Transient electromechanical dissociation in hypertrophic cardiomyopathy

Investigation of a hemodynamic basis for syncope in hypertrophic cardiomyopathy. Use of a head-up tilt test

BACKGROUND

Syncope and sudden death in hypertrophic cardiomyopathy may have a hemodynamic basis. The presence of a small ventricular cavity with high intracavity pressures may activate left ventricular baroreceptors and cause reflex hypotension as described in other populations with syncope.

METHODS AND RESULTS

To investigate this potential mechanism of syncope in hypertrophic cardiomyopathy, we studied 17 patients with a history of syncope (syncopal), 19 without syncope (nonsyncopal), and nine normal control subjects by using a head-up tilt test. Head-up tilt at 60 degrees for 45 minutes was followed by 10-minute tilts during incremental doses of isoprenaline. Heart rate, blood pressure, and two-dimensional and Doppler echocardiography were monitored throughout. On tilting, hypertrophic cardiomyopathy patients showed a decline in mean arterial pressure of -5 +/- 6 mm Hg (p less than 0.001) compared with no change in control subjects (0.2 +/- 6 mm Hg, p = 0.9). Left ventricular outflow tract velocity decreased on tilting in control subjects (-8 +/- 6 cm/sec, p = 0.004) but increased in the syncopal and nonsyncopal patients (20 +/- 50 cm/sec, p = 0.05). Reflex hypotension with or without bradycardia, associated with syncope or presyncope, was induced in seven syncopal patients, two nonsyncopal patients, and two control subjects (p = 0.05). The early response to tilt in these subjects was characterized by maintenance of blood pressure but a greater increase in left ventricular fractional shortening than in the other subjects (10 +/- 8% versus 1 +/- 1%, p = 0.002). The onset of hypotension was associated with a trend toward further decreases in left ventricular diameters, outflow tract velocity, and transmitral flow velocities. In the remaining patients who had a negative test, transient hypotension (systolic pressure less than 100 mm Hg) occurred in seven syncopal patients and three nonsyncopal patients compared with none of the control subjects (p = 0.01). In total, hypotension was demonstrated in 82% of syncopal patients compared with 26% of nonsyncopal patients and 22% of control subjects (p = 0.001).

CONCLUSIONS

Patients with hypertrophic cardiomyopathy and a history of syncope frequently display hypotension during head-up tilt. In some cases, sudden hypotension occurs and is usually associated with bradycardia and a reduced cavity size, findings compatible with activation of a ventricular baroreflex. In other cases, transient hypotension occurs and could be explained by an impairment of baroreceptor function. These mechanisms may contribute to the occurrence of syncope in daily life.

Electromechanical dissociation. Treatable causes of a dire cardiac emergency

Electromechanical dissociation (EMD) may be primary, due to disease of the heart muscle itself, or secondary to alterations in loading conditions of the heart. Factors such as internal hemorrhage, acute cardiac tamponade, tension pneumothorax, acute pulmonary embolism, and inflow or outflow obstructions of the heart may be responsible for changes in loading. Myocardial ischemia, myocardial depressant overdose, and other conditions may also contribute to secondary EMD. If detected early, these secondary forms of EMD may respond to treatment. Drugs for resuscitation of a patient with EMD include epinephrine, atropine sulfate, and, in selected instances, calcium.