The Bainbridge reflex in canine cardiac autotransplants

The effects of volume infusion were examined in cardiac autotransplants to determine the role of cardiac innervation in the Bainbridge reflex. In autotransplantation the atria are transected ventral to the vena cavae and pulmonary veins. The remaining atria ("host") are innervated. The autotransplanted atria and ventricles ("donor") are extrinsically denervated. Host and donor heart rate, arterial pressure, aortic flow, and, in several experiments, venous pressure were monitored. Sixteen volume infusions (saline or 6% dextran) of 300 to 800 cc were given to five unanesthetized animals. In three additional experiments infusions were given after atropine, 2 mg, and propranolol, 1 mg/kg.

In all of these animals, volume infusion was accompanied by a small change in donor rate and a consistent increase in the host rate. The venous pressure, when measured, increased 7 cm H 2 O, the aortic pressure mean change was +3 mm Hg, and the cardiac output increased 0.2 L/min. Atropine and propranolol both attenuated the host rate response to infusion.

A venous pressure rise distributed equally to innervated and denervated portions of the same heart resulted in an increase only in the innervated portion. It is concluded that local stretch does not appear to be responsible for the tachycardia in the intact animal.

Effect of timing of vagal stimulation on heart rate in the dog

The effects of right vagus nerve stimulation on heart rate were examined in 16 dogs anesthetized with chloralose (60 mg/kg, iv) and urethane (600 mg/kg, iv) after a bilateral cervical vagotomy.

Two protocols were followed. In the first, trains of 1, 3, or 5 stimuli, 2 msec each in duration, 10 to 15 v in magnitude, and separated by 10 msec were delivered at predetermined times after "P" wave onset as a single stimulation. In the second, the stimulation was carried out repetitively for 2 minutes at a predetermined frequency. Typically, in the first, the later in the cycle the stimulation occurred, the greater was the prolongation of the included cycle. With one stimulus in the stimulation, a typical P-P interval was 574 msec with a "P" to stimulation delay of 4 msec, and 620 msec when the "P" to stimulation delay was 200 msec. With five stimuli moving the stimulation through a range of 200 msec, the change in P-P interval was 150 msec. There was a functional dependence of the P-P cycle length on the time of arrival of the inhibitory pulse. The slope of this relationship is a function of the number of stimuli in the stimulation.

With repetitive stimulation in the second protocol, there were zones wherein the heart rate synchronized to the vagal stimulation rate not only at the fundamental frequency but also at harmonics and subharmonics. This range of entrainment was also dependent upon the number of stimuli in the stimulation. The data suggest the possibility of phase shift as a mechanism for vagal regulation of heart rate.

Acute rejection following cardiac transplantation. Phonocardiographic and ultrasound observations

Nine patients have undergone cardiac transplantation at Stanford University Hospital. Three of these showed no clinical evidence of rejection and had essentially normal physical and phonocardiographic findings. There were eight distinct early rejection episodes in the other six patients. Early rejection was accompanied by decreasing QRS voltage, an early diastolic gallop, and frequently a rightward shift of the mean electrical axis of the heart. If not treated, this progressed to severe heart failure with a right ventricular heave, biventricular gallop, marked elevation of venous pressure with rapid y descent and severe dyspnea without orthopnea. Ultrasound measurements show increasing posterior wall thickness and overall heart diameter during the rejection episode. With aggressive therapy, all of these signs usually resolve rapidly, suggesting that dysfunction of the rejecting heart is secondary to the interstitial edema, vascular engorgement, and cellular infiltration, causing restriction of diastolic filling. These cardiovascular signs appear to aid detection of early cardiac rejection and allow treatment at an earlier stage of the rejection process.

Surgical aspects of cardiac transplantation in man

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Ventricular function in cardiac-denervated and cardiac-sympathectomized conscious dogs

The role of the autonomic nervous system in ventricular function was studied in conscious animals when either the cardiac nerves or the sympathetic nerves to the heart had been eliminated. Regional neural ablation was used to denervate the heart, and blockade of the cardiac sympathetic nerves was performed with propranolol. A reduction in the plateau of the ventricular output was seen in the cardiac-denervated animals (263 ml/min per kg) and in animals with beta-blockade (240 ml/min per kg) as compared with 318 ml/min per kg for the control curves. The difference in stroke volume in the cardiac-denervated animals and the beta-blocked animals at the plateau demonstrates that the chronically cardiac-denervated animal changes its ventricular output solely by a change in stroke volume, whereas the animal with beta-blockade changes heart rate and stroke volume in response to stress. Thus the beta-blocked animals increased their heart rate by a decrease in parasympathetic activity.

Electrocardiograms of dogs with heart homografts

Serial electrocardiograms from 50 dogs which survived 4 days to 12 months with orthotopic cardiac homografts have been evaluated. Although sinus rhythm is the basic mechanism in all homografted hearts, arrhythmias are frequently encountered early in the post-transplant period and later during episodes of threatened rejection. The decrease in QRS voltage which consistently accompanies impending rejection makes the electrocardiogram indispensable in monitoring the cardiac homograft.