Effect of nitroglycerin with and without systemic hypotension on canine regional myocardial tritiated water deposition.

A marked alteration in the transmural distribution of left ventricular blood flow, with a relative increase in subendocardial and mid-wall flows, but with no change in the distribution of the relative blood flow to the two ventricles occurred when nitroglycerin was administered and the systemic arterial blood pressure in the upper body maintained near control levels in anaesthetized, open-chested dogs. The relative increase in subendocardial and mid-wall flows may have resulted from a direct action of nitroglycerin on the coronary vasculature. On the other hand, the intravenous administration of nitroglycerin, when followed by the hypotension which it produces, did not alter the transmural distribution of blood flow in the left ventricle of the dog. Blood flow to the right ventricle relative to flow to the left ventricle increased in this situation.

Left ventricular mechanoreceptors: a haemodynamic study.

1. To study the function of the left ventricular mechanoreceptors, a working left ventricle preparation was devised in dogs which permitted control of pressure and flow of the isolated perfused coronary circulation and of the flow of the isolated, separately perfused systemic circulation. The systemic circulation was perfused at a constant rate so that changes in systemic pressure reflected changes in systemic resistance.2. Increases in myocardial contractility produced by injection of catecholamines into the isolated, perfused coronary circulation produced a fall in the pressure (resistance) of the isolated, separately perfused (at a constant rate) systemic circulation.3. Completeness of isolation of the coronary and systemic circulations was shown by the marked difference in appearance times between the reflex hypotensive responses from catecholamine injections into the isolated coronary circulation and the direct hypertensive response from a similar injection when the circulations were connected as well as by the marked difference between the pressure pulses recorded simultaneously on both sides of the aortic balloon separating the two circulations.4. Myocardial beta receptor blockade produced by injection of propranolol into the isolated coronary circulation abolished or attenuated the changes in left ventricular myocardial contractility as well as the subsequent hypotensive responses following the similar injection of catecholamines.5. Electrical stimulation of a sympathetic nerve innervating the heart resulted in increases in left ventricular myocardial contractility and subsequent systemic hypotensive responses indistinguishable from those following injection of catecholamines.6. That distortion of the mechano- or stretch receptors in the left ventricular myocardium was the cause of the hypotensive responses was demonstrated by increasing left ventricular myocardial contractility by mechanically obstructing the left ventricular outflow which produced hypotensive responses similar to those following the injection of catecholamines or nerve stimulation.7. Bilateral high cervical vagotomy abolished the hypotensive responses following injection of catecholamines into the isolated coronary circulation or following left ventricular outflow obstruction in all but one instance, indicating the importance of vagal fibres to the afferent arm of the reflex.8. It is suggested that the left ventricular mechanoreceptors function normally to reduce the peripheral resistance in order to prepare the systemic circulation to receive the left ventricular output and, especially during exercise, to prepare the systemic circulation to receive the augmented cardiac output with a minimum alteration in the systemic blood pressure and to distribute this augmented output preferentially to the skeletal muscles.

Recent developments in the measurement of cardiac output and regional blood flow using indocyanine green and microspheres.

Densitometric studies show that the large flow measurement errors and inability to obtain reproducible densitometer calibrations reported with indocyanine green (ICG) in nearly isotonic saline may have arisen from two sources: (a) slowed optical stabilization, and (b) sedimentation of dye aggregates formed in the salt "solutions" of ICG, both of which are avoidable by preparing the dye in water. The assumptions of the widely used regional blood flow measurement technique using radionuclide-labeled microspheres are described. Simultaneous injection of 8mu and 15mu microspheres in turkeys and in dogs demonstrated the existence of at least 8 mu arteriovenous communications (AVCs) in the stomach and intestine, not previously described by this technique, which, in addition to their physiologic functions, may play a role in production of acute gastric mucosal ischemia and erosions. Similar AVCs, producing a lesser degree of "shunting," were also found in the heart. Loss of 8 mu relative to 15 mu microspheres continued with time in the gastrointestinal circulation.