Sometimes during manual blood pressure measurement by auscultatory method, after a few initial tapping sounds, no sound is heard for a variable duration and then the sounds are heard again. This period when no sound is heard is called as auscultatory gap.
When the cuff pressure is great enough to close the artery during part of the arterial pressure cycle, a sound then is heard with each pulsation. These sounds are called Korotkoff sounds believed to be caused mainly by blood jetting through the partly occluded vessel. The jet causes turbulence in the vessel beyond the cuff, and this sets up the vibrations heard through the stethoscope.
As long as the pressure in the cuff is higher than the systolic blood pressure of the patient, blood doesn’t jet through the completely occluded artery, hence no sound is heard. If the pressure is dropped to a level equal to that of the patient’s systolic blood pressure, the first Korotkoff sound will be heard. As the pressure is further gradually lowered down, following korotkoff sounds are heard:
Phase 1 (K1): Clear tapping sounds representing systolic pressure
Phase 2 (K2): Onset of swishing sound or murmur
Phase 3 (K3): Loud slapping sound
Phase 4 (K4): Muffled Tones sounds representing diastolic pressure
Phase 5 (K5): Tones cease
An auscultatory gap also called as silent gap is the interval of pressure where korotkoff sounds indicating true systolic pressure fade away and reappear at a lower pressure point during the manual measurement of blood pressure by auscultatory method. The auscultory gap occurs when the first Korotkoff sound fades out for about 20-50 mmHg only to return. It can result in following erroneous blood pressure reading:
- Underestimation of systolic blood pressure
- Overestimation of diastolic blood pressure
The patient’s actual systolic pressure is 200 with a gap from 170 to 140 and a diastolic of 110. You inflate the cuff to 170 and hear nothing until the manometer reaches 140, which you presume is the systolic pressure. Also if you, inflate the cuff to 200, you may read 170 as the diastolic pressure which is the beginning of auscultatory gap.
When recording a blood pressure with an auscultatory gap, always list your complete findings. eg. BP 200/110 with the auscultatory gap from 170 to 140.
Auscultatory gap has been found to occur due to venous pooling of blood. The auscultatory gap is most likely to appear in the obese arm, especially if the physician pumps up the cuff slowly and traps a great deal of blood in the arm’s venous compartment. Another way to trap blood is to pump the cuff 2nd time immediately after 1st determination, without allowing 1-2 minutes for the trapped blood to escape.
Auscultatory gap in Hypertension
An auscultatory gap is common in elderly hypertensive patients. It occurs in some hypertensive patients only. Auscultatory gaps are related to carotid atherosclerosis and to increased arterial stiffness in hypertensive patients, independent of age.
3 types of auscultatory gaps, have been identified by using wideband external pulse recording.
- G1: occurs with cuff pressure just below systolic and is characterized by the presence of K1 and K2 with intermittent disappearance of K2. G1 gaps are related to a phasic decrease of arterial (systolic) pressure.
- G2: are related to a phasic increase of arterial (diastolic) pressure, occur when cuff pressure is just above diastolic, and are characterized by the presence of K1, K2, and K3 with intermittent disappearance of K2.
- G3: occurs with cuff pressure between systolic and diastolic and are characterized by an underdeveloped or blunted K2 signal.
- The mechanism of origin of auscultatory gap has not been understood clearly.
- Auscultatory gaps, found in some patients with arteriosclerotic heart disease, can be elicited in others by reducing blood flow in the extremity. Reactive hyperemia eliminates the gap in every instance. The gap therefore appears to represent a vasospastic disturbance that limits the blood flow to the extremities, despite the presence of an enhanced pressure head. – Simon Rodbard and Jack Margolis
- Cavallani recently showed that the early loss of audible sound during cuff deflation is associated with blunted high frequency K2 signals associated with korotkoff sound (detected by wideband external pulse recording) likely related to the altered physical properties of a stiffer arterial wall.
- Determining systolic blood pressure by palpatory method before recording the blood pressure with auscultatory method.
- Inflating the blood pressure cuff to 20-40 mmHg higher than the pressure required to occlude the brachial pulse.