Letters to the Editor: Letters & Announcements
To the Editor:
We recently had a case of artifactual increase in direct arterial blood pressure measurement readings resulting from malposition of the stopcock lever arm.
A 54-yr-old male patient underwent automatic internal cardiac defibrillator change under general anesthesia with a right radial arterial pressure monitoring catheter. A normal appearing arterial waveform was present with a blood pressure of 149/116 mm Hg and a mean of 129 mm Hg. A simultaneous noninvasive blood pressure of 105/70 mm Hg and a mean of 82 mm Hg was also obtained from the right arm. The transducers were located at the midaxillary line and had been “zeroed” at that level. Attention was directed to the transducer assembly. It was noted that the stopcock lever arm was directed upward from the horizontal by approximately 15 degrees. This was corrected to horizontal and the arterial waveform shifted downward on the display monitor. The arterial blood pressures were then 100/65 mm Hg with a mean of 77 mm Hg, similar to the values obtained from the noninvasive blood pressure measurement (Fig. 1).
Based on the above findings we obtained institutional approval from our Human Subjects Committee for a prospective study of 10 adult patients undergoing a variety of surgical procedures with radial arterial monitoring. Each patient had an identical arterial monitoring set-up as in the above case report, with an 84-in disposable transducer with 3 mL squeeze flush (Abbott Critical Care Systems). Baseline waveform and pressure values were strip chart recorded. The stopcock lever arm was then slowly turned towards the horizontal position. At approximately 15 degrees above horizontal, an arterial-like waveform was still present that produced a significantly higher pressure than the baseline value. When the stopcock lever arm was turned back to the horizontal, the waveform and pressure values of the baseline returned.
Eight of ten patients had arterial blood pressures that were artifactually increased by manipulation of the stopcock lever arm. It was noted that the degree of overestimation was in direct relation to the flush bag pressure. It was also noted that the pulse pressure of the artifactual waveform was narrower than the baseline waveform.
Pressure from the flush bad admixes with arterial line pressure when the stopcock lever arm is misaligned, creating overestimation of arterial pressure. The admixed arterial waveform created an artifactual waveform that appears realistic but numerically elevated. The flush bag tubing overdampens the artifactual waveform. This is reflected in the artifactual waveform as a narrowed pulse pressure.
There are many reasons for obtaining artifactual arterial blood pressure measurements. I report a novel reason for overestimating arterial blood pressure. If the transducer stopcock is left 15 degrees elevated above horizontal, an artificial overestimation of arterial blood pressure with a realistic waveform may be obtained. Inappropriate treatment may be given if unrecognized. I wonder if your readership has seen this phenomenon.
Mark A. Eggen, MD
Metropolitan Anesthesia Network
John G. Brock-Utne, MD, PhD
Department of Anesthesia
Stanford University Medical Center