Preventing Syncope

Abstract & Commentary

Synopsis: Water ingestion increases the tolerance to upright posture by increasing peripheral vascular resistance and may serve as a simple prophylactic measure against syncope.

Source: Lu C, et al. Circulation. 2003;108:2660-2665.

Syncope is a common problem that complicates simple procedures such as blood donation. Since most forms of syncope involve an abnormal vasovagal response, Lu and colleagues tested the hypothesis that simple water ingestion would be preventative. They studied 22 healthy subjects without a history of syncope by head-up tilt table testing at 60° for 45 minutes or until syncope occurred. Each subject was tilted twice on separate days and randomized to 16 oz of water ingestion 5 minutes before tilting or nothing on the 2 days. All tilting was done in the morning after an overnight fast. Tilt table syncope was defined as systolic blood pressure < 70 mm Hg and heart rate < 50 beats/min. Tilt table presyncope was defined as a fall in the systolic blood pressure > 30 mm Hg with a decrease in heart rate of > 10 beats/min or a > 30 beats/min fall in heart rate with a > 10 mm Hg fall in systolic pressure. The primary end point was time until syncope. During the first 30 minutes of tilt, 8 of 22 without water experienced presyncope, whereas only 1 of 22 who had water did (P = .016). Water ingestion blunted the tachycardia associated with tilting (P < .001) and accentuated the increase in total peripheral resistance (P = .012). The subjects who ingested water tolerated head-up tilt 26% longer than those who did not (41 vs 33 minutes; P = .011). Water also attenuated the increase in hematocrit seen with tilting (P = .065). Plasma catecholamines increased with tilting but were largely unaffected by water ingestion. Lu et al concluded that water ingestion increases the tolerance to upright posture by increasing peripheral vascular resistance and may serve as a simple prophylactic measure against syncope.

Comment by Michael H. Crawford, MD

Current therapy for vagally mediated syncope is expensive, rife with adverse effects, and of questionable efficacy. Consider what we offer patients—drugs such as aldosterone agonists and beta-blockers and even pacemakers for some. All this for a symptom that may never occur again or be of minor significance. On the other hand, as Lu et al point out, syncope has important medical and societal significance under certain circumstances, such as blood donation and space travel. Thus, a simple, efficacious prophylactic therapy would be of value.

Is water ingestion the answer? Perhaps, since it did have a marked hemodynamic effect during tilt testing. Interestingly, previous studies by the same group have shown that similar volumes of intravenous dextrose solutions did not protect against orthostatic symptoms. Also, they have shown that water ingestion can increase systolic blood pressure up to 11 mm Hg and may represent a significant unrecognized cause of blood pressure fluctuation between visits. In addition, this suggests that studies of pharmacologic agents for hypertension may be influenced by taking the drugs on an empty stomach with water.

There are some potential limitations to the study. The crossover design resulted in a noticeable increase in orthostatic tolerance on the no-water day when it was second vs first. Also, there is no placebo for water ingestion, which may have influenced the results. Finally, no subject ever reached the primary end point of syncope as defined a priori. Finally, the mechanism of increased peripheral vascular resistance observed is not clear from this study. Plasma catecholamine levels were not influenced by water ingestion, but dopa levels were. Since the gastrointestinal tract is a major source of dopa, this may explain why water ingestion, but not intravenous infusion, increases peripheral vascular resistance.

Dr. Crawford, Professor of Medicine, Associate Chief of Cardiology for Clinical Programs University of California San Francisco, is Editor of Clinical Cardiology Alert.