BP readings should be close to cuff

Dear Dr. Roach: I recently had surgery, and my blood pressure was monitored with a catheter. The readings were higher by 30 to 40 points compared with cuff measurements taken at the same time. They were having a hard time getting the catheter readings down to 170. I am wondering which readings are to be used under blood pressure guidelines. — R.R.

A catheter (that’s just a name for a tube inserted in the body) measures blood pressure directly. When people are in the operating room or in the intensive care unit, a catheter often is placed in the artery to give instant, direct measurements of the blood pressure. Several studies have shown that these are more accurate than noninvasive blood pressure measurements.

Cuff measurements can be higher than catheter measurements in people who have stiff blood vessels with calcium in them. These do not compress properly under the pressure of a cuff, so blood will still go through even with immensely high cuff pressures.

Even arterial catheters sometimes can be inaccurate. However, a difference of 30 to 40 points higher on a catheter than a cuff is beyond my experience. I suspect a technical error, either in the measurement via arterial catheter or (more likely) via the cuff.

Errors in measuring the blood pressure by cuff can come from several mistakes. The size of the cuff needs to be appropriate for the arm: Both too small and too large cuffs will be inaccurate. The cuff should be at the level of the heart, and the arm relaxed. The cuff must be deflated at a precise rate, often slower than I see done.

Blood pressure guidelines are based entirely on cuff measurements, but there usually is not a big difference between them and arterial catheter readings.