Indirect Methods

These are most commonly based on an occlusive cuff, which is inflated to a pressure above that of the artery and then slowly deflated. Once the cuff pressure falls below that of the artery, pressure transients begin to pass beneath the cuff and can be measured. Blood pressure values are derived from these pressure transients and can be measured manually or automatically.

Manual Occlusive Cuff Methods

Manual methods rely on auscultation and palpation and are historically the earliest, but have become superseded by automated non invasive techniques, which provide a continuous display of blood pressure.

Riva-Rocci (1896) described the use of an occlusive cuff to measure systolic pressure by palpation.

Using an occlusive cuff, Korotkov (1905) first described the measurement of blood pressure by auscultation. 'Korotkov sounds' heard over the artery as the cuff pressure falls, are the result of turbulent blood flow, vibration in the arterial wall and pressure transients created as the extent of arterial occlusion decreases (Figure CM.17).

Phase I—clear tapping synchronous with the pulse

Phase II—sounds become softer about 5-10 mmHg below phase I

-as the diastolic point approaches, sounds become more intense sounds suddenly become muffled sounds disappear liOi

Amplitude

Figure CM.17 Representation of Korotkov sounds von Recklinghausen (1931) described a dual cuff (occlusive, and sensing) technique, employing aneroid valves in series within a sealed metal block, the oscillotonometer. This provided a visual measure of systolic, diastolic, and mean arterial pressures displayed on a dial, connected by levers to an aneroid gauge.

• Advantages—simple and well-established. Do not require sophisticated equipment or power supplies. Encourage patient contact

• Disadvantages—dependent on operator technique and require manual intervention Automated Occlusive Cuff Methods (Oscillometry)

Oscillometry is the most common method of automatic blood pressure measurement in clinical practice. It is a development of von Recklinghausen's oscillotonometer. The accuracy of blood pressure measurement has been improved by coupling the rate of cuff deflation to heart rate. A single occlusive cuff is employed using a dual sensing connection, which replaces the double cuff of the oscillotonometer. A pneumatic pump periodically inflates the cuff to a point 25-30 mmHg above the systolic pressure and allows air to escape through a bleed valve producing controlled deflation (about 2-3 mmHg/s). Vibrations of the arterial wall produce pressure transients that are transmitted via a sensing channel to an electrical transducer in the apparatus. The data is then analysed by a microprocessor. Using algorithms that relate the rate of change of pressure transient amplitude to blood pressure, systolic, diastolic, and mean arterial pressures are calculated. The effects of electrical noise are reduced by comparing successive arterial pulsations as the cuff pressure decreases. If these do not correlate the data is rejected.

Systolic pressure corresponds to the point where the amplitude of pulsations is increasing, and is about 25-50% of maximum. Diastolic pressure, corresponds to the point where the amplitude of pulsations has declined to 80% of the maximal pulse amplitude. Mean arterial pressure is the maximum amplitude point.

Phase

III—

Phase

IV—

Phase

V—

• Advantages—principal advantages of these instruments is that they free the operator's hands, allowing measurements to be obtained conveniently when access to the patient is difficult, allow calculation of the mean arterial pressure, provide alarm capabilities, and the capacity for data transfer

• Disadvantages—while correlating fairly well with invasive measurements, they are less accurate at extremes of blood pressure (will over read at low pressures, and under read at high pressures), and should not be regarded any more accurate than manual techniques. All these instruments assume the presence of a regular cardiac cycle, when absent, e.g. atrial fibrillation, blood pressure measurements become inconsistent. The automatic cuff increases the risk of underlying tissue damage (in the elderly, particularly when the frequency of measurement is high, and the instrument is used for prolonged periods). Incorrect cuff placement may be responsible for nerve entrapment injuries, i.e. the ulnar nerve at the elbow

Penaz Technique

In oscillometry blood pressure measurement relies on gradual deflation of a cuff, which limits the frequency of measurement. To overcome this limitation, Penaz first described a continuous non invasive technique in 1973. This monitors the diameter of the digital artery using an infrared plethysmograph, which is mounted in a pneumatic cuff. The infrared signal responds to arterial dilation and contraction during each cardiac cycle. By using a pump servo-controlled by the infrared signal, the infrared signal is maintained constant, at a value corresponding to mean arterial pressure, by inflating and deflating the cuff. Thus, as the artery dilates in systole cuff pressure is increased, and as arterial diameter reduces during diastole, cuff pressure decreases. This duplicates the arterial pressure waveform in the cuff, which is then displayed on the machine.

• Advantages—method provides a record of the changing trends in blood pressure, and in patients with normal, or vasodilated fingers, correlates well with invasive methods

• Disadvantages—results are less reliable in patients with peripheral vascular disease. Small differences in cuff positioning or tightness result in significant changes in the measured pressure. These measurements display a downward drift because of relocation of tissue fluid, necessitating repeated calibration. When used for > 20-30 min, the cuff causes discomfort. If peripheral blood flow is poor, there is the potential for vascular occlusive damage

Doppler Ultrasound

Employs an encapsulated array of transducer crystals that can transmit and receive ultrasound waves. These are coupled to the skin by a layer of silicone gel (preventing excessive reflection), and positioned directly over the artery. Movements in the arterial wall caused by pressure transients as they pass beneath the cuff cause Doppler shifts in the frequency of the transmitted ultrasound waves. The amplitude of the shift provides a measure of the systolic and diastolic pressures.

• Advantages—can be used for patients of all ages

• Disadvantages—requires the accurate positioning of the transducers, and the use of the correct ultrasound coupling medium. The signals are prone to movement artefact, and are distorted by diathermy and arrhythmias

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