Twelfth lecture

Pressure pulses in the cardiac chamber and related vessels. The heart beat in a rate of about /min. At the beginning of the cardiac cycle both atria and ventricle are relaxed and filled with blood. Atrium and ventricle on each side are in continuity as the A-V valves are open and the pressure in each cavity is almost identical. Following the impulse generation of SA node, the atrial muscle contract with Pa rises with Pv following.

As Pa is greater then Pv atrial contraction adds to the diastolic volume of the ventricle. The atrial contraction was lasts about 0.1 sec. in a cardiac cycle of 0.8 sec. As it passes off, the pressure in both atrium and ventricle falls, and the ventricle has meanwhile been invaded by the excitation process which has spread from SA node across the atrial muscle to the AV node and hence via bundle of His and the purkinje tissues.

Ventricular contraction begin and promptly Pv exceed Pa and the A-V valve close (causing first heart sound). The ventricle is now a closed chamber and the pressure rises promptly during the isometric phase which lasts about 0.05 sec. The ventricular pressure then rapidly exceeds than that of the artery and Semilunar valve open.

The arterial pressure rises because blood enters the vessels faster than it can escape via the peripheral arterial brandies. The pressure decline as the ventricular contraction begins to subside which follow from the artery to its peripheral branches continues to be high. The total period of ventricular systol is 0.3 sec. and when this ends the ventricular pressure drops sharply. The arterial pressure is better sustained however owing to elastic recoil of the vessels and almost immidiately the arterial pressure exceeds that in the ventricle, thereby causing closure of the semilunar valve and the sharp second heart sound.

The initial part of ventricular diastole which follow is isometric lasting some 0.08 sec. ending in opening of the A-V valve because the arterial pressure exceeds that in the ventricle. Rapid filling of the ventricle then occur for sec., although the pressure in both chambers still falls' owing to continued rapid relaxation of the ventricle. Finally the phase of slow filling or Diastasis lasting 0.19 sec. terminates the cardiac cycle

This slow filling is due to continued venous return filling both atrium and ventricle and the readjusting the diastolic volume of the ventricle. ventricular diastole lasts 0.5 sec. (0.08 sec. isomctic relaxation ; 0.12 sec. rapid filing ; 0.2 sec. diastasis ; 0.1 sec. atrial cystol) The atrial pressure curve shows three well marked waves : 1 - Is due to atrial cystol. 2 - With the onset of vetricular cystol, there is a rise of pressure due to bulging of the A-V valve into the atrium. This is soon succeded by a sharp fall in pressure which attributed to the pressure lengthening of the atrium caused by the downword movement of the A-V as the ventricular contraction proceedes. 3 - the rising phase of this wave is due to venous return.

The EGG waves P, Q RS,and T depict the sequential events in the cardiac cycle: P is the electrical activity associated with atrial systol. Its duration 0.1 sec. It is some guide to the functional activity of the atrial muscle. QRS is the electrical activity aroused by invasion of the ventricle. ts duration in man 0.08 sec. The P - R interval sec It represent the stage of ventricular deplarization.. The time interval from the begining of P to the commencement of R Is a guide to the conduction of the bundle of His and,T is a wave caused by the repolarization of the ventricle. Its duration is about 0.27 sec.