Pulmonary artery catheter

The idea for a sail or balloon tip modification of Ronald Bradley's simple portex tubing method came about from Swan's observation from the Laguna Beach CA shore of sail boats on the water on a relatively calm day. Boats with conventional slot sails were still; one with a spinnaker was able to make reasonable headway. The concept of using thermodilution to measure cardiac output was originally the idea of Arnost Fronek.[3] As a former colleague of Fronek, Ganz added the thermistor modification after Swan showed him the initial balloon design, which was fabricated by Edwards Laboratories, which had previously contracted with Swan as a consultant.

After Swan developed the initial balloon tip, Ganz used Fronek's idea and added a small thermistor (temperature probe) about 3 cm behind the tip. Either cold 10 ml of saline (0.9% NaCl) under 10° Celsius or room temperature (not as accurate) is injected into an opening in the right atrium. As this cooler fluid passes the tip thermistor, a very brief drop in the blood temperature is recorded. A recent variation in design is the incorporation of a heating coil on the catheter (30 cm from the tip, residing in the atrium area) which eliminates the cold fluid bolus, a major factor in human technique variation.

By attaching both the injector site and the ventricular thermistor to a small computer, the thermodilution curve can be plotted. If details about the patient's body mass index (size); core temp, Systolic, diastolic, central venous pressure CVP (measured from the atrium by the third lumen simultaneously) and pulmonary artery pressure are input, a comprehensive flow vs pressure map can be calculated.

In crude terms, this measurement compares left and right cardiac activity and calculates preload and afterload flow and pressures which, theoretically, can be stabilized or adjusted with drugs to either constrict or dilate the vessels (to raise or lower, respectively, the pressure of blood flowing to the lungs), in order to maximize oxygen for delivery to the body tissues.

The ability to record results is not a guarantee of patient survivability.

Modern catheters have multiple lumina — five or six are common — and have openings along the length to allow administration of inotropes and other drugs directly into the atrium. Drugs to achieve these changes can be delivered into the atrium via the fourth lumen, usually dedicated to medication. Common drugs used are various inotropes, norepinephrine or even atropine. A further set of calculations can be made by measuring the arterial blood and central venous (from the third lumen) and inputting these figures into a spreadsheet or the cardiac output computer, if so equipped, and plotting an oxygen delivery profile.

One further development in recent years has been the invention of a catheter with a fiber-optic based probe which is extended and lodged into the ventricle wall providing instant readings of SvO2 or oxygen saturation of the ventricle tissues. This technique has a finite life as the sensor becomes coated with protein and it can irritate the ventricle via the contact area.

Various other techniques have largely relegated the PA catheter to history, e.g. the lithium dilution technique; the external bio-resistance monitor or the very simple and reliable technique of esophogeal doppler measurements of the descending aorta.

Several studies in the 1980s seemed to show a benefit of the increase in physiological information. Many reports showing benefit of the PA catheter are from anaesthetic, and Intensive Care Unit (ICU) settings. In these settings cardiovascular performance was optimized thinking patients would have supra-normal metabolic requirements. In 2005, a multi-center randomized controlled trial found no difference in mortality or length of stay in ICU patients who received pulmonary artery catheters, though it did find a 10% incidence of complications related to the procedure.[5]

Contrary to earlier studies there is growing evidence the use of a PA catheter (PAC) does not necessarily lead to improved outcome.[6] One explanation could be that nurses and physicians are insufficiently knowledgeable to adequately interpret the PA catheter measurements. Also, the benefits might be reduced by the complications from the use of the PAC. Furthermore, using information from the PAC might result in a more aggressive therapy causing the detrimental effect. Or, it could give rise to more harmful therapies (i.e. achieving supra-normal values could be associated with increased mortality).

This interpretation of Adolph Ficks' formulation for cardiac output by time/temperature curves is an expedient but limited and invasive model of right heart performance. It remains an exceptional method of monitoring volume overload leading to pulmonary edema in an ICU setting.

A feature of the pulmonary artery catheter that has been largely ignored in the clinical setting is its ability to monitor total body oxygen extraction by measuring the mixed venous oxygen saturation. Regardless of the value obtained by measurements of the cardiac output, the mixed venous oxygen saturation is an accurate parameter of total body blood flow and therefore cardiac output. The assumption that a low mixed venous oxygen saturation (normal = 60% except for the coronary sinus where it approximates 40% reflecting the high metabolic rate of the myocardium) represents less than adequate oxygen delivery is consistent with physiological and metabolic observations.[7] High oxygen extraction is associated with low cardiac output and decreased mixed venous oxygen saturation. Except during hypothermia and in severe sepsis, low mixed venous oxygen saturations are indication of inadequate hemodynamics. The ability of the pulmonary artery catheter to sample mixed venous blood is of great utility to manage low cardiac output states.

Non-invasive echocardiography and pulse-wave cardiac output monitoring are concordant with (and much safer) if not better than invasive methods defining right and left heart performance. The advent of MRSA and similar hospital based catheter infections now clearly limits the utility of this type of invasive cardiac ICU intervention.