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 blood flow to the distal part of that limb and possible limb loss in extreme cases.
Waveform artifacts due to technical errors include damping (overdamping vs. underdamping), fling (whip). See normal arterial waveform in Figure 9-1.
Damping is when the waveform amplitude is reduced. Some amount of damping is desired in order to reduce the inherent resonant frequency of the monitoring system or to eliminate the contributions of the physical forces of the system and only measure the patient’s pressure waveform. If the system is overdamped (See Fig. 9-2) (by an air bubble, partially closed stopcock, or soft tubing that absorbs the pressure waves), then the pressure will be underestimated with a low-amplitude waveform. An underdamped system occurs when there is too high a frequency response in the system, causing a large-amplitude waveform and an overestimation of the systolic blood pressure.
Arterial catheter fling (whip) (See Fig. 9-3) occurs when the peak pressure “overshoots,” resulting in a falsely high systolic blood pressure. This can result from significant catheter tip movement or overfilling of the transducer.
Resonance: The arterial waveform is composed of multiple sine waves with each wave having a specific frequency. If any of these frequencies are the same as the resonant frequency inherent in the transduction system, distortion of the signal will occur, leading to erroneous measurements. The equipment used is specifically designed to maintain a natural frequency above the frequencies of the arterial waveforms to eliminate this problem.
CENTRAL VENOUS PRESSURE (CVp) MONITORING (SEE FIG. 9-4)
Indications: In patients with hemodynamic instability, CVp monitoring provides an assessment of intravascular volume as well as an assessment of right ventricular end diastolic pressure (RVEDp) in patients without tricuspid stenosis. Central lines also offer stable vascular access for infusion of vesicants, inotropes, and parenteral nutritional support, as well as a means of obtaining blood samples for laboratory testing.
Potential complications: Infection, pain, bleeding, vascular injury, pulmonary embolus, air embolus, hemothorax, and pneumothorax.
Sites: Femoral, internal jugular, subclavian. Note: Due to line characteristics that lead to inaccuracy (i.e., length and distensibility), peripherally inserted central catheter (PICC) lines are generally undesirable for CVp monitoring.
The normal CVp tracing. The waveform consists of a, c, and v waves, as well as a systolic x descent and diastolic y descent.
a wave = atrial contraction
c wave = ventricular contraction
v wave = atrial filling driven by systemic venous return
x descent = atrial relaxation
y descent = rapid emptying of atrium following opening of tricuspid valve
An understanding of the components of the CVp is crucial, as the waveform undergoes characteristic changes in certain pathologic states: