Plumbing system

The components comprising the plumbing system must always be kept sterile because the fluid contained within them comes into direct contact with the patient's blood. Most of these components are rather inexpensive and thus are single-use devices.

Catheter

Arterial catheters should be as small in diameter as is practical. The smaller the diameter, the less is the risk of obstruction of the artery. However, the wall must be thick enough to prevent kinking and obstruction of the catheter. The length of the catheter insertion should be sufficient to prevent spontaneous ejection from the artery. The diameter and length of the pulmonary artery catheter are set by the number of lumens required and the length of catheter required to reach the pulmonary artery. The diameter of each lumen is small and, coupled with the long length of the catheter, results in a large resistance which makes it difficult to achieve adequate dynamic response.

Stopcocks

A stopcock is usually located near the catheter connection and is used as a site for blood withdrawal for blood gas or similar analyses. When filling the plumbing system great care should be taken to ensure that all the central cavities of all stopcocks are filled with fluid. Air bubbles can lead to embolism if they are 'flushed' into the patient and can also diminish the fidelity of the pressure recording. In addition, stopcocks are a vulnerable site for patient contamination. Hands should be washed before stopcocks are touched, open stopcock ports should never be touched, and ports not in active use should be covered. Each connection in the plumbing system, particularly those involving stopcocks, can be problematic. The connections present a size discontinuity in the fluid pathway and become sites for entrapment of air bubbles. Unfilled central cavities of stopcocks are a frequent source of air bubbles that distort dynamic response.

Pressure tubing

The catheter and stopcock are normally connected to the pressure transducer with pressure tubing. Since the fluid-filled column between the catheter tip and the transducer must faithfully transmit the patient's pressure waveform, the tubing should be short (typically less than 50 cm) and non-elastic (soft venous tubing should not be used). Long lengths of tubing add resistance to the pressure waveform as it is transmitted from the catheter tip to the pressure transducer. Such resistance distorts the transmission of the pressure waveform. With elastic tubing, the pressure signal is distorted, 'pulsating' the pressure tubing, and therefore the signal detected by the external pressure transducer is distorted.

Continuous-flush device and pressurized fluid source

The continuous-flush device with its attached pressurized fluid source is used to fill the plumbing system. The device contains a tiny controlled 'leak' so that it infuses from 1 to 3 ml/h of fluid through the plumbing system and helps prevent clot formation at the catheter tip. The fluid source is usually an intravenous bag of normal saline pressurized in a cuff to 300 mmHg. The bag of saline should have the air expelled before connecting it to the plumbing system to prevent air embolism as it empties. A microdrip chamber should be attached to the fluid-filled source so that the flow rate can occasionally be monitored to ensure that the bag is pressurized and to detect when it is empty. Since it is impossible to clean and sterilize continuous-flush devices thoroughly, and to ensure that the tiny 'leak' functions properly for continuous flushing, they should not be reused.

Pressure transducer

Pressure transducers are used to convert the patient's pressure signal into an electrical signal. Modern blood pressure transducers have resulted from a remarkable set of advances in technology. Semiconductor technology has produced the transistor, processor chips for digital watches, and personal calculators. The same technology is now used to make pressure transducers. These transducers are calibrated to within ±3 per cent accuracy and maintain their zero set-point. Modern pressure transducers have a sensitivity of 5 pV/mmHg applied per volt of excitation voltage. The transducers are remarkably rugged, accurate over time, temperature stabilized, and relatively inexpensive. Since it is impossible to clean and sterilize disposable transducers thoroughly, they should not be reused.

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