while peak systolic (p= 0.027), diastolic (p= 0.014), and mean pressures (p= 0.029) decreased more at the carotid sinus than at the aortic sinus. For the calculated gain (6 dogs) there were no differences among means either by day (p=0.14) or by time (p=0.992), nor was there a day-time interaction (p=0.751). The recover from each anesthetic episode was not turbulent or violent. Analysis of the blood constitutes tissues evaluated in the histology showed no alterations due to anesthetic regimen. There were no differences of significance in gain. An important limitation is the lack of assurance that constant levels of anesthesia were maintained over the 3 recordings of each day, or if constant levels were achieved each day. Recovery from this anesthetic protocol is relatively long compared to other anesthetics (e.g. propofol, isoflurane). Finally buffering of the baroreceptor response elicited from changing pressure at the carotid sinus may alter, profoundly, the role of only the carotid sinus receptors. Therefore it was concluded that 1.repeated anesthesia with morphine/chloralose results in no observable pathological changes monitored by histopathology, blood chemistry, and analysis of blood cellular components; 2.gain of high-pressure baroreceptor reflex may be assessed using the passive head-up tilt; 3.gain did not change significantly within an anesthetic period or on subsequent days; 4.for head-up tilt, the change in pressure on the carotid sinus was greater than the change of pressure on the aortic sinus.