The measurement of arterial pressure in man. I.—The auditory method

In a previous communication we showed that when an artery, exposed in a living animal, is compressed in a glass compression tube full of water (Ringer’s solution), the pulse, distal to the compression, is not obliterated until the pressure of the water is raised just above the systolic pressure of the blood in the artery, whereas when the same artery, placed on bone, wood or glass, is compressed by the bag of Leonard Hill’s pocket sphygmometer, or by the armlet of the sphygmometer, so arranged that it does not embrace the surrounding pulsating tissues, the pulse is abolished by pressures under, and even much under, the diastolic pressure of the blood stream. These facts are correlated with the manner in which the artery is compressed in each case. Enclosed in the compression tube the artery is compressed by the water equally in a circular fashion so that the rise of external pressure, up to the diastolic pressure, has no effect in producing deformation of the artery. Ultimately, when the compression becomes greater than the diastolic pressure, the artery flattens and changes to the oval shape during diastole. It is flattened during systole when the external pressure rises above the systolic pressure. When the carotid artery of the living animal is freed from the surrounding tissues and placed on a watch-glass and compressed by the bag of the pocket sphygmometer, or by the armlet so arranged as not to embrace the pulsing tissues of the neck, the oval deformation sets in at far lower pressures and is complete in relatively thin-walled labile arteries at pressures much under diastolic pressure. Consequently the blood flow is cut down by an external pressure less than diastolic to a mere ineffective trickle of blood, and the pulse is completely damped out. If a branch of the carotid artery distal to the bag were connected with a C-spring manometer, the record would show a progressive lowering of both the systolic and the diastolic pressure almost to zero, till with an external pressure much less than the diastolic pressure in the aorta the pulse was damped out and the blood flow became a trickle.