Mean left atrial pressure is higher for any given left ventricular end diastolic pressure, as the heart rate is progressively increased, for two reasons. First, by encroaching on ventricular diastole, the relative period during which inflow into the atrium continues while the egress of blood from the atrium is blocked is increased. Second, with more severe tachycardia, atrial systole can be seen to occur either partially or completely while the mitral valve is still closed from the previous ventricular systole. At a constant heart rate, mean left atrial pressure is higher for any given left ventricular end diastolic pressure during efferent vagal nerve stimulation than during control conditions. This is due to a negative inotropic effect on the atrium and not to a change in the performance characteristics of the ventricle. At a constant heart rate, mean left atrial pressure is lower for any given left ventricular end diastolic pressure during cardiac sympathetic nerve stimulation than during control conditions. This is due to a positive inotropic effect on both the atrium and the ventricle. The relation between left ventricular end diastolic pressure and stroke work is determined only by the performance characteristics of the ventricle; the relation between mean left atrial pressure and stroke work is determined by the performance characteristics not only of the ventricle but also to a substantial extent by those of the atrium. Atrial contractility and the changes therein induced by central nervous system activity are of importance to circulatory regulation both because of the influence of atrial systole on ventricular end diastolic pressure and because of the effect of such changes on central venous pressure at any given level of ventricular stroke work. The possibility is suggested that depressed atrial function, i.e., atrial failure, may be a significant contributory factor in the elevated venous pressure observed in congestive heart failure.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine