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Free Essays - Biology Essays

QRS Complex, ECG and Heart Sounds

The ECG is a tracing of the net effect of all of the changes arising from the various depolarisation potentials that occur in the heart during its activity.

The P wave represents the triggering of the sequence of systole, which is the cardiac contraction. It is caused by the depolarisation of the sino-atrial node in the wall of the right atrium. It typically lasts about 0.1 seconds

The brief iso-electric (flat) gap after the P wave is the P-R interval. It represents the time that the impulse takes to travel within the AV node (which is slower than across the myocardium). This is typically 0.1-0.2 seconds long

The QRS complex is the result of ventricular depolarisation. This is a rapid event typically lasting about 0.06 to 0.1 seconds. It is rapid because the wave of depolarisation is carried primarily by the Purkinje fibres between and across the ventricles in an anchor shaped structure before it spreads out across the myocardium.

It is this anchor shape that carries the wave of depolarisation from the A-V node along the inter-ventricular septum, around the tip of the heart and then returns ( back towards the recording electrode) to run up the side of each ventricle that is responsible for the changes in the apparent direction of the electrical potential that gives rise to the three wave pattern of the ECG.

The iso-electric (flat) S-T segment represents the time that the entire ventricle is depolarised and going through its absolute refractory period. There is no net electrical activity at this time.

The T-wave is coincident with ventricular repolarisation. It is longer than therefore repolarisation wave as it is not assisted by the Purkinje system.

The Q-T interval therefore represents the total time of ventricular depolarisation and repolarisation. This is typically 0.2-0.4 seconds. It is shorter during fast heart rates.

There is no distinctly visible wave representing atrial repolarisation in the ECG because it occurs during ventricular depolarisation. Because the wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), it is masked by the much larger amplitude of the ventricular-generated QRS complex.

The QRS wave is a combination of deflections in both directions because of the multidirectional Purkinje fibres. The T wave does not involve this system and the net electrical charge of repolarisation effectively comes from the rear of the tip of the left ventricle upwards across the heart towards the upper recording electrode thereby giving an upward deflection of the isoelectric line

In reality there is an atrial repolarisation event, but it is overpowered (literally) by the ventricular depolarisation which is occurring at the same time

Exercise increases heart rate by a process of sympathetic autonomic stimulation. Sympathetic (adrenergic) nerves increase the excitability of the sino-atrial node and reduce the P-R interval. As exercise continues, the physiological changes in the body are continuously monitored by a number of physiological systems and the balance of activity of the sympathetic system (speeding up) and the parasympathetic system (slowing down) is constantly adjusted. When exercise is over, the heart rate does not drop immediately as the body has to undergo a period of readaption to return to the resting state. The adjustment in the autonomic system activity reflects this.

The P-R interval is typically reduced in a linear relation to the increase in heart rate - typically about 6.9 msec. per increase in rate of 10 beats per minute. (Lee et al. 1995) It reduces in men to a greater degree than in women. It is a reflection of the adrenergic activity of the sympathetic system described in question 5.

The P-T interval effectively measures the time it takes for the active part of the heart's cycle to be initiated. During exercise, the most noticeable change is the reduction of the P-R interval together with the reduction in the P-P intervals (Tachycardia)

Cardiac output is the term used for the total volume of blood (in mls.) pumped by the heart in one minute. Which is a reflection of the amount of venous return in the same time period. The cardiac output (and therefore venous return) increases during exercise primarily through the mechanism of sympathetic stimulation. The increased venous return dilates the atrial and ventricular chambers further which results in greater contraction (and therefore greater stroke volume ) - the Frank-Starling Law of the Heart

In temporal terms, these three effects arising from the same cardiac systole are separated by the time that it takes the different effects to reach, and be recorded by, their particular sensor.

The ECG is - to all practical purposes- synchronous with the electro-muscular activity of the heart. And is indicative of the actual baseline timing of the various event of the cardiac cycle as outlined above. The heart sounds and plethysmograph wave are the result of cardiac systole which is co-incident with the QRS complex of the ECG

The first cardiac sounds are caused by the closure of the Mitral and Tricuspid valves at the onset of systole so this will be slightly after the QRS complex is seem on the ECG. The second set of sounds, due to the closure of the Aortic and Pulmonary valves, is heard after systole when the intra-aortic pressure starts to exceed the falling intra-ventricular pressure, and the blood tries to flow back into the ventricle. This clearly takes place some time after the QRS complex has passed.

The plethysmograph wave is the result of the transmission of the systolic pressure wave along the arterial tree and through the body. It's detection is a directly proportional to the distance between the heart and the positioning of the monitor. The closer to the heart the monitor is, the quicker the systolic pressure wave will reach it. It follows from this that the sensor on the ear - being nearer to the heart than the finger, will detect the systolic pressure wave first.

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