1. Stroke volume and cardiac output were measured using the Doppler ultrasound technique in 16 normal subjects immersed to the neck in water at 33°C, 35°C, 37°C and 39°C. A standard aortic diameter was assumed and results were expressed as percentage changes from pre-immersion resting values.
2. Cardiac output rose progressively at higher temperatures, increasing by 30% at 33°C and by 121% at 39°C. At thermoneutral temperatures (33°C and 35°C) this was achieved by an increase in stroke volume of 50% despite a significant decrease in heart rate. There was a further rise in stroke volume and pulse rate at higher temperatures and a mean tachycardia of 109 ± 4 beats/min was noted at 39°C. Calculated peripheral resistance reduced progressively with increasing temperature of immersion.
3. This non-invasive and simple technique may provide a non-exercise-related cardiovascular stress test to study cardiovascular responses in a variety of pathophysiological states.