Chapter 8, cont… (Page 2)

Pulmonary circulation

SWAN­GANZ CATHETERIZATION

The first cardiac catheterization is now legendary. Working alone in 1929, a German physician named Werner Forssmann threaded a urologic catheter through a vein in his arm and into his heart. This procedure was done clandestinely since he did not have official approval for such a daring experiment. Dr. Forssmann took a chest x­ray, which demonstrated the catheter's position, and he published the procedure as a brief report. Cardiac catheterization did not become a clinically useful test, however, until the late 1940's, following the work of Dr. Dickinson W. Richards and Dr. Andre Cournand. For cardiac catheterization's revolutionary effect on cardiac diagnosis, all three physicians shared the 1956 Nobel Prize for Medicine.

Fig. 8­4. The quadruple channel Swan­Ganz catheter. The most distal channel (distal injection port) is for pulmonary artery pressure measurement; blood can also be aspirated from this channel for mixed venous oxygen measurements. A second channel (balloon inflation valve) is used to inflate/deflate the distal balloon. A third channel (proximal injection port), which exits 30 cm from the catheter tip, is used for central venous (right atrial) pressure monitoring and fluid infusion. The fourth channel (extra injection port), which is not present on all catheters, can be used for continuous infusion of hyperalimentation fluids. The thermistor connector plugs into a bedside cardiac output computer.

In 1970 Swan, Ganz, Forrester, et al. published their now famous study of a special flow­directed, balloon­tipped catheter. The Swan­Ganz catheter made bedside catheterization of the right side of the heart a feasible procedure.^* The catheter was originally introduced to assess patients suffering from acute myocardial infarction. Subsequently it has been used to assess patients who have a wide variety of hemodynamic and fluid problems. A general approach is to use the catheter in unstable patients when, by noninvasive means, the hemodynamic or fluid status is uncertain. The catheter provides measurements of right­sided heart pressure, pulmonary artery pressure, pulmonary artery wedge pressure, and cardiac output. An unstable patient's hemodynamic status cannot be reliably assessed clinically, so bedside catheterization is an important procedure in critical care units.

The Swan­Ganz catheter is shown in Fig. 8­4. The catheter is basically a thin, flexible tube with an inflatable rubber balloon surrounding the distal end. The balloon is inflated by injecting a maximum of 1.5 cc air through a part on the proximal end. The catheter's position within the pulmonary artery, with the balloon inflated, is shown in Fig. 8­5. In the center of the catheter runs a channel through which fluids can be infused or blood aspirated. One or more additional channels in the catheter can be used to administer fluids.^**

Fig. 8­5. Swan­Ganz catheter in wedge position. The heart, shown here separated into its right and left chambers, is in diastole. The Swan­Ganz catheter, with its balloon inflated, is gradually floated into the wedge position (dotted circle), occluding a pulmonary artery.

In the preceding chapters, interaction between the heart and the lungs has not been a point of emphasis. However, anyone managing patients with the aid of the Swan­Ganz catheter can appreciate the heart­lung interaction. Often, changing one parameter to provide improved cardiac function has a profound effect on gas exchange, for better or worse.

Swan­Ganz catheterization is a sophisticated technique that requires an experienced person to insert the catheter and to wisely interpret and use the hemodynamic data obtained. A brief summary follows of the indications for use of the Swan­Ganz catheter, the catheter's route of insertion, and the measurements obtained from the catheterization.

Indications

In general terms, Swan­Ganz catheterization should be used when a reasonably accurate hemodynamic assessment cannot be made noninvasively. Specifically, the catheter should be use, when knowing any of the following values ma affect therapy and benefit the patient: cardiac out put; pulmonary artery pressures; pulmonary artery wedge pressure; mixed venous partial pressure o oxygen (PvO2), or mixed venous oxygen saturation (SvO2). These measurements may be obtained one only or, as is more common, over a period of hours to days (hemodynamic monitoring). Catheters equipped with a special fiberoptic bundle can continuously monitor pulmonary artery oxygen saturation.

Route of insertion

Any large vein can be used for insertion of the Swan­Ganz catheter. The most commonly used veins are the internal jugular, the subclavian, the external jugular, and the brachial. Catheterization is performed under sterile conditions (the physician wears a sterile gown and sterile gloves) at the patient's bedside.

Measurements

To obtain pressure measurements, the catheter is connected to a transducer, which is calibrated by using a mercury manometer. The transducer electronically converts the vascular pressure readings so that they can be displayed digitally or graphically on chart paper. All pressures are recorded in mm Hg.

Cardiac output is measured by using the thermodilution technique. Ten milliliters of saline are rapidly injected through the catheter's central venous pressure port, which exits in the right atrium. The saline, which can be either room temperature or iced to 0° C, changes the temperature of the blood in the pulmonary artery. The change in temperature is sensed by a thermistor at the distal end of the catheter (Fig. 8­4), which is located in the pulmonary artery. This thermistor is connected, through a port on the proximal end, to a small computer that sits at the patient's bedside. Almost immediately after the saline is injected the computer calculates cardiac output based on the rapidity of the temperature change in the pulmonary artery.

Mixed venous oxygen measurement is performed on a pulmonary artery blood sample that is obtained from the most distal port of the catheter; this sample should be obtained only when the catheter is in the nonwedged position. If the catheter is equipped with a special fiberoptic sensor, the SvO2 can be continuously monitored.

INSERTION OF SWAN­GANZ CATHETER­­PRESSURE TRACINGS

The flow­directed catheter requires careful bedside monitoring of vascular pressure tracings during its insertion. The pressure tracings are used to determine the catheter's position. In practice, these pressure tracings are continuously monitored on an oscilloscope as the catheter is advanced; in addition, the ECG is continuously observed for any disturbance of cardiac rhythm.

A guide to catheter insertion follows, with reference to the pressure tracings at each point of insertion as shown in Fig. 8­6. This is not meant to be a specific guide to catheter insertion. The catheter manufacturer's instructions should always be observed, and the catheter should only be inserted, or the insertion should be directly supervised, by an experienced physician. Distances refer to the position of the catheter as shown in Fig. 8­6. Each distance is from the point of insertion. assuming that the internal jugular vein is the portal of entry and that the patient is an average­sized adult. The use of other venous access routes results in different catheter distances to the right atrium. the right ventricle, and the wedge position.

Superior vena cava. The first pressure reading obtained, approximately 5 to 10 cm from the point of insertion, is the central venous pressure (CVP). Normal CVP is less than 10 mm Hg. The operator continues to advance the catheter into the right atrium .

Right atrium. The right atrium is approximately 15 cm from the point of insertion. Right atrial pressure is normally less than 10 mm Hg, and the pressure tracing also represents the CVP. At this point the balloon tip is safely inflated with 1.5 cc of air. As the catheter is gently advanced, the balloon tip "floats'' across the tricuspid valve into the right ventricle.

Fig. 8­6. A, Swan­Ganz catheter position in heart. B, As monitored by pressure tracings. (RA, pressure tracing from right atrium; RV, pressure tracing from right ventricle: PA, pressure tracing from pulmonary artery; PAWP, pulmonary artery wedge pressure tracing.)

Right ventricle. The right ventricle is approximately 25 cm from the point of insertion. The RV pressure tracing is distinctive, marked by a systolic pressure of approximately 25 mm Hg and an enddiastolic pressure close to zero. From this point the catheter is continually advanced. (If more than 15 cm of catheter is advanced and if it remains within the right ventricle, the catheter is coiling or knotting upon itself; the balloon should be deflated, and the catheter should be pulled back to the right atrium and advanced again.) As the catheter crosses the pulmonic valve, another distinctive pressure tracing heralds its entry into the main pulmonary artery.

Main pulmonary artery. The main pulmonary artery (PA) is approximately 35 cm from the point of insertion. Normal PA pressure is approximately 24/10 mm Hg, with a PA diastolic pressure distinctly higher than the right ventricular end­diastolic pressure. From this point the catheter is continually advanced until it wedges in one of the pulmonary artery branches. Pulmonary artery branches. The pulmonary artery branches are approximately 40 cm from the point of insertion. The wedge pressure tracing is also distinctive and is marked by an overall flattening when compared to the pulmonary artery tracing. The "hills" and "valleys" of the normal wedge pressure tracing reflect left atrial diastole and systole. The actual wedge pressure reading is usually taken as the mean of this pressure curve and ranges from 6 to 12 mm Hg. At this point a chest x­ray is obtained to check for catheter position and to make sure no complication occurred during the insertion, such as knotting of the catheter or pneumothorax. Fig. 8­7 shows a chest x­ray with the Swan­Ganz catheter in proper position.

Fig. 8­7. Chest x­ray showing Swan­Ganz catheter in the patient's left pulmonary artery. A ballpoint pen is pointing to the catheter.

HEMODYNAMIC MEASUREMENTS AND CALCULATIONS

The various measurements that are obtained from Swan­Ganz catheterization are used to calculate several derived values. Table 8­3 lists the usual measurements obtained by this technique. plus those measurements available from peripheral arterial cannulation. Table 8­4 lists the most common values that are calculated from the basic measurements.

Table 8­ 4. Common values calculated from hemodynamic measurements
Value	Formula*	Normal range
Cardiac index		2.8 to 4.2 L/min/m2
Stroke volume		50 to 80 ml/beat
Stroke index (SI)		30 to 65 ml/beat/m2
Left ventricular stroke work index		43 to 61 gm­meters/m2
Right ventricular stroke work index		7 to 12 gm­meters/m2
Systemic vascular resistance†		11 to 18 mm Hg/L/min
Pulmonary vascular resistance		1.5 to 3.0 mm Hg/L/min
% Shunt		<5%
Oxygen uptake		150 to 300 ml 02/min
* MSAP, mean systemic arterial pressure; CVP, central venous pressure; MPAP, mean pulmonary artery pressure; PAWP, pulmonary artery wedge pressure (mean); m2, square meters of body surface area; QT, cardiac output; CcO2, CaO2. and CvO2, oxygen content in end­capillary blood, arterial blood, and mixed venous blood, respectively. † In many textbooks, the resistance formula is multiplied by a conversion factor of 80 to obtain resistance units of dynes x sec x cm­5 Either formula is correct. When dynes x sec x cm­5 is used the normal range for systemic vascular resistance (SVR) is approximately 880 to 1440 and for pulmonary vascular resistance (PVR) is approximately 150 to 240.

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