The dynamic responses of liquid-filled catheter systems for direct measurements of blood pressure

Direct measurement of blood pressure with a fluid-filled catheter and transducer is widely accepted in clinical practice. However, errors associated with the measurements are often not appreciated. The system frequently is unable to reproduce rapidly changing waveforms and overshoots to produce higher peak pressures. The most common causes of this phenomenon are trapped small air bubbles and long connecting tubing. To assess the inaccuracy in pressure measurements, we calculated the weighted sum of the percentage difference between reference and recorded amplitudes of sinusoidal waveforms for several catheters and connecting tubings. We found that when the connecting tubing was shorter than 3 feet long and no air bubbles were trapped readings were accurate. On the contrary, connecting tubings 7 feet long or longer, and/or air bubbles, were frequently associated with inaccurate results. For example, minimal air bubbles (0.25 ml) in a system exaggerated the systolic pressure measurement by 41 torr when simulated blood pressure was 150/50 torr.