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PAC Q&A 1

Henry Fessler, MD
Associate Professor of Medicine
John Hopkins Hospital

Question 1

A patient is to be monitored with a pulmonary artery catheter, and his newly hired nurse assembles the transducer system. The transducer is clamped to an IV pole beside the bed, and is 10 cm below the level of the supine patient's right atrium. A bag of saline is pressurized to 300 mmHg, connected to an IV drip chamber and connected to the appropriate flush port of the transducer. The transducer is connected to the hub of the distal lumen of the pulmonary artery catheter by 100 cm of high-pressure extension tubing, which is filled with saline and flushed free of bubbles. The transducer is zeroed by opening to air a stopcock which is held at the level of the right atrium. After zeroing, the stopcock is closed and placed on the mattress. The catheter is then inserted.

The single-best-answer questions below (#1-4) refer to this system, which is illustrated here:

Flushbag

1. As shown, the flush bag:

A. Does not cause bubble formation in the absence of fast flushing.
B. Has been over pressurized, causing a significant over-estimation of pulmonary artery pressure.
C. Risks air embolization from turbulence in the drip chamber during fast flushing.
D. Should have been connected to the transducer by high-pressure tubing.

Answer

Answer is C. The flush system consists of a pressurized bag of saline, often heparinized, connected to the transducer through a high resistance flush valve. The resistor reduces the pressure on its downstream end to only 1-2 mmHg, even when the bag is pressurized to 200-300 mmHg. Therefore, it has trivial effects on the pressure recorded by the transducer. There is no need to use special tubing between the flush bag and transducer because IV tubing can withstand a much higher pressure than 300 mmHg, and this segment of tubing is not part of the path for pulse wave transmission.

The flush system maintains a slow but continuous flow of saline, which helps keep the catheter free of clots. When the resistor is removed by depressing a lever or button on the flush valve, the catheter is exposed to the full force of the upstream pressure, allowing a rapid flush. If an air-filled drip chamber is attached to the saline bag, turbulence during a rapid flush can entrain bubbles of air. These will degrade the fidelity of the wave recording if they become trapped in the tubing downstream from the resistor, and will cause air embolism if flushed into the patient. The latter is obviously more dangerous with systemic arterial than pulmonary arterial catheters. Even without flushing, the flush device contributes to the formation of microbubbles in the downstream tubing. The high pressure in the saline bag drives gas into solution. This gas comes out of solution in the lower pressure system beyond the resistor, and will accumulate into small bubbles over several hours.

References:

Gardner RM, Warner HR, Toronto AF, Gaisford WD. Catheter-flush system for continuous monitoring of central arterial pulse waveform. J Appl Physiol 1970:29;911-3.

Gardner RM, Bond EL, Clark JS. Safety and efficacy of continuous flush systems for arterial and pulmonary artery catheters. Ann Thorac Surg 1977;23:534-8.

Soule DT, Powner DJ. Air entrapment in pressure monitoring lines. Crit Care Med 1984; 12:520-2


Question 2

Which of the following is true regarding the zeroing of this transducer: A. The zeroing was made inaccurate when the stopcock, previously used to zero, was placed on the mattress.
B. The zeroing was inaccurate from the start, because the transducer was not leveled to the right atrium.
C. The zeroing will be made inaccurate when the bed, together with the stopcock previously used to zero, is raised.
D. After the PA catheter is placed, zero errors can be corrected by raising the transducer to the right atrium.

Answer

Answer is C. "Zeroing" a transducer means defining a reference pressure as ambient, from which all vascular pressures will be measured. By standard convention, vascular pressures are measured relative to the level of the right atrium. In the supine patient, this is approximated by the mid-axillary line. At that level, the transducer system is opened to the room using a three-way stopcock. It is not necessary that the transducer be at that same level. If the transducer is below the mid-axillary line, a hydrostatic pressure equal to the height of the water column above the transducer will be applied to it. The transducer will likewise be exposed to a small negative pressure if it is above the open zero port. However, when the system is then electronically zeroed, the appropriate pressure will be added or subtracted to set the transducer to zero, regardless of its height.

Once the zero port is closed, the height of the port becomes unimportant. However, the height of the transducer must now be kept constant, relative to the mid-axillary line. If the bed is raised but the transducer is not, the additional hydrostatic pressure will be recorded as an artifactual elevation of vascular pressure. The transducer would require re-zeroing to regain its accuracy. Some situations, such as intraoperative use, may require that the transducer be kept a distance from the patient. However, the transducers may often be conveniently strapped to the patient's arm, and the port molded into the transducer body used for zeroing. This has the dual advantage of keeping the transducer and zero port at a fixed relationship to one another, and keeping them both near the mid-axillary line as the bed is raised or lowered.

References:

Pederson A, Husby J. Venous pressure movement: choice of zero level. Acta Med Scand 1951;141:186-94.

Yang SS, Bentivoglio LG, Maranhao V, Goldberg H. From cardiac catheterization data to hemodynamic parameters. Philadelphia: Davis, 1988.

Courtois M, Fattal PG, Kovacs SJ, et al. Anatomically and physiologically based reference level for measurement of intracardiac pressures. Circulation 1995;92:1994-2000.


Question 3

The 100 cm length of high-pressure tubing could be improved upon because: A. The low-pressure PA does not require the narrow, high pressure tubing.
B. Standard IV tubing would decrease the natural frequency of the recording system.
C. Two, 50 cm lengths with an extra stopcock between them would make it easier to remove bubbles.
D. Shorter tubing would improve the natural frequency of the recording system

Answer

Answer is D. A critically important feature of a transducer/catheter system is that its natural frequency suit the pressure waveform that is being recorded. In a simple oscillating system like a pendulum, the natural frequency is the frequency with which the pendulum will swing back and forth when given a push. If a pendulum is pushed repetitively at its natural frequency, its oscillations will get higher and higher. A transducer and catheter system oscillates, because a pressure wave introduced into the catheter will travel down and deflect the transducer membrane. The membrane recoils back, and sends a reflected wave back up the catheter. When it reaches the end, another reflected wave is generated, headed back to the transducer. If the transducer is repetitively stimulated at the same frequency with which waves reflect back and forth, the oscillations of the transducer membrane will become larger and the amplitude of the pulse pressure will be exaggerated. Thus, the systolic pressure will be over-estimated and diastolic pressure under-estimated.

A pulse wave has a complex form, but can be decomposed into a series of simple sine waves of increasing frequency, called harmonics, which summate to produce the original wave. In order to reproduce a wave accurately, a transducer/catheter system must be able to record accurately up to the 6-10th harmonic. For a pulse of 120 beats per minute, for example, (2 Hz), a transducer must be able to record up to at least 12 Hz. If the natural frequency of the transducer/catheter system is less than that, the recorded wave will be distorted because harmonics near the natural frequency will be amplified.

The natural frequency of transducer/catheter systems is affected by several physical properties. The stiffer the catheter, the higher the natural frequency. Thus, the importance of high-pressure tubing is not that it will not burst, but that it is more rigid than standard IV tubing. The length of tubing is another determinant of natural frequency, since the oscillations complete a cycle faster when the distances are short. Therefore, the transducers should be positioned as close as possible to the site of pressure measurement. By the time a transducer is attached to the usual assemblage of tubing, catheters and connectors, natural frequency is reduced to only about 12-15 Hz, barely adequate for the task.

It is important to clear all bubbles from the tubing, because they decrease the natural frequency. However, it is also important to minimize the number of connections and stopcocks. Any irregularity in the path of the waves creates additional reflections, and creates blind pockets for bubbles to collect.

0701b

The figure on the left shows an arterial pressure tracing in which the natural frequency of the catheter system is too low, together with a fast flush. The figure on the right is the same patient moments later, after bubbles have been cleared from the system. The systolic pressure differs by 20-30 mmHg.

References:

Milnor WR. Pulsatile blood flow. N Engl J Med 1972;287:27-34.

O'Rourke MF, Yaginuma T. Wave reflections and the arterial pulse. Arch Intern Med 1984;144:366-71.

Kleinman B. Understanding natural frequency and damping and how they relate to the measurement of blood pressure. J Clin Monit 1989;5:137-47.

Gardner RM. Direct blood pressure measurement-dynamic response requirements. Anesthesiology 1981;54:227-36.

Heimann PA, Murray WB. Construction and use of cathetermanometer systems. J Clin Monit 1993;9:45-5.



Questions #4 and 5 refer to the simulated waveform below which shows the pulmonary artery pressure and the response to a brief fast flush in a patient in the intensive care unit.

0701c

Question 4

Which one of the following statements is true:
A. The pulmonary artery mean pressure will be underestimated.
B. The pulmonary artery diastolic pressure will be underestimated.
C. The pulmonary capillary wedge pressure will be underestimated.
D. The pulmonary artery pulse-pressure will be underestimated.

Answer



Question 5

Which one of the following conditions is NOT likely to cause a tracing like that above: A. A bubble of air in the tubing.
B. A partially wedged catheter.
C. A partially closed stopcock.
D. A fibrin clot on the catheter tip.

Answer

Answers: D, B. Another characteristic of transducer/catheter systems essential for high fidelity recordings is damping. Returning to the example of a pendulum, damping is the characteristic that slows the pendulum to a stop. In a transducer with a wave oscillating back and forth, it is the quality that makes the amplitude of the wave decay with successive reflections. A certain amount of damping is useful to prevent excessive oscillations when important harmonics of a wave approach the natural frequency of a recording system. Too much damping begins to filter out important features of the wave, making it appear more like a sine wave or eventually, like a mean pressure.

Damping is provided by electronic filters, and by physical characteristics of the recording system. A high resistance, such as provided by a catheter partially occluded by clot or partially closed stopcock, increases the damping. Increased compliance of the conducting system, due either to the inadvertent use of standard IV tubing or the presence of air bubbles, also increases damping. The degree of damping can be qualitatively estimated by inspection of the response to a brief fast flush, as the flush is released and the wave comes back within scale. In a properly damped system, one should observe 2-3 quick oscillations and then a return to the underlying wave. In an over damped system, the pressure will gradually return to the waveform, without oscillations. Alternatively, it may go off-scale in the negative direction, and gradually return. While the waveform itself may be confused with a "partial wedge," the response to a fast flush will be similar in a properly damped system whether or not the catheter is wedged.

0701d

The figures above show an overdamped (left) and properly damped (right) arterial pressure and fast flush. The tracings were recorded from the same patient within minutes of each other. In the tracing on the right, the time between successive peaks at the end of the fast flush is 1/natural frequency, and the decreased amplitude of successive peaks is a function of damping. The waveform recorded by an over-damped system will approach the mean pressure. Mean pulmonary artery pressures will remain accurate (as will mean wedged pressures). Systolic pressures will be underestimated, and diastolic pressures overestimated. When an over damped pressure is recognized, reversible causes may often be found.

References:

Kleinman B. Understanding natural frequency and damping and how they relate to the measurement of blood pressure. J Clin Monit 1989;5:137-47.

Morris AH, Chapman RH, Gardner RM. Frequency of technical problems encountered in the measurement of pulmonary artery wedge pressure. Crit Care Med 1984;12:164-

Pulmonary Artery Catheter Primer