Medical Blog

Although in this thesis we focus on invasive arterial blood pressure measurements, in some situations we may use the non-invasive alternatives. These all have in common that the assessment of the blood pressure is not very accurate, but, on the other hand, an arterial cannulation is not required. The oscillometric method is the oldest one used in man. In 1866 this method was described by Marey [Marey, 1866]. He used a small cuff, snugged around a limb, which was inflated with water above systolic pressure. During deflation he registrated the oscillations of a mercury column on a carbon drum. Pressure was read from a mercury manometer. The point of maximal oscillations was decided to be the mean arterial pressure. In 1896 Riva-Rocci introduced the palpatory method, only suitable to determine the systolic pressure. He used a cuff with air connected to a mercury manometer to measure the pressure applied to the brachial artery. The pulse was determined by palpation of the a. radialis at the wrist аnd the pressure was increased until the pulse disappeared [systolic pressure]. The oscillation of the cuff pressure with the heartrate, when the cuff was smoothly being deflated, was noted. It results from pressure transmission from the artery to the cuff, which is the basis of all oscillometric measurements. The problem where to locate the point of diastolic pressure in the sequence of
oscillations could not be solved. In 1905 a solution was proposed by Korotkoff, who introduced the auscultatory blood pressure measurement [Korotkoff 1905]. He described the characteristic sounds [named after him] heard with a stethoscope in the “elbow” when the pressure in the inflated cuff was lowered through systolic аnd diastolic pressure. He used the same cuff as Riva-Rocci. This method still is the basis of the auscultatory method used today. The method slightly improved after 1940, when intra-arterial measurements had become available that could be used as a “gold standard”. In particular attention was given to the small size of the cuff, which especially in the obese turned out to overestimate the blood pressure level. The American Heart Association recommends for the cuff-bladder dimension a width of 40% of the circumference of the limb, long enough to encircle 80% of the circumference, to be placed such that it is centered over the artery to be compressed. For premature neonates this leads to the use of a smaller cuff.

The automated oscillometric devices currently used still follow the oscillometric principle described by Marey, using a cuff snugged around a limb. A detailed description, characteristic for many of these devices, is given by Ramsey [Ramsey 1991]. The cuff is inflated to approximately 160 mmHg for the first determination, or to 30 mmHg above the previous systolic pressure found. Oscillations in the cuff
pressure are sampled [by a microprocessor] at a constant cuff pressure to improve artefact reduction. If no oscillations occur the pressure is lowered a few mmHg, аnd sufficient sample time is allowed. If oscillations occur the amplitude of two consecutive pulses should only differ a small amount, аnd the time interval between them has to match closely the previous time intervals. When two consecutive pulses meet this criterion their amplitude is averaged аnd stored for later determination of systolic,
diastolic or mean pressure. This criterion avoids many of the artefacts arising from strong respiratory variation, premature ventricular contraction, аnd external motion. If artefacts occur, the measurement time will be increased by the time waiting for two acceptable pulses. If the delay is too long the actual cuff pressure level is skipped. The cuff pressure is lowered again a few mmHg аnd sampling for the oscillations starts again. In general no oscillations are found any more after going several deflation steps through the diastolic pressure. An algorithm is applied to the average values, stored at each deflation step, to determine the systolic аnd diastolic arterial pressures. The mean is also calculated from the averaged values. The systolic [diastolic] pressure normally is associated with the cuff pressure at which the increase [decrease] of the oscillation amplitude has a maximum. The algorithm is an important part of the measuring device, because it has to reduce artefacts from the set oscillation amplitudes of many different patients аnd patient groups.

Problems in blood pressure measurements may occur due to:

• physiologic variation [shock, large blood pressure variation during the measurement, variation in pulse rate [>15%]]
• anatomic variation [conically shaped arm, calcified arteries, subclavian compression];
• cuff compression variation [movement [of cuff or arm], shivering, bumping the cuff, vehicular or helicopter vibration].

In neonates additional care has to be taken in the interpretation of blood pressure values. Studies in which oscillometric аnd invasive arterial measurements are compared report a significant disagreement between them. Many report an overestimation of low pressures [Gevers 1994, Diprose 1986] аnd unacceptably large errors for the individual infant [Briassoulis 1986, Wareham 1987]. Furthermore, attention is required to the cuff size used: too small sizes yields too high pressure values, too large sizes only have a small effect. The cuff has to be applied snugly enough аnd all air has to be squeezed out before applying the cuff. The cuff hoses may not be kinked аnd the air system may not leak. The patient must be still аnd quiet, th e cuff should be at heart level [if no hydrostatic compensation is applied] аnd subclavian compression by the arm or retraction of the chest should be avoided.

Some automated devices use microphones or a Doppler signal to determine the systolic, diastolic аnd mean pressure points during deflation of a cuff. The microphone instruments use the abrupt change of frequency content of the Korotkoff sound. A filter circuit is able to detect the muffled end point of the sound at diastolic pressure, which is more reliable than the detection of disappearance of sound [McCutcheon аnd Rushmer, 1967]. Nevertheless, errors may occur due to ambient sounds or sounds generated by patient movement. These errors can be reduced either by ECG triggering or by use of a second microphone that only registrates the ambient sounds. Subtraction of this signal from the original with the Korotkoff sound gives a purer Korotkoff registration. Doppler blood flow detection under the cuff can be used to determine the systolic pressure in specific small arteries. It is useful in the evaluation of peripheral vascular disease [Sumner 1984]. Only recently a continuous non-invasive measurement of finger arterial blood pressure in adults аnd children older than one year has become available [Smith et al., 1985, Peñáz 1973]. Up till now this method is considered to be inappropriate for use with neonates.

Summarizing we can state that automatic non-invasive blood pressure measurements currently are performed by the oscillometric method. In manual blood pressure measurements the auscultatory method is used. In both cases a significant deviation from the intra-arterial pressure cannot be excluded, especially not in case of a patient with several affected organs аnd systems.