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INTRODUCTION

The aim of this chapter is to introduce the reader to commonly deployed patient monitoring devices in critical care medicine. It is necessary to have a basic understanding of how these devices work to have an ability to recognize potential pitfalls in interpretation of the data they generate.

HEMODYNAMIC MONITORING

Invasive hemodynamic monitoring remains the accepted reference standard for blood pressure monitoring of hemodynamically unstable patients.

  • Transducers: Integral to monitoring of all pressure waveforms is the transducer. Transducers contain a fluid-filled interface that detects changes in pressure. The transducer contains a diaphragm that interfaces with a column of fluid extending from the cannula inserted into the blood vessel, through the pressure tubing, to the transducer.

  • How it works: Changes in intravascular pressure result in pulsations in the column of saline in the tubing between the cannula in the blood vessel and the transducer. The pulsations displace the diaphragm on the transducer, which transmits the waveform to the monitor and converts this waveform into an electrical signal displaying the pressure.

  • Zeroing/positioning: All transducers must be zeroed and placed in the appropriate location relative to the patient.

    • Zeroing negates the influence of external pressures (like atmospheric pressure) and occurs when the stopcock connecting the cannula to the noncompressible pressure tubing is opened to ambient atmospheric pressure. Consequently, all pressures displayed account for the external pressures like atmospheric pressure.

    • Transducer position is often aligned either with the blood vessel or cavity against which the pressure will be measured or at the level of cannula insertion. For example, the central venous pressure (CVp) transducer should be aligned with the upper fluid level of the right atrium (typically 5 cm posterior to the right sternal border at the fourth intercostal space). If the transducer changes position, then the blood pressure will be incorrect secondary to the effects of hydrostatic pressure from the tubing. For example, if the transducer is too low, the fluid in the tubing above the transducer will exert a greater pressure on the transducer than at the location at which it was zeroed, resulting in a falsely high blood pressure.

INVASIVE ARTERIAL PRESSURE MONITORING (SEE FIG. 9-1)

  • Indications include the following: Blood pressure monitoring in the hemodynamically unstable patient, frequent sampling of arterial blood gases or other laboratory tests.

  • Potential complications of arterial cannulation: Infection, pain, bleeding, embolus, impaired arterial circulation in the extremity cannulated. Retroperitoneal hematoma may occur in femoral arterial line placement.

  • Locations for arterial cannulation: Radial, dorsalis pedis, and posterior tibial arteries are most commonly the first sites accessed. If unable to cannulate these areas, alternative areas to consider include ulnar (smaller, more challenging, and less desirable if the radial artery is in the same side previously cannulated, as this would compromise blood flow to that hand), femoral, or axillary arteries. Brachial arterial lines are often avoided, as compromise to this artery may result in impaired ...

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