CARDIAC MONITORING. Electrocardiography (EKG) and phonocardiography are the two most important techniques for observing the condition of the heart and its associated arteries.
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On the other hand, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is near the apex, which is the direction of electropositivity during depolarization.
It represents the time taken from the start of the excitation at the pacemaker (sinoatrial node) to the beginning of ventricular depolarization (i.e. depolarization of the atrium, conduction through the atrioventricular node and through the conduction system to reach the ventricular muscle).
In contrast, the strength of the contraction depends on the amount and state of the muscle contractile substance.
Thus, it is a mistake to expect that the amplitude of the electrocardiogram can tell us, except in extreme cases, anything about the strength of contraction or the force of the heartbeat (e.g. level of arterial pressure pulse produced).
The amplitude of the waves is also affected by the electrical resistance of the pathways to the distant electrodes; in addition, as has been already pointed out, the recorded voltages represent only the difference between the influences of simultaneous and opposite electrical dipoles in a complicated pattern.
The resultant depends as much on the synchrony or asynchrony of the component dipoles as on the magnitude of the original potentials.
Bundle-branch block means that the impulses have come from the atrial pacemaker (sinoatrial node), reached and passed the atrioventricular node, but travel down only one of the two main branches of the conduction system (left or right bundle-branch block).
The QRS complex of the electrocardiogram is greatly prolonged from the normal 0.06 - 0.10 sec to more than 0.12 sec (First Degree).
This is because conduction takes longer route to the block ventricle and because the velocity is less in the muscle than in the conduction system.
In total sinoatrial block (Third Degree), the ventricles may continue to beat in a new rhythm, called a "nodal, or ideoventricular, rhythm," according to whether the initiation of the impulse is taken up by the sinoatrial node itself or an ectopic focus in the ventricular muscle becomes the pacemaker.
Such varieties of block are diagnosed from the electrocardiogram by noting a dissociation of the P waves (atrial activity) from the QRST waves (ventricular activity).
The P wave may be absent (atrial standstill) or present from an abnormal origin in the atrium.
Only every second, third, fourth, or fifth of these waves may be followed by the ventricular complex (QRST complex).
One explanation for flutter is that the wave of depolarization is following some unusual path in the atrial tissue and is returning to re-excite repetitively the pacemaker tissue, which is capable of responding to stimuli at intervals much shorter than those of its own normal spontaneous rhythm.
The atrioventricular node is thus bombarded with hundreds of impulses every minute, and only now and then does an impulse find the atrioventricular node out of its refractory state (i.e. susceptible to stimulation).
The whole base line of the electrocardiogram shows tiny irregular waves, less regular and more frequent than those of flutter.
The atria no longer achieve a significant pumping of blood; but it turns out that this does not, per se, reduce the output of the heart greatly, although the accompanying irregularity of the ventricular contractions does significantly reduce the output.