Monitoring of a patient’s blood pressure (BP) is crucial to the safe conduct of any surgical procedure. This chapter focuses on invasive arterial-line BP monitoring and some of the common errors associated with inaccurate readings.

For an arterial monitoring system to provide an accurate reading of the measured waveform, it must have an appropriate natural frequency and damping (Fig. 28.1A). Briefly, each measuring system has a natural frequency about which it oscillates. This frequency is proportional to the stiffness of the tubing and the transducer diaphragm and to the cross-sectional area of the catheter. It is inversely proportional to the catheter length and fluid density. If the frequency of the pressure wave being measured approaches the natural frequency of the system, the system distorts the measurements by excessive amplification (ringing or resonance) of the incoming waveform. This may overestimate the systolic BP by as much as 25% and underestimate the diastolic BP by 10% (the mean pressure is not affected). Damping counteracts this phenomenon.

Several problems can occur in this interaction between the natural frequency and damping parameters. Overdamping decreases the frequency response too much, underestimating the systolic BP and overestimating the diastolic BP (Fig. 28.1B). Blood clots, air bubbles in the tubing, and kinked catheters are common causes of an overdamped system. Underdamping, or hyperresonance, occurs when long connecting lines (>1.4 m) or smalldiameter tubing (<1.5 mm internal diameter) are used or when the catheter is too large for the vessel (e.g., 18-gauge catheter in a small radial artery). This overestimates the systolic BP and underestimates the diastolic BP. Underdamping may also cause the appearance of additional, small, nonphysiologic pressure waves on the tracing (Fig. 28.1C). Slow degradation in the dynamic response also may occur over time (decrease in the natural frequency with increase in the damping coefficients), causing underdamping. Fortunately, this problem can be easily rectified with periodic manual flushing of the system.

Problems within the system that lead to inaccurate measurements are usually due to problems with the mechanical connection components (i.e., the catheter, fluid-filled tubing, or the stopcock). To prevent these mechanical problems, several steps should be followed.