Irrespective of what the pre-a wave pressure is, the a wave height in the atrial pressure is determined by the strength of the atrial contraction first and foremost because the a wave rise results from atrial contraction. In atrial fibrillation, both the atrial contraction and relaxation become ineffective and disorganized and feeble, leading to loss of the a wave peak and the x descent (40).
The atrium tends to contract strongly when there is resistance to ventricular filling. The extreme form of resistance to ventricular filling would occur in tricuspid stenosis or obstruction because of a tumor (e.g., a myxoma in the right atrium). These would be expected to cause very high a wave pressures (18). In fact they do. However, these conditions are extremely rare and therefore not to be thought of first when considering causes of prominent a wave.
The most common reason for resistance to ventricular filling is decreased ventricular compliance. The decreased compliance may be the result of any pathological process that affects the wall of the RV, such as hypertrophy, ischemia, infarction, inflammation, infiltration and/ or fibrosis (Fig. 8).
Ifthe atrium itself becomes involved in the disease process, which leads to a decrease in its systolic contraction, high a wave pressure may not be generated despite the presence of decreased ventricular compliance.
Whereas the normal a wave pressure rise is slow and small and therefore not appreciated in the jugulars, a strong atrial contraction causing a quick and rapid rise in pressure may actually cause flow reversal in the jugulars and become recognizable in the jugulars as an abnormal sharp rising wave preceding the x' descent, which can be timed with the radial pulse. The short duration of this wave is another distinguishing feature (see JVP Videofile [1st-4th patient] in Jugular Venous Pulse, Normal and JVP Videofile 8 on the Companion CD).
If atrial contraction were to occur in a haphazard relationship to ventricular systole as in atrioventricular dissociation (as in complete A-V block with a ventricular pacemaker driving the ventricles with atria beating on their own from sinus depolarizations), a fortuitous relationship could develop that could result in simultaneous atrial and ventricular contraction. Because atrial contraction would be occurring at the time of a closed tricuspid valve due to ventricular systole, it would result in a sharp and quick rise in atrial pressures, termed "cannon waves'' These will cause flow reversal in the jugulars (40) and be recognizable as sharp rising waves of short duration at the time ofthe radial pulse. They will be irregular. Rarely regular cannon waves may occur in junctional rhythms with retrograde P waves and very short PR interval (Fig. 7H).
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