In the intensive care unit or operating room, continuous readout of blood pressure is often required, usually to monitor for changes in stability or the effects of vasoactive drugs.
In most cases, a small-bore (no greater than 20-gauge), single-lumen, specially designed catheter is placed; intravenous catheters are not appropriate. The catheter is connected to special tubing that is stiff and relatively short, and this tubing attaches to a transducer via three-way stopcock. The entire tubing and catheter are filled with saline under pressure from a flush bag, creating a hydraulic system. It is critical that the saline pressure be kept at about 300 mm Hg to maintain fidelity of transmission of pressure waves from the artery to the transducer. The transducer sits on a pole or the bedrail at the estimated level of the center of the patient's right atrium. Inside the transducer are a flexible diaphragm and a silicon chip. The diaphragm moves with the pulsations of the fluid column originating at the arterial catheter tip. The chip detects movements, amplifies them and converts them to electrical signals, which are then displayed on a monitor as waveforms.
Can you see any points at which serious data errors can be introduced into this system? I can find at least 10. (See .) Most are human errors. No wonder we frequently confirm the invasive pressure measurement with a noninvasive one, and struggle to decide which reflects the patient's true systemic blood pressure. The two methods use different principles, too. Noninvasive cuff measurement is based on flow-generated oscillation, and invasive intra-arterial readings come from pressure-generated waveforms. Remember that flow equals pressure only if resistance is constant. Critically ill or anesthetized patients rarely have stable vascular resistance.
Fortunately, there is a measurement that is reasonably consistent between the two: mean arterial pressure (MAP). The noninvasive oscillometric method actually measures MAP and calculates systolic and diastolic pressures. The invasive method calculates MAP from the area under the curve of the arterial waveform (Figure 1).
In a perfect world, the two MAPs are relatively close, within 10 mm Hg. In reality, many technical, operator, and patient-related factors contribute to inaccuracies with either method. You must exercise considerable clinical judgment and understand the limitations of both methods to puzzle out whether one, the other, or neither measurement accurately captures your patient's hemodynamic status.
Most nursing literature recommends using the invasive arterial pressure for clinical decisions, as long as it is set up properly, has good waveforms and passes the “square wave test.” With the square wave test, the hydraulic system is opened to the pressurized flush for a few seconds, then quickly closed. The monitor tracing should shoot up to its maximum, sharply flatten out, and as sharply fall back to a little below baseline, then rebound up and down in quickly attenuating waveforms until returning to normal.
The normal arterial pressure waveform has several sections: rapidly increasing tracing up to a peak, then dropping down to a “shoulder” called the dicrotic notch, then somewhat less rapid declining to baseline or until the next cycle. The dicrotic notch represents closure of the aortic valve. The time from beginning of the upstroke to the dicrotic notch reflects the entire systolic period. The peak of the tracing is often read by the monitor as the systolic pressure, and the lowest point of the tracing is read as diastolic. The entire area under this curve is the MAP.
All waves or vibrations have a natural resonant frequency (RF). Additional energy applied to the waves at or near that frequency will dramatically increase the size of the waves. Peripheral arterial waveforms have a natural RF of about 3 to 5 Hz. If the fluid-filled hydraulic system also vibrated at 3 to 5 Hz, the result would be an overexcited or augmented waveform pounding on the silicon chip and causing amplification or “ringing” in the electrical tracing. The peak of the tracing would be much higher than the actual pressure inside the artery, or “systolic overshoot” (Figure 2). This can lead to incorrect assumptions of hypertension and physiologically inappropriate treatment. So, the hydraulic system has an RF of 10 to 20 Hz, at least three to four times the physiologic RF. In patients whose RF is higher, a damping device can be added to the system to reduce the vibrations and overshoot.
Hydraulic systems can be overdamped too. Compliant or excessive tubing, air bubbles, catheter malposition, an underpressurized system and other factors may greatly reduce the vibrations reaching the transducer, creating a flattened waveform that does not reflect the patient's pressures (Figure 3). A waveform that appears to indicate hypotension may result in inappropriate fluid challenges and vasopressors. In such a case, the cuff pressure may be higher than the arterial pressure, a clue that the system is overdamped.
Interpreting an “art line” pressure takes considerable skill and an understanding of the physiology, physics and physical setup that produce the numbers. Frequently, the physician must base his or her estimate of the true blood pressure on clinical judgment, and not rely so heavily on the numbers.