The Direct Measurement of Pressures

When there is a clinical need for the direct measurement of a patient’s pressure, many devices have to work together to get accurate results.

The direct measurement of pressures is an invasive procedure during which a clinician places a catheter into a vessel or chamber to measure the pressures at that point. With any direct-pressure measurement, the patient needs to be protected against infection. Good site-protection techniques are very important.

All direct-pressure measurements have the same basic components: a catheter that is fed into the vessel or area to be measured, a fluid column that connects the catheter to the transducer, a transducer that converts the mechanical variations of the pressures to electrical signals, an amplifier that processes the transducer signals, a display, and, possibly, a recording.

The biomed should not touch the catheter or the fluid column; they should be left to the clinical personnel. A common problem with fluid columns is that they clot (clog) up over time. A small disposable flush valve connecting a pressurized solution and the catheter generally prevents clotting. The valve regulates the flow of a flush solution that keeps the catheter from clotting. The solution is in a bag pressurized to 250 to 300 mm Hg. The flow through the valve is less than 10 mL per hour; on an infusion pump it would be called the KVO (keep vein open) rate. If the waveform on the display is overdamped, it is an indication that the catheter needs to be “flushed.” This is done by squeezing the valve or pulling the pigtail, depending on the design. The flush solution is a drug and must be administered only by a licensed person. Technicians should not touch the flush valve but have the nurse or physician flush the line.

Q U E S T I O N S

1 Invasive and noninvasive systolic pressures are_____________.
    A    always the same
    B    never the same
    C    within 5 mm Hg of each other
    D    rarely the same

2 The frequency response of a pressure amplifier is______________.
    A    0.5 to 35 Hz
    B    0.1 to 100 Hz
    C    0 to 1,000 Hz
    D    0 to 15 Hz

Answers: 1–D; 2–D

Most transducers now in use are disposable and come with a flush valve. The transducer must be mounted so that its diaphragm is level with the catheter tip inside the body. If the transducer is 1 inch too low, the pressure indicated will be 5 mm Hg higher; and if it is 1 inch too high, it will give a pressure 5 mm Hg lower than actual. The transducer cable is often a problem source—as are most cables—and should be inspected before use. If there is a problem with the cable, it should be thrown away, not put back in the drawer.

The amplifier is generally part of a patient-monitoring system that may offer several channels for pressure monitoring. The amplifier gain is 5,000, with a frequency response of 0 to 15 Hz. Both the gain and frequency response may vary with manufacturers. The amplifier also processes the pressure waveform and derives the systolic, diastolic, and sometimes the mean pressures for display. Most manufacturers use a level detector to derive both the systolic and diastolic pressures to be displayed. The systolic is the highest point on the waveform and the diastolic is the lowest. Using level detectors can present a problem. If the dP/dT (change in pressure /change in time) of the waveform is very fast, one could get a false high reading. Conversely, if the dP/dT is slow, a low reading is possible. An air bubble in the line, the catheter occluding, or a leaking connection port can cause a low dP/dT. Technicians should direct the clinician to check the ports for leaking and to flush the line. In 95% of the cases this will correct the problem. A high dP/dT is most common with heart rates greater than 120. There is very little that can be done to correct that problem. To test a system while the patient is attached is not something that can be done with normal test equipment since sterility has to be maintained.

What a technician can do is take an IV extension tube and measure the length, which is often 40 inches. The patient line is turned off at the transducer, and the extension tube is attached to the vent port of the transducer. The vent tube should be held straight up and the tube filled from the flush bag. When the tube is filled, the readout should be 95 to 105 mm Hg, with 2.5 mm Hg per inch of filled tubing above the transducer. This will provide a safe method of testing the accuracy of the transducer/amplifier. Once the testing has been done, the vent port is shut, the patient port opened, and the extension tubing carefully removed so the fluid is not spilled.

Most amplifiers will also compute the mean blood pressure as 0.707 of the systolic pressure on arterial measurements. Venous pressures are only in mean, and that is usually computed as 0.707 of any variations in the pressure.

On pulmonary capillary measurements, it is possible to have a negative pressure displayed on a screen with the digital display of the reading showing 00. This occurs when the clinician has advanced the catheter too far into the capillary, and, as the patient is ventilated, each inspiratory effort pulls on the catheter. To confirm that it is respiratory coupling, watch the waveform and note that the waveform returns to normal after each inspiratory effort.

Other points to remember:
•Direct and indirect blood pressure measurements are almost never the same.
•The high dP/dT can cause an artificially high systolic reading, especially when the patient is in tachycardia.
•Biomeds should never put a flush system on an intracranial pressure measurement; it can cause very severe brain damage—even death.
•Unless biomeds have licenses to administer drugs, they should not flush the pressure lines.

David Harrington is director of staff development and training at Technology in Medicine Inc in Holliston, Mass. Contributing to this article is Ed Bober, a staff biomed for Technology in Medicine at Caritas Holy Family Hospital in M ethuen, Mass. Direct questions to [email protected].