2.3.1      Method of Korotkoff

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The most common procedure to get the systolic and diastolic arterial pressure, that is the maximum and the minimum of the pressure waveform respectively, was started by Riva-Rocci in the 1896 and, in principle, it remained the same till nowadays. Usually with this method we measure blood pressure at the brachial artery. An elastic bladder, contained in a tissue case is wrapped around the arm of the subject. The tissue case is stiff outside, and softer in the part in contact with the arm; a ball pump allows to inflate the bladder.

The bladder is quickly inflated, until the pressure inside it overcomes the maximum pressure in the artery. The cuff squeezes the arm of the subject and the radial artery, compressed against the humerus, occludes. At this point, the air in the cuff is slowly released so that the pressure inside the cuff decreases (Figure 2.7). When the pressure in the cuff becomes just lower than the maximum arterial pressure, the blood, at systole, is able to overcome the occlusion of the cuff. This event must be detected since it is the trigger to measure the pressure in the cuff which, in this moment, is equal to the systolic pressure (actually it is slightly lower).

 

Figure 2.7. Sphygmomanometric measurement. A cuff is wrapped around a arm and quickly inflated above maximum pressure.  It is then deflated at 3 mm Hg/s and appearance and disappearance of Korotkoff sounds indicates systolic and diastolic pressures.

 

Korotkoff sounds

In order to detect the moment the pressure in the cuff equals the systolic pressure, Riva-Rocci was feeling the pulse at the wrist while deflating the cuff. As long as the pressure in the cuff was above the systolic pressure he couldn’t feel any pulse, as soon as the pressure in the cuff was becoming lower than the systolic pressure, he could feel the pulse again and, therefore, he could measure the systolic pressure. In 1905 Korotkoff, a doctor or the Russian army, noticed that placing a stethoscope on the brachial artery, just below the cuff, inflating the cuff and then deflating it, it was possible to hear very peculiar sounds, lately called Korotkoff sounds. In 1935 these sounds were correlated with the extremes of the pressure waveform and also nowadays the pressure of the cuff is measured synchronously with these sounds, in order to get the systolic and diastolic pressure.

Figure 2.8. Phases of Korotkoff sounds. ‘S’ means silence.

Figure 2.8 gives a short description of Korotkoff sounds and their relation with the events in the artery. The upper part represents the pressure in the cuff that slowly decreases. Initially it is greater than the maximum arterial pressure and no sound is audible. Then, when it reaches the maximum of arterial pressure, the Korotkoff sounds start. This is the trigger to measure the pressure in the cuff and get the systolic pressure. Hence, the sounds change in intensity and quality as the pressure in the cuff decreases. From snapping tones they become murmurs (faint blowing sounds), then thumping sounds, like when knocking at a door and after muffled (as when a alarm clock is put under a pillow). In most of the people they finally disappear and this generally is considered the right moment to get the diastolic pressure. According to Gary Drzerwiecki, it is a long-held misconception that the origin of Korotkoff sound is due to flow turbulence in the partially occluded artery. He justifies this statement assessing that the sounds occur in a low-blood-flow situation*, while the peak of the flow is successive to sound occurrence as demonstrated by Doppler ultrasound measurements[1].

Use of sphygmo-manometers

Whenever systolic and diastolic pressures are sufficient information, it is typical to measure them with a sphygmomanometer, i. e. a set including the cuff, the ball pump (with a valve to release the air) and a mercury manometer to measure the pressure in the cuff.  A stethoscope is also needed, to listen to Korotkoff sounds. Figure 3.9 shows an aneroid sphygmomanometer, where a dial replaces the classical mercury-column manometer, for reading the pressure measurements.

Figure 2.9. Anaeroid sphigmo-manometer. Instead of a mercury column it includes a dial to show the pressure measurement.

It wasn’t until well after the Second World War that nurses were permitted to take blood pressure readings, being considered “doctor’s work” until that time. Indeed, although the procedure is clearly harmless for the patient and not even difficult to be understood and performed, there are several considerations which must be kept in mind when measuring blood pressure with a sphygmomanometer. Besides, sphygmomanometric measurements are of basic importance in the diagnosis and the treatment of hypertension; their results are used to decide weather to proceed with further investigations in the diagnostic phase, or they provide feed-back during the treatment of the disease. Considering then the fact that this disease affects a non negligible percentage of the world population, it is understandable the attention that is paid nowadays to identify standard procedures, in order to confer the maximum reliability and repeatability to the measurements. Several factors need to be taken into account. Concerning the patient, for instance, he should be relaxed and feel comfortable. Concerning the cuff, it should be chosen with a proper size according to the circumference of the arm, or the limb where arterial pressure has to be measured. Table 3-1 resumes suggested guidelines in taking blood pressure measurements using a sphygmomanometer.

It would be too long to enter in details for all the suggested steps and for the most the enclosed rationale might be sufficient an explanation. Here we remind that Bernoulli’s law suggests the importance of measuring pressure at the same height of the source of the pressure. When we measure blood pressure at the brachial artery, generally we actually want to estimate the pressure at the left ventricle. When the patient is sitting with a supported arm (step 3), the left ventricle and the arm are more or less at the same height, besides, since the brachial artery is relatively close to the heart, the pressure recorded is similar to the aortic pressure. An analogous consideration recommends to keep the transducer at the same level of the catheter tip using fluid-filled catheters, see paragraph 2.2.1.

The auscultatory gap (step 8 and 15a) is a time interval during which Korotkoff sounds become very low and in some cases cannot be heard at all. When this happens, although Korotkoff sounds seem to disappear, the pressure in the cuff didn’t reach the systolic pressure yet. Taking a reading in this moment, therefore, leads to overestimation of the diastolic pressure (step 15a). A critical point, is to determinate until which pressure to inflate the cuff, at the beginning of the procedure. In fact, it must be greater than the systolic pressure, which actually is the object of the measurement and therefore it is unknown. For this purpose, the maximum pressure is firstly roughly estimated by palpation of the pulses at the wrist but, since also the pulses, as Korotkoff sounds become faint in correspondence of the auscultatory gap, it is possible to wrong this preliminary estimation of the maximum pressure. Adding 30 mm Hg to the estimated value we get a good confidence that the measurement will start from above the real maximum pressure. Besides, inflating the cuff to such a pressure should not cause excessive discomfort to the patient.

Cuff deflating velocity

The velocity the cuff is deflated at is a main determinant of the accuracy of the measurement (step 12). In fact, we ever sample the systolic pressure in correspondence of a heart beat, the first heart beat at which Korotkoff sounds became audible. In order to hear this sound, however, the pressure in the cuff must have become at least a bit lower than the systolic pressure. The entity of this “bit lower” determines the accuracy in measuring the systolic pressure and, in turn, it is determined by the deflation velocity. If we deflate the cuff too fast, the pressure in the cuff might become significantly lower than the systolic pressure, in the time between the last heart beat we don’t hear Korotkoff sound, and the first heart bit we do hear it and we take the measurement. Therefore we underestimate the systolic pressure (Figure 2.10).

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Figure 2.10. Deflating the cuff too fast we underestimate the systolic pressure because at the heart beat Korotkoff sound appear, the pressure in the cuff has become excessively lower than the systolic pressure.

Releasing the air too slowly we get a very precise measurement of systolic pressure but we may have undesired consequences as well. We risk in fact to cause a venous congestion because, when the pressure in the cuff becomes lower than the systolic pressure, the artery opens for some time and the blood flows in the limb. At the same time it cannot leave the limb, because the pressure in the veins is lower than the pressure in the cuff. As long as this situation persists, the venous pressure increases more and more at every heart bit and, a part patient’s complaints, it happens that also the pressure at the capillaries will increase. Then the mean pressure at the artery increases as well, therefore, in the time we wait for Korotkoff sounds to disappear, the whole blood waveform shifts to greater values of pressure and, when the sounds finally disappear, we measure an “inflated” diastolic pressure.

The suggested deflation velocity is about 2-3 mm Hg per second; between a heart beat and the successive more or less 1 second passes, therefore during this time the pressure in the cuff changes about 2-3 mm Hg. If other sources remain within 2 mm Hg, we can assess that the accuracy of sphygmomanometric measurements is approximately 5 mm Hg.

Automated equipment
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Nowadays a lot of equipment have been developed to measure blood pressure non invasively, which are of easier use than a sphygmomanometer and even allow self-measurements of blood pressure. Basically all of them present a kind of cuff which wraps either around the arm, the wrist or a finger, then they differ in the degree of automation. Fully automated devices inflate and deflate the cuff at the appropriate speed and perform the measurement at the appropriate instants. Semi-automated equipment, on the contrary, live the patient to inflate the cuff but they control its deflating and measure the pressure. In any case, the measured signals are A/D-converted and displayed on a digital readout.

Such devices present evident advantages with respect to the classical sphygmomanometer with mercury column and stethoscope, since they don’t require neither good hearing and sight, nor personnel training. On the other hand, they need to be periodically calibrated and they are less accurate in performing the measurement than a trained physician could be using sphygmomanometer and stethoscope.

Automated equipment accomplishes the measurement of blood pressure   either detecting the instants of maximum and minimum pressure (e. g. identifying somehow Korotkoff sounds), or following different procedures. The method of Korotkoff, however, is considered an appropriate non-invasive blood pressure reference by which other methods may be evaluated.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    The method of Korotkoff has been automated identifying electronically the sounds. Korotkoff sounds are characterised by a particular frequency content (20-300 Hz), which varies in a specific way as the sounds change in time (see previous paragraph). In order to listen to the sounds, the cuff incorporates microphones, inserted at its distal extremity; the detected sounds are recorded, digitised and elaborated. These devices identify the instant of maximum and minimum pressure according to the appearance and disappearance of the sounds or, even better, thanks to their characteristic change in frequency content. Handling microphones, however, presents two main difficulties: the first concerns their use in a noisy environment, since ways must be found to discriminate between Korotkoff sounds and noise (also noise generated by patient movements limits the application area of this technique); the second difficulty originates from their need to be precisely positioned upon the artery. Consequently, other techniques have been developed as well to detect when the blood is flowing through the vessel; they make use of many different kind of sensors as photoelectric, thermometric, for tissue impedance. Also Doppler ultrasounds have been employed, either to detect vessel walls movement (but this technique is not used anymore) or to detect when blood flows inside the vessel, after the cuff occlusion. This method however is not employed for automatic measurement, but it is used to measure systolic pressure in distal small arteries, in order to identify possible vascular diseases.