PULMONARY ARTERY CATHETERIZATION
The pulmonary artery (PA) catheter (or Swan-Ganz catheter) was introduced into routine prac-tice in operating rooms and intensive care units in the 1970s. It quickly became common for patients undergoing major surgery to be managed with PA catheterization. The catheter provides measure-ments of both CO and PA occlusion pressures and was used to guide hemodynamic therapy, especially when patients became unstable. Determination of the PA occlusion or wedge pressure permitted (in the absence of mitral stenosis) an estimation of the left ventricular end-diastolic pressure (LVEDP), and, depending upon ventricular compliance, an estimate of ventricular volume. Through its ability to perform measurements of CO, the patient’s stroke volume (SV) was also determined.
CO = SV × HR
SV = CO/HR
Blood pressure = CO × systemic vascularresistance (SVR)
Consequently, hemodynamic monitoring with the PA catheter attempted to discern why a patient was unstable so that therapy could be directed at the underlying problem.
If the SVR is diminished, such as in states of vasodilatory shock (sepsis), the SV may increase. Conversely, a reduction in SV may be second-ary to poor cardiac performance or hypovolemia. Determination of the “wedge” or pulmonary cap-illary occlusion pressure (PCOP) by inflating the catheter balloon estimates the LVEDP. A decreased SV in the setting of a low PCOP/ LVEDP indicates hypovolemia and the need for volume adminis-tration. A “full” heart, reflected by a high PCOP/ LVEDP and low SV, indicates the need for a positive inotropic drug. Conversely, a normal or increased SV in the setting of hypotension could be treated with the administration of vasoconstrictor drugs to restore SVR in a vasodilated patient.
Although patients can present concurrently with hypovolemia, sepsis, and heart failure, this basic treatment approach and the use of thePA catheter to guide therapy became more or less synonymous with perioperative intensive care and cardiac anesthesia. However, numerous large obser-vational studies have shown that patients managed with PA catheters had worse outcomes than similar patients who were managed without PA catheters. Other studies seem to indicate that although PA catheter-guided patient management may do no harm, it offers no specific benefits. Although the PA catheter can be used to guide goal-directed hemodynamic therapy to ensure organ per-fusion in shock states, other less invasive methods to determine hemodynamic performance are available, including transpulmonary thermodilution CO mea-surements and pulse contour analyses of the arterial pressure waveform. Both methods permit calcula-tion of the SV as a guide for hemodynamic manage-ment. Moreover, right atrial blood oxygen saturation, as opposed to mixed venous saturation (normal is 75%), can be used as an alternative measure to dis-cern tissue oxygen extraction and the adequacy of tissue oxygen delivery.
Despite numerous reports of its questionable utility and the increasing number of alternative methods to determine hemodynamic parameters, the PA catheter is still employed perioperatively more often in the United States than elsewhere. Although echocardiography can readily inform hemodynamic decision-making through imag-ing the heart to determine if it is full, compressed, contracting, or empty echocardiography requires a trained individual to obtain and interpret the images. Alternative less invasive hemodynamic monitors have gained acceptance in Europe and may expand in the United States, further decreasing the use of PA catheters.
Until other alternatives are available, PA cath-eterization should be considered whenever cardiac index, preload, volume status, or the degree of mixed venous blood oxygenation need to be known. These measurements might prove particularly important in surgical patients at high risk for hemodynamic instability (eg, those who recently sustained myo-cardial infarction) or during surgical procedures associated with an increased incidence of hemody-namic complications (eg, thoracic aortic aneurysm repair).
Relative contraindications to pulmonary artery catheterization include left bundle-branch block(because of the concern about complete heart block) and conditions associated with a greatly increased risk of arrhythmias, such as Wolff–Parkinson–White syndrome. A catheter with pacing capability is better suited to these situations. A PA catheter may serve as a nidus of infection in bacteremic patients or throm-bus formation in patients prone to hypercoagulation.
Although various PA catheters are available, the most popular design integrates five lumens into a 7.5 FR catheter, 110-cm long, with a polyvinylchlo-ride body (Figure 5–20). The lumens house the
following: wiring to connect the thermistor near the catheter tip to a thermodilution CO computer; an air channel for inflation of the balloon; a proximal port 30 cm from the tip for infusions, CO injections, and measurements of right atrial pressures; a ventricular port at 20 cm for infusion of drugs; and a distal port for aspiration of mixed venous blood samples and measurements of PA pressure.
Insertion of a PA catheter requires central venous access, which can be accomplished using Seldinger’s technique, described above. Instead of a central venous catheter, a dilator and sheath are threaded over the guidewire. The sheath’s lumen accommodates the PA catheter after removal of the dilator and guidewire ( Figure 5–21).
Prior to insertion, the PA catheter is checked by inflating and deflating its balloon and irrigating all three intravascular lumens with saline. The distal port is connected to a transducer that is zeroed to the patient’s midaxillary line.
The PA catheter is advanced through the intro-ducer and into the internal jugular vein. At approxi-mately 15 cm, the distal tip should enter the right atrium, and a central venous tracing that varies with respiration confirms an intrathoracic position. The balloon is then inflated with air according to the manufacturer’s recommendations (usually 1.5 mL) to protect the endocardium from the catheter tip and to allow the right ventricle’s CO to direct the catheter forward. The balloon is always deflated during withdrawal. During the catheter’s advance-ment, the ECG should be monitored for arrhyth-mias. Transient ectopy from irritation of the right ventricle by the balloon and catheter tip is common and rarely requires treatment. A sudden increase in the systolic pressure on the distal tracing indi-cates a right ventricular location of the catheter tip (Figure 5–22). Entry into the pulmonary artery nor-mally occurs by 35–45 cm and is heralded by a sud-den increase in diastolic pressure.
To prevent catheter knotting, the balloon should be deflated and the catheter withdrawn if pressure changes do not occur at the expected distances. In particularly difficult cases (low CO, pulmo-nary hypertension, or congenital heart anomalies),flotation of the catheter may be enhanced by having the patient inhale deeply; positioning the patient in head-up, right lateral tilt position; injecting iced saline through the proximal lumen to stiffen the cath-eter (which also increases the risk of perforation); or administering a small dose of an inotropic agent to increase CO. Occasionally, the insertion may require fluoroscopy or TEE for guidance.
After attaining a PA position, minimal PA cath-eter advancement results in a pulmonary artery occlusion pressure (PAOP) waveform. The PA trac-ing should reappear when the balloon is deflated. Wedging before maximal balloon inflation signals an overwedged position, and the catheter should be slightly withdrawn (with the balloon down, of course). Because PA rupture may cause mortality and can occur because of balloon overinflation, the frequency of wedge readings should be minimized. PA pressure should be continuously moni-tored to detect an overwedged position indicative of catheter migration. Furthermore, if the catheter has a right ventricular port 20 cm from the tip, distal migration can often be detected by a change in the pressure tracing that indicates a pul-monary artery location.Correct catheter position can be confirmed by a chest radiograph.
The numerous complications of PA catheter-ization include all complications associated with central venous cannulation plus bacteremia, endo-carditis, thrombogenesis, pulmonary infarction, PA rupture, and hemorrhage (particularly in patients taking anticoagulants, elderly or female patients, or patients with pulmonary hypertension), catheter knotting, arrhythmias, conduction abnormalities, and pulmonary valvular damage (Table 5–2). Even trace hemoptysis should not be ignored, as it may herald PA rupture. If the latter is suspected, prompt placement of a double-lumen tracheal tube may maintain adequate oxygenation by the unaffected lung. The risk of complications increases with the duration of catheterization, which usually should not exceed 72 hr.
The introduction of PA catheters into the operat-ing room revolutionized the intraoperative man-agement of critically ill patients. PA catheters allow more precise estimation of left ventricular preload than either CVP or physical examination, as well as the sampling of mixed venous blood. Catheters with self-contained thermistors
can be used to measure CO, from which a multitude of hemodynamic values can be derived (Table 5–3). Some catheter designs incorporate electrodes that allow intracavitary ECG recording and pacing. Optional fiberoptic bundles allow con-tinuous measurement of the oxygen saturation of mixed venous blood.
Starling demonstrated the relationship between left ventricular function and left ventricular end-diastolic muscle fiber length, which is usually proportionate to end-diastolic volume. If compli-ance is not abnormally decreased (eg, by myocar-dial ischemia, overload, ventricular hypertrophy, or pericardial tamponade), LVEDP should reflect fiber length. In the presence of a normal mitral valve, left atrial pressure approaches left ventricu-lar pressure during diastolic filling. The left atrium connects with the right side of the heart through the pulmonary vasculature. The distal lumen of a correctly wedged PA catheter is isolated from right-sided pressures by balloon inflation. Its distal open-ing is exposed only to capillary pressure, which—in the absence of high airway pressures or pulmonary vascular disease—equals left atrial pressure. In fact, aspiration through the distal port during balloon inflation samples arterialized blood. PAOP is an indirect measure of LVEDP which depending upon ventricular compliance approximates left ventricu-lar end diastolic volume.
Whereas central venous pressure may reflect right ventricular function, a PA catheter may be indicated if either ventricle is markedly depressed, causing disassociation of right- and left-sided hemodynamics. CVP is poorly predictive of pul-monary capillary pressures, especially in patients with abnormal left-ventricular function. Even the PAOP does not always predict LVEDP. The relation-ship between left ventricular end-diastolic volume (actual preload) and PAOP (estimated preload) can become unreliable during conditions associated with changing left atrial or ventricular compliance, mitral valve function, or pulmonary vein resistance. These conditions are common immediately follow-ing major cardiac or vascular surgery and in criti-cally ill patients who are on inotropic agents or in septic shock.
Ultimately, the information provided by the PA catheter is like that from any perioperative moni-tor dependent upon its correct interpretation by the patient’s care givers. In this context, the PA catheter is a tool to assist in goal-directed perioperative ther-apy. Given the increasing number of less invasive methods now available to obtain similar informa-tion, we suspect that PA catheterization will become mostly of historic interest.