Mechanism of skeletal muscle underperfusion in a dog model of low-output heart failure

Alteration of humoral and peripheral vascular responses during graded exercise in heart failure

We hypothesized that performance of exercise during heart failure (HF) would lead to hypoperfusion of active skeletal muscles, causing sympathoactivation at lower workloads and alteration of the normal hemodynamic and hormonal responses. We measured cardiac output, mean aortic and right atrial pressures, hindlimb and renal blood flow (RBF), arterial plasma norepinephrine (NE), plasma renin activity (PRA), and plasma arginine vasopressin (AVP) in seven dogs during graded treadmill exercises and at rest. In control experiments, sympathetic activation at the higher workloads resulted in increased cardiac performance that matched the increased muscle vascular conductance. There were also increases in NE, PRA, and AVP. Renal vascular conductance decreased during exercise, such that RBF remained at resting levels. After control experiments, HF was induced by rapid ventricular pacing, and the exercise protocols were repeated. At rest in HF, cardiac performance was significantly depressed and caused lower mean arterial pressure, despite increased HR. Neurohumoral activation was evidenced by renal and hindlimb vasoconstriction and by elevated NE, PRA, and AVP levels, but it did not increase at the mildest workload. Beyond mild exercise, sympathoactivation increased, accompanied by progressive renal vasoconstriction, a fall in RBF, and very large increases of NE, PRA, and AVP. As exercise intensity increased, peripheral vasoconstriction increased, causing arterial pressure to rise to near normal levels, despite depressed cardiac output. However, combined with redirection of RBF, this did not correct the perfusion deficit to the hindlimbs. We conclude that, in dogs with HF, the elevated sympathetic activity observed at rest is not exacerbated by mild exercise. However, with heavier workloads, sympathoactivation begins at lower workloads and becomes progressively exaggerated at higher workloads, thus altering distribution of blood flow.

Mechanical characteristics of tachycardia-induced left-ventricular failure as evaluated in isolated dog hearts

Left ventricles of control dog hearts and dog hearts failing due to chronic tachycardia were examined in vivo by echocardiography for systolic function and size, then subsequently studied with an isolated-heart system (artificial perfusion, artificial loading). During 3 weeks of tachycardia (250 bt/min), area ejection fraction fell by 58%, while end-diastolic transverse area increased by 56% (measurements at 120 bt/min). Judging from post-perfusion left-ventricular weights, the dilation occurred with no hypertrophy, raising the question whether the failure model may be associated with anabolic dysfunction. End-diastolic pressure-volume (P-V) relations occurred at higher volumes in failing chambers than in controls, and this was marked by increases in two indices of chamber size (candidate reference volumes): the volume resulting in a diastolic stress of 16 g/cm2, and the volume at which the nearly straight, low-stiffness segment of the end-diastolic P-V relation meets the upward bending, high-stiffness segment. Developed P-V relations of failing chambers were shifted to higher volumes and to lower pressures, the lower pressures being due more to reduced stress-developing ability (contractility) than to reduced wall/cavity ratio (pressure/stress ratio). On average, shortening ability (normalized difference between reference volume and extrapolated volume-axis intercept, i.e., apparent ejection fraction from reference volume in absence of afterload) was not different from that of controls. Isovolumic pressure waves of the failing and dilated chambers were of almost normal duration and shape, extending further the range of conditions where isovolumic pressure can be predicted by fitting a model isovolumic wave function to the isovolumic phases of ejecting beats.

Spontaneous and induced arterial disease in the dog: pathology and pathogenesis

The spontaneous arterial diseases of the dog relevant to safety assessment studies of drugs are the extramural coronary arteritis of Hartman, intramural coronary arteriosclerosis (with amyloid deposition) occurring in older dogs with cardiac disability, intramural arteriosclerosis without amyloid deposition in the left ventricle of dogs with congenital subaortic stenosis or in the right ventricle in dogs with severe pulmonic stenosis, and necrotizing polyarteritis (nodosa). Experimentally induced lesions include right atrial necrosis and arteritis produced by minoxidil and theobromine, extramural coronary arteritis produced by positive inotropic/vasodilator drugs, intramural coronary arteriosclerosis associated with decreased peripheral resistance and tachycardia induced by hypotensive drugs (including antihypertensive and positive inotropic/vasodilator agents), and intramural arteriosclerosis associated with rapid ventricular pacing. The pathogenesis of none of these lesions is known. The left ventricular subendocardial and papillary muscle intramural coronary arterial lesions are associated with hyperdynamic activity and, in the case of drugs and subaortic stenosis, the possibility of lowered perfusion pressure and tachycardia. This has led to the supposition that these are ischemic lesions, but the evidence available either does not support or refutes that conclusion since subendocardial coronary flow and perfusion pressure are adequate with pacing tachycardia and in toxicity trials. Necrotizing polyarteritis appears to be an immune mediated disease that may appear in genetically prone beagles when they are placed under the stresses of experimental manipulation and/or a new environment. Since the right atrial minoxidil lesion can also be produced by theobromine in dogs and minoxidil can cause a left atrial lesion in swine, it is neither species nor drug specific. Its cause, however, escapes us. There appears to be little in common between the extramural coronary artery Hartman lesion and that caused by positive inotropic/vasodilator drugs. Left ventricular subendocardial and papillary muscle intramural coronary arterial lesions induced by ventricular pacing at 250 beats/min for 2 months are generally similar to those seen in toxicity trials with peripheral vasodilator drugs that induce tachycardia in electrocardiograms exceeding 200 beats/min, although in acute pacing experiments subendocardial perfusion is adequate at these heart rates. Coronary artery autoregulation may be compromised or so destabilized by the drugs that episodes of underperfusion can account for these lesions.(ABSTRACT TRUNCATED AT 400 WORDS)