Let's now analyze what we observed by listening at the upper left sternal edge. You see, we had to correlate it with breathing. The inspiratory "didup, didup" of the second sound - the expiratory "lub dub, lub dub". So, when the patient breathed in, it was "lub didup, lub didup, lub didup", and when he breathed out, "lub dup, lub dup, lub dup".
What you were hearing was asynchronous closure of the pulmonary and aortic valves - .06 seconds asynchronous closure. You can get to judge that really well - "didup, didup, didup" - .06 seconds, whereas "didp, didp, didp" - more like .03 seconds.
Everyone listen. Watch breathing. As the patient breathes in... The splitting occurs, and we'll describe the reason for this in just a little while. Everyone listen. Watch breathing.
Graphic example
This is a graphic example of normal inspiratory or physiologic splitting of S2 in which the first component, A2, is normally louder than the second component, P2. Let us listen together.
Inspiratory splitting of S2
The normal respiratory variation of the second heart sound, that is, inspiratory splitting, is attributable to two mechanisms: changes in venous return and changes in pulmonary vascular compliance.
1st mechanism
Inspiration causes a drop in intrathoracic pressure with greater venous return to the right ventricle and less venous return to the left ventricle. The increase in right ventricular volume prolongs right-sided ejection time, and the decrease in left ventricular volume reduces left-sided ejection time.
2nd mechanism
An additional factor relates to an inspiratory increase in pulmonary vascular compliance that allows the pulmonary arteries to receive a greater volume of blood during inspiration, resulting in an inertial delay of the pulmonary second sound. This has been called "hangout", a description that relates to the character of the pulmonary and right ventricular pressure curves during inspiration.
The net effect of inspiration is a later P2 and an earlier A2.
Abnormal splitting
Abnormal splitting of the second heart sound occurs in a variety of pathologic conditions. It may be the key to a diagnosis, as in patients with atrial septal defects who have fixed splitting.
Normal children and young adults may have persistence of expiratory splitting when supine, but this typically disappears when they sit or stand.
Relative intensities
The relative intensity of the second heart sound may reflect underlying pathology.
Normally, the aortic sound is louder than the pulmonary, because it closes at a higher pressure.
In systemic hypertension the aortic sound may be further accentuated and in pulmonary hypertension, the pulmonary sound may be increased.
In aortic and pulmonary stenosis, one component may be diminished due to valve immobility.