Measurements Obtained from a PAC

Values obtained from a pulmonary artery catheter (PAC) are crucial when deciding how to treat ICU patients. There are some important limitations that should be considered before the measured PAC values are used. One should know that pressure readings fluctuate during the respiratory cycle. Therefore, measurements should be taken at the end of expiration, this is when pleural pressure is closest to atmospheric pressure. Next, for a pressure change to be considered significant, it must be at least a 4-mm Hg change. Caution should be used to see that the pulmonary capillary wedge pressure (PCWP) never exceeds the pulmonary artery diastolic pressure. If this occurs, either the balloon is overinflated and should be deflated immediately to prevent artery rupture, or excessive alveolar pressures are effecting the reading. Also, if the patient's pulse is over 120 beats per minute the pulmonary artery pressure can be surreptitiously elevated.

There are four basic measurements that are obtained from the PAC. These measurements are represented by the four "pig-tail ports" that are on the non-patient end of the catheter. Each "pigtail" when used correctly will measure one specific piece of monitoring information-- pulmonary artery pressure, right atrial pressure, pulmonary capillary wedge pressure, cardiac output.

The first of the 4 'pigtail's' is known as the distal lumen. Attaching a transducer/ocilloscope system to this catheter "pigtail" measures the pulmonary artery pressure (PAP). This is the anatomic site in which the tip of the PAC terminates. The PAP is the pressure of the blood within the pulmonary vasculature. This pressure also represents right ventricular afterload. The second 'pigtail' is connected to the right atrial port. When this 'pigtail' is connected to a transducer/ocilloscope system, it measures the right atrial pressure (RAP). The RAP is the body's central venous pressure. The terminal end of the venous circuit is the right atrium. The RAP is the preload pressure for the right heart. The third 'pigtail' is the pulmonary capillary wedge pressure (PCWP). This pressure is obtained when the balloon located at the catheter's end is inflated with air. The inflated balloon causes catheter wedging downstream in the pulmonary artery. This static pressure reflects the back pressure from the pulmonary veins which is equivalent to the pressure in the left atria. The PCWP is an indirect reading of the left atrial pressure and during ventricular diastole is equivalent to the left ventricular end-diastolic filling pressure. The PCWP is the preload pressure for the left heart. The fourth 'pigtail' is a thermistor used to determine cardiac output (CO). Before this measure can be made the thermistor must be attached to a CO computer. As a part of the procedure an 5-10 ml of IV solution at a known temperature is injected into the right atrial port of the PAC. The thermistor measures the temperature change of the blood column when it reaches the thermistor wires downstream in the in the pulmonary artery. The changing wire resistance enables the computer to calculate a temperature/time curve which indicates the patient's CO. This patient has a PAC that has more monitoring ability than the standard catheter. This catheter also has an additional "pigtail" for measuring the S_vO₂. This S_vO₂ catheter lumen has fiberoptic threads that allow determination of the oxygen saturation of the blood within the pulmonary artery. This value varies with changes in the patient's oxygen delivery and/or oxygen consumption.

Derived Parameters from a PAC

From these measurements there are several parameters that can be calculated (derived) from the measured variables of the PAC and the arterial line.

This patient had several abnormal hemodyamic measurements from the PAC -- the RA pressure, Mean PAP, and PCWP. The patient's post-operative RA pressures were increased and ranged from 10 to 15 mmHg (normal levels 1-6 mmHg). The RA pressure is elevated in conditions that increase right ventricular preload, or right ventricular afterload or decrease right ventricular function. The mean PAP is also elevated at 25 mmHg (normal 10-20 mmHg). The patient's PAP ranged from 21-35 through his clinical course. Conditions that cause the PAP to increase are decreased left ventricular function, increased pulmonary blood flow, or increased PVR. The patient's PCWP is also elevated at 25 with a range of 14-25 mmHg (normal 4-12 mmHg). The PCWP will elevate in cases of excessive circulating volume and/or decreased left ventricular function. The cause of this patient's hemodynamic abnormalities is secondary to left ventricular failure. His heart failure is complicated by a myocardial infarction that also affected ventricular function. He developed cardiogenic pulmonary edema post-operatively and this is consistent with high PCWP and increased PAP. This patient's right ventricular function was also affected by the left ventricular failure as evidenced by his elevated RA pressure.

Common Complications of a PAC

There are complications associated with the PAC. One complication commonly mentioned is pneumothorax. When using the subclavian vein as the site for inserting a PAC, there is a higher risk of lacerating the right apex of the lung. A chest x-ray is recommended after PAC insertion to check for pneumothorax as well as correct placement of the catheter. Other complications of central lines are infection, phlebitis, thrombosis, catheter malposition, arrhythmias, and electrical microshock. Because of the complication rate with central lines, it is recommended that invasive lines be removed when they are no longer needed.

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