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A PA catheter refresher course


RN/DREXEL Home Study Program
CE CENTER

A PA catheter refresher course

This article is approved for 2.0 ANCC/AACN contact hours.
After reading it you should be able to:

1. Describe how pulmonary artery (PA) catheters work.
2. Discuss how to ensure accurate PA catheter data.
3. Identify potential complications associated with PA catheters.


CE credit is no longer available for this article. (Expired April 2005)


Originally posted April 2003

SALLY BEATTIE DULAK, RN, MS, CNS, GNP

SALLY BEATTIE DULAK is the principal of CV Health Promotion, a cardiovascular consultancy in Columbia, Mo., and a member of the RN editorial board.

KEY WORDS: pulmonary artery (PA) catheter, hemodynamics, right heart catheterization, pulmonary artery wedge pressure (PAWP), pressure transducer, phlebostatic axis, dynamic response, square wave test

Pulmonary artery catheters are still the most commonly used tool for in-depth hemodynamic assessment. This primer will help you brush up on how PA catheters work, how to ensure the data they provide is accurate, and which potential complications are associated with their use.

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Components of a fluid-filled pressure monitoring system include an IV fluid flush bag connected to IV tubing; a pressure sleeve/bag around the fluid flush bag with pressure gauge manometer and pump; low-compliance tubing with a pre-attached fast-flush device; a pressure transducer with enclosed diaphragm; a transducer stopcock; and an electric cable from the transducer to the monitor.

Since its introduction in the 1970s, the flow-directed pulmonary artery (PA) catheter has led to major advances in monitoring pressure and volume forces that affect circulating blood throughout the body. It has enabled clinicians to directly measure parameters that could not be detected from bedside physical assessment alone.

For example, level of consciousness and physical signs such as low blood pressure are late indicators of changes in blood flow and tissue oxygenation.1 Being able to access more precise hemodynamic data via the PA catheter makes it easier for clinicians to diagnose and manage critically ill patients. This in turn allows a more rapid evaluation of the effectiveness of interventions used to manipulate circulatory volume and pressure, including the administration of fluids and diuretics as well as vasoactive and inotropic medications.

Although new technologies are being developed to measure hemodynamic data, PA catheters—also called Swan-Ganz catheters—are still the most commonly used tools for that purpose.1 As a result, while PA catheters were initially used to manage patients with complicated MIs, their use has broadened to other areas such as critical care/ pulmonary medicine, nephrology, and surgery.2 Questions remain, however, about their precise indications, cost effectiveness, and effect on clinical outcomes, and the extensive training required to accurately interpret the hemodynamic data they provide.3

In order to use PA catheters most effectively, nurses in critical care and stepdown units need to understand how they work, how to ensure the data they measure is accurate, and what kind of complications might arise. Although a comprehensive discussion of PA catheter management is beyond the scope of this article, I will provide you with the key concepts that are essential to understanding this technology and the hemodynamic data it can provide.

When direct monitoring is indicated

Hemodynamics refers to the forces produced by volume and pressure that affect circulating blood throughout the body. The delicate balance of volume and pressure within the right and left heart chambers, and within the pulmonary and peripheral vasculature necessary for blood flow, ensures maximum blood and oxygen delivery to the major organs and peripheral tissues. Taking a blood pressure or palpating a pulse to measure heart rate and rhythm are two very basic—but indirect—assessments of the stability of volume and pressure within the circulatory system. They are used to guide hemodynamic therapies such as those used to treat hypertension or non-life-threatening arrhythmias.

When direct monitoring of hemodynamic parameters is desired, a PA catheter can provide a wide range of specific volume and pressure measures; a summary of some of the more important ones, their normal ranges, and their clinical relevance appears in the "PA catheter measurements and what they reveal" box. Direct monitoring of these parameters may be indicated when events such as an acute MI or acute renal or pulmonary failure lead to massive hemodynamic instability.3 This instability can also occur as a result of hypovolemic shock secondary to trauma, when the patient does not respond to conventional hemodynamic therapy in a predictable manner.3 Accurately interpreting this data, which is displayed as waveforms and numeric values, allows clinicians to fine-tune assessment of patients' cardiovascular status and to provide more precise interventions.

A look at the anatomy of a PA catheter

A PA catheter is a flexible, balloon-tipped, flow-directed catheter that is guided through the right side of the heart and into a branch of the pulmonary artery. Under sterile conditions, the catheter may be inserted through the subclavian, internal or external jugular, femoral, or antecubital vein. The procedure, known as right heart catheterization, may be performed with or without the use of fluoroscopy.

The typical PA catheter is 110 cm long and has four lumens—the distal, proximal, thermistor, and inflation lumen—each of which leads to a specific port. (Some PA catheters have a second proximal lumen and port.) The catheter is marked incrementally with black bands that indicate the length of insertion.

The distal lumen port is located at the tip, or end, of the catheter. Through this port you can monitor systolic, diastolic, and mean pressures in the pulmonary artery. It can also be used for drawing mixed venous oxygen saturation (SvO2) samples.

The proximal port, located approximately 30 cm from the tip of the catheter, is used to monitor right atrial (RA) pressure and central venous pressure (CVP), and to inject the solution used to assess intermittent cardiac output (CO) by thermodilution.

Thermodilution involves injecting a known quantity of solution, either 0.9% normal saline or 5% dextrose in water, at a known temperature (usually room temperature) into the right atrium. This fluid cools the surrounding blood. The temperature of the blood is measured downstream in the pulmonary artery by a thermistor—a thermometer that can measure extremely small changes in temperature—located in the thermistor lumen port approximately 4 cm from the tip of the catheter.

This information is plotted on a time-temperature curve that represents the time it takes for the blood to change from a warmer baseline temperature to a cooler temperature after the injection and then back to the baseline temperature as the blood circulates. When CO is low, it takes longer for the blood temperature to return to baseline; when CO is high, the cooling fluid is carried faster through the heart, and the temperature returns to baseline faster.

The inflation lumen connects to a balloon that's located less than 1 cm from the catheter tip. When this balloon is inflated with a specific amount of air (typically 1.5 ml) during catheter insertion, it helps the PA catheter float into the pulmonary artery and prevents trauma to the cardiac structures as it moves through the heart chambers and valves. It's used to obtain pulmonary artery wedge pressure (PAWP), which is also called pulmonary artery occlusion pressure (PAOP) or pulmonary capillary wedge pressure (PCWP).

The balloon is deflated once the PA catheter is positioned in the pulmonary artery. It is then re-inflated to allow the catheter to float and "wedge" into a smaller branch, or capillary, of the pulmonary artery, where measurement of the PAWP is recorded.4,5 Once lodged in the pulmonary artery, the inflated balloon serves as an artificial valve, creating an unrestricted channel from the catheter tip through the pulmonary vein, left atrium, and open mitral valve into the left ventricle. (See Figure 1.) This allows the distal lumen to indirectly measure left ventricle pressure. Because it can block the flow of blood to lung tissue, the balloon should never remain inflated in the wedged position for more than eight to 15 seconds, or it will act as a pulmonary embolus.4

PA catheters may have other features, including electrodes for cardiac pacing and designs that allow continuous SvO2 monitoring and CO measurement. Some designs permit measurement of right ventricular ejection fraction (RVEF), an indicator of right ventricular (RV) function that's expressed as the percentage of diastolic volume ejected during systolic contraction.

How a PA catheter measures pressures

Hemodynamic pressures can be measured by connecting a PA catheter's distal and proximal ports to a closed, fluid-filled pressure monitoring system. This system is based on the principle that changes in pressure at any point in the unobstructed system result in equal pressure changes at all other points in the system. This system is composed of several connected parts.

IV tubing from a flush bag of fluid (usually 500 ml of 0.9% saline or dextrose 5% in water) is connected to low-compliance, non-compressible tubing that has a pre-attached flush device that's used to "clear," or flush, the system intermittently. The flush bag is slipped into a sleeve that has an attached pressure gauge manometer (similar to a blood pressure cuff with attached manometer).

The sleeve, or pressure bag, is inflated to 300 mm Hg. This setup allows 1 – 3 ml per hour of flush solution to be delivered through the catheter to maintain patency, minimize clot formation, and prevent backflow of blood from the catheter as it sits in the pulmonary artery.4

The low-compliance tubing from the flush system is in turn connected to a pressure transducer—a device that translates one form of energy into another. As the catheter moves through the right heart chambers, pulsating waves created by changes in the systolic and diastolic pressures within the heart are transmitted from the catheter tip through the low-compliance tubing to the diaphragm of this transducer.4,6 These pulsations induce motion on the diaphragm and are converted to low-voltage electric signals that are transmitted via the cables to the bedside monitor.

At the monitor, these signals are amplified, filtered, and displayed as pressure waveforms and numerical pressure readings.5,7 The monitor displays this data on a grid. The horizontal lines are numerically identified on the left to show pressure in mm Hg; the vertical lines denote time in seconds.

These waveforms change as the catheter progresses through the heart during insertion, letting clinicians know exactly where in the heart the catheter is as it's being advanced to the pulmonary artery. (See Figure 2.) It's important to continuously monitor these waveforms to ensure that the catheter doesn't slip back into the right ventricle. A chest radiograph is always used to verify placement.

While the PA catheter remains in place (usually not more than four days per a facility's infection control policy), correct placement demonstrates a PAWP tracing when the balloon is inflated, and a pulmonary artery tracing when the balloon is deflated. EKG data is displayed continuously and simultaneously on the same screen during PA catheter monitoring, as the timing of mechanical and electrical events that occur in the heart are interrelated. Graphic printouts of the simultaneous EKGs superimposed above or below the pressure waveforms are examined together to fine-tune waveform analysis and to determine the precise moment during systole and diastole in which the pressures are clinically relevant.

Ensuring the accuracy of the data

Accurate data is essential to interpreting waveforms and determining appropriate therapy. (To learn more about waveform interpretation, consult the Pulmonary Artery Catheter Education Project— www.pacep.org —an online educational program sponsored by the American Association of Critical-Care Nurses and several other organizations.) Acquiring accurate data hinges not only upon correct placement of the PA catheter but upon proper equipment preparation and transmission of hemodynamic waveforms, which are outlined below.

Equipment preparation. The catheter, tubing, flush system, and transducer must remain free of any air or blood, and all connections need to be tightly secured. There should be no air leaks from the balloon when a test inflation is done right before the catheter is inserted.

You must zero and level the transducer to ensure its reliability. Zeroing and leveling are performed simultaneously during initial catheter setup and insertion. These steps are critical to ensuring that the transmitted pressures are derived purely from inside the closed pressure monitoring system without interference from other sources.

Zeroing eliminates the effects of pressure alterations that occur inside the chest cavity during normal or mechanically assisted inspiration and expiration.5,8 It's similar to establishing a reference point on, for example, a bathroom scale or thermometer. To zero the transducer, first expose the pressurized fluid/flush system (which is attached to the transducer) to atmospheric pressure (air) by turning the stopcock closest to the transducer to the "off' position in the direction of the patient and removing the dead-end cap. Then push and release the zero function button on the monitor and observe the digital reading until it gives a value of "0." The zero reference point needs to be reestablished if the transducer cable becomes disconnected from either the transducer or the monitor, or if the pressure values obtained don't fit the patient's clinical presentation.4

At the same time, the stopcock used for obtaining the zero-reference pressure needs to be aligned, or "leveled," with an external landmark on the patient—called the phlebostatic axis—that marks the location of the right atrium and the PA catheter tip. This point occurs at the mid anterior/posterior line, which is usually the intersection of the midaxillary line and fourth intercostal space with the patient in the supine position. It changes when the patient is in a lateral position, and remains constant with the head of the bed raised from 0 to 45 degrees.

The phlebostatic axis represents the precise anatomical point of origin of the hemodynamic pressures being measured.6,8 To ensure consistency, the point of the phlebostatic axis should be marked on the patient's chest. (See Figure 3.)

Leveling the zero-referenced stopcock accounts for the weight of the fluid (hydrostatic pressure) within the tubing flush system, which is proportional to the height of the fluid column. In order for the system to provide accurate data, the zero-referenced stopcock must remain at the level of the phlebostatic axis. If it's positioned below the phlebostatic axis, readings will be artificially elevated; if it's above this point, the pressures will be falsely low. Leveling needs to be redone whenever the stopcock is moved from the position where the original referencing was done.

All pressures are interpreted at the end-expiration phase of respiration, whether or not the patient is spontaneously breathing or on assisted mechanical ventilation. This is the point in the respiratory cycle when intrathoracic pressure is equal to atmospheric pressure and exerts minimal effect on intracardiac pressures (unless a patient is on a ventilator with positive end-expiratory pressure, which can alter the baseline intrathoracic pressure).4

Waveform transmission. To obtain reliable hemodynamic pressure values, the pressure monitoring system must be able to transmit an accurate pulsating waveform to the transducer without distortion or interference. For example, air bubbles or blood in the tubing or transducer will absorb parts of the waveform and distort its appearance on the monitor.

The system's ability to reproduce the pressure variations within the patient's vasculature is referred to as the dynamic response, which is evaluated by performing a square wave test. This is done by activating and releasing the fast-flush device, which exposes the transducer to the amount of pressure in the flush solution—300 mm Hg. This fast flush interrupts the pulmonary artery waveform observed on the monitor. It should create the appearance of a sharp, upstroke, "squared off" wave, or peak. The square wave should be followed by a rapid downstroke extending below the baseline with one or two oscillations. (See Figure 4.) It should return quickly (no more then 0.04 seconds) to the baseline waveform configuration. This response to a square wave test is referred to as a normal dynamic response, and indicates that the waveforms the system is displaying are accurate.

A slower, slurred dynamic response that does not extend below the baseline or a dynamic response that demonstrates several oscillations before returning to normal indicates that the waveforms you have been observing on the monitor have been inaccurate. In the first case, the waveforms will appear "overdamped"—the amplitude will be lower and the waveform wider. In the latter case, the waveforms will be "underdamped" and will appear higher and narrower.4,5,8 The dynamic response should be tested every eight to 12 hours, when the pressure monitoring system is open to air, or when the accuracy of readings is in question.4 Causes of inaccurate readings include loose tubing connections, air bubbles in the system, and a pressure bag that is not delivering 300 mm Hg.

Complications are rare but potentially fatal

The risks associated with PA catheters have been significantly reduced since the 1980s.3 Although the overall incidence of complications is low, knowing what could go wrong during catheter insertion or use will enable you to anticipate a problem and intervene quickly.

The most common complication associated with insertion is ventricular arrhythmias (isolated or bursts of premature ventricular contractions), created by irritation as the catheter moves through the right ventricle.2 These arrhythmias are usually transient and rarely convert to sustained ventricular tachycardia. Ventricular arrhythmias can also occur if the catheter migrates back into the right ventricle, which is why it's important to monitor the waveforms for catheter position.

Pneumothorax could also occur during catheter insertion. The overall risk is low (about 1% to 2%), but is higher for insertion through the subclavian vein than through the internal jugular vein.2 Catheter knotting can occur either within a vessel or inside the heart and may result if the length of catheter advanced to reach the pulmonary artery and wedge position is excessive. If knotting occurs, the catheter will not perform as anticipated, and the chest X-ray taken to confirm its position should show the knotting or a pneumothorax.

Pulmonary infarction and pulmonary artery rupture are rare but potentially fatal complications while the catheter remains in place.2 The risk of pulmonary infarction increases if the catheter is left in the wedge position for a prolonged period, as this can impede blood flow to the peripheral lung tissue. Thromboemboli from the catheter tip can also lead to infarcted lung tissue. Institutional policy or clinician preference will dictate whether heparin is used in the flush solution; it has not yet been established if this precaution is necessary to maintain catheter patency.4

Pulmonary artery rupture may occur if the PA catheter balloon is overinflated while in the wedged position. While the complication occurs in only an estimated 0.2% of patients, it is associated with a 50% mortality rate.2

The threat of infection increases if the catheter is left in place for more than 72 to 96 hours. Sterile technique is mandatory during catheter insertion, while handling any component of the pressure monitoring system, and during insertion-site dressing changes. It's generally accepted that the hemodynamic flush system can be safely used for 72 hours.4

Institutional policy determines how often the PA catheter dressing should be changed and the site inspected for signs of infection. The Centers for Disease Control and Prevention makes no recommendation as to the frequency of routine PA catheter dressing changes, but suggests it should be done if the dressing becomes damp, loosened, soiled, or when inspection of the site is necessary.9 If infection is suspected or confirmed, the catheter needs to be removed immediately, since this complication can lead to bacteremia and sepsis.2,4

The requirements for assisting in the insertion and management of PA catheters are substantial. Nurses clearly play a pivotal role in ensuring their correct and safe application. Confidence and skill in working with PA catheters will grow with mentoring, continuous education, and of course, practice.

REFERENCES

1. Ott, K., Johnson, K., & Ahrens, T. (2001). New technologies in the assessment of hemodynamic parameters. J Cardiovasc Nurs, 15(2), 41.

2. Cruz, K., & Franklin, C. (2001). The pulmonary artery catheter: Uses and controversies. Crit Care Clin, 17(2), 271.

3. Prentice, D., & Ahrens, T. (2001). Controversies in the use of the pulmonary artery catheter. J Cardiovasc Nurs, 15(2), 1.

4. Lynn-McHale, D. J., & Carlson, K. K. (Eds.). (2001). AACN procedure manual for critical care. Philadelphia: W. B. Saunders.

5. Schell, H. M., & Puntillo, K. A. (Eds.). (2001). Critical care nursing secrets. Philadelphia: Hanley & Belfus.

6. Mayer, B. H., Duksta, C., et al. (2002). Nursing procedures made incredibly easy. Philadelphia: Springhouse.

7. Daily, E. K. (2001). Hemodynamic waveform analysis. J Cardiovasc Nurs, 15(2), 6.

8. Quaal, S. J. (2001). Improving the accuracy of pulmonary artery catheter measurements. J Cardiovasc Nurs, 15(2), 71.

9. Centers for Disease Control and Prevention. "Guidelines for the prevention of intravascular catheter-related infections." MMWR. 2002. www.cdc.gov/mmwr/preview/mmwrhtml/rr5110a1.htm (14 Jan. 2003).



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PA catheter measurements and what they reveal

Parameter Normal range Clinical relevance
Central venous pressure (CVP) 0 – 6 mm Hg Used to determine volume status and right ventricle (RV) function; correlates with right ventricular end-diastolic pressure (RVEDP)
Right ventricular pressure (RVP) 20 – 30/0 – 6 mm Hg Used to determine RV function and volume
Pulmonary artery pressure (PAP) 20 – 30/6 – 10 mm Hg Used to determine state of resistance in pulmonary vasculature and RV function
Pulmonary artery wedge pressure (PAWP) 4 – 12 mm Hg Used to determine left ventricle (LV) function; correlates with left ventricular end-diastolic pressure (LVEDP)
Stroke volume (SV) 60 – 80 ml/beat Amount of blood ejected during systole; decreased SV indicates ventricular dysfunction
Cardiac output (CO)*
(SV x heart rate)
4 – 8 L/min Describes blood flow through tissues; reflects adequacy of overall cardiac function
Stroke volume index (SVI) 33 – 47 ml/beat/m2 SV adjusted for patient’s body surface area (BSA)
Cardiac index (CI) 2.5 – 4 L/min/m2 CO adjusted for patient’s BSA
Pulmonary vascular resistance (PVR) 20 – 120 dynes/sec/cm5 Describes state of resistance in pulmonary vasculature
Systemic vascular resistance (SVR) 770 – 1,500 dynes/sec/cm5 Describes state of resistance in systemic vasculature
Right ventricular stroke work (RVSW) 10 –15 g-m/beat Defines how hard right ventricle is working to pump blood
Left ventricular stroke work (LVSW) 60 – 80 g-m/beat Defines how hard left ventricle is working to pump blood
Mixed venous oxygen saturation (SvO2)* 60% – 75% Index of oxygenation status that measures the relationship between O2 delivery and O2 demand; reflects cardiovascular tissue perfusion

*Some new PA catheter designs feature continuous CO and SVO2 monitoring capabilities.

Sources: 1. Schell, H. M., & Puntillo, K. A. (2001). Critical care nursing secrets. Philadelphia: Hanley & Belfus, Inc. 2. Ahrens, T. (1999). Hemodynamic monitoring. Crit Care Nurs Clin North Am, 11(1), 19.



 

Emil Vernarec, ed. Sally Beattie Dulak. A PA catheter refresher course. RN 2003;4:28.

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