The presence of atrial relaxation is the foremost prerequisite for the pressure fall termed x descent. A strong healthy atrial contraction can be expected to be followed by a good x descent (Fig. 7C).
In terms of timing, the x descent comes just before the more dominant systolic x' descent. It usually precedes the x' descent as a minor hesitation before the major fall during systole, as observed in the jugulars during periods of normal PR relationship. If the PR interval is prolonged, it could occur long before ventricular systole and, in fact, before end-diastole. When it becomes diastolic in timing, it may be difficult to distinguish this at the bedside from the usual diastolic y descent in the absence of an electrocardiogram (40,47) (Figs. 2C, 9, and 11).
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