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Blood pressure can be measured directly or indirectly. While direct methods provide the maximum quantity of reliable information from probes inserted into the blood stream, indirect methods produce much less disturbance to the subject. Indirect methods are based on the adjustment of a known external pressure to equal the vascular pressure. Systolic and diastolic pressure can be determined intermittently from the pressure that will just collapse the vessel; an approximation of the instantaneous pressure level is obtained from a surrounding chamber adjusted to remove all vessel wall tension. Direct methods can provide continuous, high fidelity recordings of the absolute vascular pressure via a catheter either to transmit the blood pressure through liquid to an external sensor or to carry the signal leads from a miniature internal sensor. External sensors require careful adjustment of the catheter dimensions to obtain optimum dynamic response. Internal sensors provide the maximum dynamic response and avoid acceleration artifacts. Convenience of electrical signal manipulation, display and recording have made electrical transducers increasingly popular.
Hampton W. Shirer (Mon,) studied this question.