Biology Essays - QRS Complex & ECG


QRS Complex, ECG and Heart Sounds

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

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

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

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

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It is this anchor shape thatcarries the wave of depolarisation from the A-V node along the inter-ventricularseptum, around the tip of the heart and then returns ( back towards therecording electrode) to run up the side of each ventricle that is responsiblefor the changes in the apparent direction of the electrical potential thatgives rise to the three wave pattern of the ECG.

The iso-electric (flat) S-T segmentrepresents the time that the entire ventricle is depolarised and going throughits absolute refractory period. There is no net electrical activity at thistime.

The T-wave is coincident with ventricularrepolarisation. It is longer than therefore repolarisation wave as it is notassisted by the Purkinje system.

The Q-T interval thereforerepresents 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 waverepresenting atrial repolarisation in the ECG because it occurs duringventricular depolarisation. Because the wave of atrial repolarization isrelatively small in amplitude (i.e., has low voltage), it is masked by the muchlarger amplitude of the ventricular-generated QRS complex.

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

In reality there is an atrialrepolarisation event, but it is overpowered (literally) by the ventriculardepolarisation which is occurring at the same time

Exercise increases heart rate by aprocess of sympathetic autonomic stimulation. Sympathetic (adrenergic) nervesincrease the excitability of the sino-atrial node and reduce the P-R interval.As exercise continues, the physiological changes in the body are continuouslymonitored by a number of physiological systems and the balance of activity ofthe sympathetic system (speeding up) and the parasympathetic system (slowingdown) is constantly adjusted. When exercise is over, the heart rate does notdrop immediately as the body has to undergo a period of readaption to return tothe resting state. The adjustment in the autonomic system activity reflectsthis.

The P-R interval is typicallyreduced in a linear relation to the increase in heart rate - typically about6.9 msec. per increase in rate of 10 beats per minute. (Lee et al. 1995) Itreduces in men to a greater degree than in women. It is a reflection of theadrenergic activity of the sympathetic system described in question 5.

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

Cardiac output is the term used forthe total volume of blood (in mls.) pumped by the heart in one minute. Which isa reflection of the amount of venous return in the same time period. Thecardiac output (and therefore venous return) increases during exerciseprimarily through the mechanism of sympathetic stimulation. The increasedvenous return dilates the atrial and ventricular chambers further which resultsin greater contraction (and therefore greater stroke volume ) - theFrank-Starling Law of the Heart

In temporal terms, these threeeffects arising from the same cardiac systole are separated by the time that ittakes the different effects to reach, and be recorded by, their particularsensor.

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

The first cardiac sounds are causedby the closure of the Mitral and Tricuspid valves at the onset of systole sothis will be slightly after the QRS complex is seem on the ECG. The second setof sounds, due to the closure of the Aortic and Pulmonary valves, is heardafter systole when the intra-aortic pressure starts to exceed the fallingintra-ventricular pressure, and the blood tries to flow back into theventricle. This clearly takes place some time after the QRS complex has passed.

The plethysmograph wave is theresult of the transmission of the systolic pressure wave along the arterialtree and through the body. It's detection is a directly proportional to thedistance between the heart and the positioning of the monitor. The closer tothe 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 heartthan the finger, will detect the systolic pressure wave first.