1. In order to study the role of atrial pressure and atrial stretch on the release of atrial natriuretic peptide we have measured plasma atrial natriuretic peptide concentration, urine output and haemodynamic variables in eight patients during and 30 min after the relief of cardiac tamponade. This condition is characterized by high atrial pressure with little or no atrial stretch.

2. Relief of tamponade was associated with a rise in urine output (53 ± 27.9 to 101 ± 24.5 ml/h, mean ± sem; P = 0.09), systolic blood pressure (95 ± 9.6 to 126 ± 7.0 mmHg, P < 0.0001), and plasma atrial natriuretic peptide concentration (369.5 ± 70.9 to 490.3 ± 94.7 pg/ml, P < 0.05) despite a large fall in right atrial pressure (18.6 ± 1.6 to 9.5 ± 1.3 mmHg, P < 0.001).

3. These results suggest, therefore, that an increase in atrial stretch, rather than in atrial pressure, stimulates the release of atrial natriuretic peptide.

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