Biology

 Since hypertension in sinoaortic denervated dogs has been reported by many investigators in the past, this study was done in order to verify these reports. The result of the study showed no major amount of hypertension but great variations in arterial blood pressure instead. They also exhibited transient hypertension episodes during postural changes. Thus it was necessary to reevaluate its effects on all important hemodynamic variables. 24-hour recording of pressure and removal of disturbing factors are also necessary.
  
The methodology involved preparation of 12 normal and 15 denervated dogs. Baroreceptor denervation was performed by surgically stripping the nerve and adventitia from the aortic arch and carotid sinus areas. Cervicovagal trunk was reduced by 1.5 cm and medial bundle of right vagus was cut. An electromagnetic flow transducer was also implanted and calibrated in vitro. A chronic indwelling of polyinyl-Tygon catheter was placed in the femoral artery and tunneled to the cephalad position. Plasma renin activity was measured using radioimmunoassay technique. An arterial blood pressure transducer and a Crass polygraph were used to record blood pressure, cardiac output, aortic pulse flow and position of the dog. The heart rate was used using Crass techygraph, while the cardiac output was obtained using pulse logic Biotronex blood flowmeter.
   
The polygraph records were analyzed by a PDP-9 computer. Four separate fiber optic scanning pens were used with three channels of Crass recorders to obtain two identical recordings of pulse and arterial pressure. Cardiac output was also recorded using similar scanning technique.The digitalized information was used to calculate average hourly blood pressure, cardiac output, total peripheral resistance, and heart rate.
   
The 96-minute recording comparison of a normal and baroreceptor denervated dog showed fluctuations in the arterial blood pressure of the denervated dog between 220180 mm Hg and 7545 mm Hg with minimal excitement. This showed the need of new method to quantify arterial blood pressure in denervated dogs. A continuous 24-hour     recordings of all the hemodynamic factors, which illustrates events that most frequently altered the arterial blood pressure were obtained.  These shows large elevations in heart rate, pressure and cardiac output that frequently occurred.
   
The comparison of 24-hour mean arterial blood pressure in normal and denervated dogs showed that the mean arterial blood pressure of a normal dog was distributed around a mean pressure of 98 mm Hg with a range of 75 to 125 mm Hg. On the contrary baroreceptor denervated dogs exhibited a wide range of pressure with a mean of 104 mm Hg. Important features of the composite graphs and tabular data showed a difference of 11.1 mm Hg between normal dogs and denervated dogs, which was quite significant statistically. The same was in the case of systolic and diastolic pressures. Minute to minute fluctuation and average 24-hour deviation was also significantly different. Mean arterial blood pressure was elevated to high levels in denervated dogs many times in a day even because of slight disturbances. There were also hypotensive episodes recorded of very low mean arterial blood pressure which lasted for 15-20 minutes.
   
The pattern of 24-hour heart rate distribution was similar to the pattern of arterial blood pressure. The heart rates of normal dogs varied less than the heart rates of denervated dogs. Normal dogs mean heart rate was 89.0 beatsmin compared to 105.0 beatsmin of denervated dogs. The cardiac output and peripheral resistance was similar in normal and denervated dogs in a 24-hour period, though with a different characteristics on short periods. The denervated dogs exhibit more abrupt changes. This led to the conclusion that the absence of baroreceptor reflexes did not alter the daily variability in cardiac output and total peripheral resistance.
   
Despite the initial rise in mean arterial blood pressure immediately after the surgery it fell after few hours but rose gradually after a week and then did not show further changes in the pattern. In baroreceptor denervated dogs diurnal variation in arterial blood pressure was evident twice a day. On the contrary normal dogs exhibit only a slight decrease in pressure in early morning. The analysis of hemodynamic changes resulting during the postural changes of both normal and denervated dogs showed a rise in pressure in both groups when standing up but a more rapid fall in denervated dogs. Cardiac output and total peripheral resistance was same in both groups but with a rapid decrease in peripheral resistance in denervated dogs. Moreover it took more time for the denervated dogs for their blood pressure to return to pre-standing levels. The average blood volume and plasma electrolytes was nearly the same in both groups. There was no difference in arterial renin activity in both groups.
   
This study led to the conclusion that presence of neurogenic hypertension in baroreceptor denervated dogs is not valid. The mean 24-hour pressure of an undisturbed dog was not far from normal. On the contrary it led to variations in arterial blood pressure. Thus the baroreceptor system does not maintain the arterial blood pressure for prolonged periods. The cardiac output and total peripheral resistance were the least influenced by the baroreceptor system. The study also suggested that the evidence of high blood pressures in previous studies was the result of psychic stimuli resulting from the monitoring techniques used previously. Moreover some denervation techniques also result in prolonged elevation of pressure like to excise the entire area of common carotid bifurcations. Furthermore these dogs show a 24-hour wide arterial blood pressure frequency never observed in normal dogs.
   
Diurnal variation of arterial blood pressure was difficult to explain as it did not associate with daily routines or surrounding noise. Elevations of pressures during sleep might be due to increase in peripheral resistance. There was little difference between the cardiac output and total peripheral resistance between both groups. Moreover the hemodynamic effects of postural changes reflect twice fall in blood pressure in denervated dogs. Denervated dogs unlike their normal fellows were unable to maintain the total peripheral resistance.
   
The overall results obtained from the study led to the conclusion that previously reported hypertension in denervated dogs was a bit exaggerated and baroreceptor system s primary function is to minimize arterial blood pressure variations.

What is learnt from this article
The primarily objective of this study was to verify the previously reported neurogenic hypertension in baroreceptor denervated dogs. The findings of this report clearly states that the previous studies, which proved this phenomenon to be true were not that valid,  but on the contrary highly exaggerated. The study proved that with the help of better technique and computer analysis is was possible to determine that the primary function of baroreceptor system was not to maintain the chronic level of pressure but to minimize systemic arterial blood pressure variations.
   
The study clearly made its point that the reason behind these exaggerated reports regarding hypertension in baroreceptor denervated dogs were primarily because of the methods of monitoring the blood pressure or because of the techniques used to denervate the baroreceptor reflex system, most importantly by excising the entire area of common carotid bifurcations. Since the most prominent difference in the readings were the variations in the 24-hour distribution of mean arterial blood pressure between the two groups it s imminent that the mean arterial blood pressure is the most highly influenced function by the baroreceptor reflex system.