Which is true about measurement of BP with sphygmomanometer versus intra-aerial pressure measurements?
**Core Concept**
The measurement of blood pressure (BP) using a sphygmomanometer is a non-invasive method that relies on the compression of a blood vessel to estimate the pressure within the circulatory system. Intra-arterial pressure measurements, on the other hand, involve direct insertion of a catheter into an artery to obtain a more accurate and direct measurement of blood pressure.
**Why the Correct Answer is Right**
When a sphygmomanometer is applied to a limb, it compresses the artery and temporarily stops blood flow. As the cuff is slowly deflated, blood flow resumes, and the pressure at which the first Korotkov sound is heard is considered the systolic blood pressure. However, because the sphygmomanometer is compressing the artery, the measured pressure is higher than the actual intravascular pressure. This is because the cuff is not only measuring the pressure generated by the heart but also the pressure exerted by the external compression of the artery. In contrast, intra-arterial pressure measurements do not involve external compression and therefore provide a more accurate measurement of the actual intravascular pressure.
**Why Each Wrong Option is Incorrect**
**Option A:** Less than intravascular pressure - This option is incorrect because the sphygmomanometer is compressing the artery, which would increase the measured pressure, not decrease it.
**Option C:** Equal to intravascular pressure - This option is incorrect because the external compression of the artery by the sphygmomanometer artificially raises the measured pressure, making it higher than the actual intravascular pressure.
**Option D:** Depends upon blood flow - This option is incorrect because while blood flow is necessary for the measurement of blood pressure, the relationship between sphygmomanometer measurements and intra-arterial pressure is not directly dependent on blood flow.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that sphygmomanometer measurements are estimates of intravascular pressure, and while they are widely used in clinical practice, intra-arterial pressure measurements are considered the gold standard for accurate blood pressure assessment.
**✓ Correct Answer: B. More than intravascular pressure**