578

Sietsema KE, Cooper DM, Perloff JK, et al: Control of ventilation during exercise in patients with central venous-to-systemic arterial shunts. J Appl Physiol64:234-242, 1988

Alterations of the ventilatory response to hypoxemia have been reported in patients with cyanotic congenital heart disease. The authors studied the ventilatory responses to exercise in 18 patients with right-to-left intracardiac shunts. Even at rest, minute ventilation was greater in patients with shunts than in controls and increased with exercise. End-tidal PaCO, decreased during the early stages of exercise, but arterial pH and PaCOZ were unchanged. Thus, the ventila- tory cost of clearing carbon dioxide is increased in patients with right-to-left shunts. Arterial oxygen saturation decreased almost immediately with exercise indicating an increase in shunt fraction, but could not be correlated with the increased minute ventilation. Minute ventilation increased more than oxygen consumption, indicating that the ventilatory response is unrelated to increased pulmonary blood flow with exercise. Likewise, the presence of pulmo- nary hypertension was not essential to the ventilatory responses. These findings suggest that humoral factors such as PaCO* are responsible for the increased ventilatory requirements for given metabolic rates in patients with intracardiac right-to-left shunts.

Brien JF, McLaughlin BE, Kohus SM et al: Mechanism of glyceryl trinitrate-induced vasodi- lation. I. Relationship between drug biotransfor- mation, tissue cyclic GMP elevation, and relaxa- tion of rabbit aorta. J Pharmacol Exp Ther 244~322-327, 1988

Kawamoto JH, Brien JF, Nakatsu K: II. Lack of evidence for specific binding of GTN to bovine pulmonary vein. J Pharmacol Exp Ther 244~32%334,1988

In the first paper, the authors propose a new mecha- nism for the vasodilation produced by nitroglycerin. Based on time-dependent biotransformation of glyceryl trinitrate to glyceryl dinitrate in rabbit aortic strips and a time-dependent increase in cyclic guanyl monophosphate (GMP), they sug- gest that nitroglycerin-induced vasodilation proceeds through the following steps: preferential biotransformation of trini- trate to 1,2 dinitrate and inorganic nitrite anion by a direct interaction of one of the terminal nitrate groups with a ferrous hemoprotein in vascular smooth muscle; inorganic nitrite anion is converted to nitric oxide which then combines with and activates guanylate cyclase; intracellular cyclic GMP increases and mediates vasodilation.

The second paper suggests that specific binding sites (receptors) for nitroglycerin are absent in vascular smooth muscle such as bovine pulmonary vein. After 60 minutes of incubation of the radioligand [‘HI nitroglycerin, more than 95% and, after 90 minutes, more than 83% of the added radioactivity was associated with nitroglycerin.

CAROL L. LAKE

Gengo PJ, Bowling N, Wyss VL, Hayes JS: Effects of prolonged phenylephrine infusion on cardiac adrenoceptors and calcium channels. J Pharmacol Exp Ther 244:100-105,1988

Infusion of phenylephrine, 3 mg/kg/h, into rats for six days resulted in depletion of myocardial norepinephrine concentrations (0.49 rg/g v 3.8 cg/g in controls). Modest cardiac hypertrophy also occurred. Alpha,- and beta-adren- ergic binding sites and calcium channels were reduced by 39%, 3 l%, and 32%, respectively, during chronic phenyleph- rine infusion. Changes in calcium channel density may result from direct activation of the alpha-adrenoceptor or a com- pensatory response to the 50% increase in blood pressure during phenylephrine infusion.

Kuchar DL, Thorburn CW, Sammel NI, et al: Surface electrocardiographic manifestations of tachyarrhythmias: Clues to diagnosis and mechanism. Pace 11:61-82,1988

Many of the electrophysiologic concepts derived from the surface ECG have been confirmed by intracardiac map- ping and programmed stimulation. However, improvements in signal processing permit enhanced interpretation of the surface ECG. Computer-assisted signal processing allows recognition of the P wave even with very rapid tachycardias or when the P wave lies within the T wave or ST segment. The authors detail the findings on the surface ECG in supraven- tricular tachycardias and the features differentiating supra- ventricular from ventricular tachycardia. Criteria helpful in diagnosing the origin of a tachycardia include: (1) AV dissociation, (2) width of the QRS complex, and (3) QRS axis and morphology. Ventricular tachycardia is suggested by the presence of a superior frontal axis, AV dissociation, and QRS duration greater than 0.14 seconds. Despite these sophisticated techniques, which have a 90% predictive accu- racy, there are still patients who require invasive intracardiac studies for definitive diagnosis and long-term therapy.

Young MA, Knight DR, Vatner SF: Auto- nomic control of large coronary arteries and resistance vessels. Progr Cardiovasc Dis 30:211- 234,1987.

This review of the autonomic control of the coronary circulation emphasizes the regulation of coronary tone pro- vided by the alpha, beta, and cholinergic mechanisms. Both alpha, and alpha,-receptors mediate coronary vasoconstric- tion, although their effects on large coronary vasomotion are uncertain. In addition to vasoconstriction produced by exoge- nous catecholamines, reflex activation of the sympathetic nervous system increases coronary vascular resistance. Dur- ing exercise, enhanced alpha-adrenergic tone mediates coro- nary vasoconstriction, which may be sufficient to limit myo- cardial function. However, the aBerent stimulus for the increased alpha response during exercise is unclear, but chemoreceptor reflexes, arterial baroreceptors, or central neural sites are possibilities.

Beta-adrenergic stimulation dilates resistance coro-