The diagnostic importance of blood pressure differences between right and left arms has been enhanced in recent years by the recognition of supravalvular aortic stenosis and this "choana effect" in children and of the subclavian steal syndrome in adults.158 Most patients with the former have greater than 20 mmHg higher blood pressure in the right arm. The subclavian steal syndrome, often accompanied by symptoms of cerebrovascular insufficiency, usually results in a pronounced lowering or absence of brachial artery pressure in the ipsilateral extremity.125
A progressive increase in systolic pressure normally occurs as the point of measurement is moved peripherally from the central aorta (Fig. 10-41), and the increment in systolic pressure is equivalent in the large arteries of the upper arm and the thigh. Direct recordings of femoral and brachial arterial pressures (systolic, diastolic, and mean) in adults159 and children160 and indirect measurement of popliteal and brachial artery pressures using appropriate pressure cuffs161 have demonstrated that mean pressures are equal at these sites. A difference in arm and leg pressures may occur because of coarctation of the aorta or acquired disease such as aortic dissection, aortic arch syndrome, or the subclavian steal syndrome.125
Pulsus alternans may be detected by palpating a peripheral artery. The femoral artery is probably best for this purpose. One must, of course, be certain the heart rhythm is normal. The sphygmomanometer can be used to measure accurately the beat-to-beat variation in pressure that characterizes pulsus alternans.
Pulsus alternans, which is discussed at greater length later in this chapter, occurs in patients with severe heart disease who exhibit impaired LV contraction. It also can occur for a few beats following supraventricular tachycardia in normal persons or when the respiratory rate is half the pulse rate. This may be apparent when pulsus paradoxus is present in patients with cardiac tamponade.
A normal person may exhibit a 10- to 12-mmHg drop in systolic pressure during normal inspiration. A fall in pressure greater than this amount may be identified in patients with acute cardiac tamponade, constrictive pericarditis, severe obstructive lung disease, and restrictive cardiomyopathy.
Pulsus paradoxus is best detected by inflating the blood pressure cuff above systolic pressure and then slowly releasing it. As the cuff pressure is gradually reduced, the blood pressure sounds become audible during expiration. The difference in pressure between the first audible sound heard on expiration and the pressure level at which the sounds are heard during all phases of respiration gives a measurement of magnitude of pulsus paradoxus. The mechanism of pulsus paradoxus is discussed in Chap. 72.
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