Simultaneous recording of the electrocardiogram and of the pressure transmitted through the drainage needle in patients with haemopericardium provided confirmation that the needle tip was in the pericardial cavity. Proper position was indicated by a positive pressure wave beginning before the QRS complex, reaching its maximum value in the first part of the ST segment and ending with the T wave. In contrast, ventricular penetration was signalled by a positive pressure wave beginning in the first part of the QRS complex, reaching its maximum during the T wave and ending between the T and P waves.