CONTINUOUS MIXED VENOUS OXIMETRY PULMONARY ARTERY CATHETERS

Although the formal Fick cardiac output method is not applied widely in clinical practice, the physiologic relationships described by the Fick equation form the basis for another PAC-based monitoring technique termed continuous mixed venous oximetry. Rearrangement of the Fick equation reveals the four determinants of mixed venous hemoglobin saturation (Sv̄O₂ ):





where Sv̄O₂ = mixed venous hemoglobin saturation (%)
SaO₂ = arterial hemoglobin saturation (%)

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To the extent that arterial hemoglobin saturation, oxygen consumption, and hemoglobin concentration remain stable, mixed venous hemoglobin saturation may be used as an indirect indicator of cardiac output. For example, when cardiac output falls, tissue oxygen extraction increases and the mixed venous blood will become more desaturated. However, as noted in this equation, mixed venous hemoglobin saturation also varies directly with arterial hemoglobin saturation and hemoglobin concentration and varies inversely with oxygen consumption. When any of these other variables change significantly, one cannot assume that a change in mixed venous hemoglobin saturation results solely from a change in cardiac output. Although these considerations may confound the use of mixed venous hemoglobin saturation as an indicator of cardiac output, monitoring this variable provides more comprehensive information about the balance of oxygen delivery and consumption by the body—not just cardiac output, but also the adequacy of cardiac output.^[675]

Although mixed venous hemoglobin saturation may be determined by intermittent blood sampling from the pulmonary artery, a specially designed PAC can provide this information reliably and continuously. Fiberoptic bundles incorporated into the PAC determine the hemoglobin saturation in pulmonary artery blood based on the principles of reflectance oximetry and the use of either a two- or three-wavelength system. A special computer connected to this PAC displays mixed venous hemoglobin saturation continuously. In addition, these continuous mixed venous oximetry PACs allow standard thermodilution cardiac output measurements or are combined with warm thermal CCO capability, thereby providing both continual cardiac output and venous oximetry data.

Technical problems with continuous mixed venous oximetry are generally limited to improper positioning of the PAC tip against the vessel wall or inaccurate calibration.^{[671] [675]} Multi-wavelength fiberoptic technology and reflection intensity algorithms help reduce wall artifacts caused by spurious reflections from a PAC thrombus or the pulmonary arterial walls. These catheters are calibrated at the bedside before use but may also be calibrated in vivo from a pulmonary artery blood gas sample if the mixed venous saturation values are questionable.^[685]

Most clinical trials comparing PAC-derived continuous venous oximetry values with laboratory analysis of pulmonary artery blood samples have show good agreement with little bias between techniques.^[671] Despite the apparent accuracy of this technique, there are significant questions about the clinical value of this additional monitoring. Although normal values of mixed venous hemoglobin saturation are well known, the appropriate therapeutic target value for any individual patient remains uncertain.^[671] Clearly, low values of mixed venous hemoglobin saturation indicate an overall inadequacy of oxygen delivery relative to total-body oxygen consumption. However, many critically ill patients with normal or high values for mixed venous hemoglobin saturation have regionally inadequate blood flow and tissue oxygen delivery that is not detected by global measurements such as mixed venous oximetry.

The continuous mixed venous oximetry PAC may be used to measure thermodilution cardiac output and arterial and mixed venous oxygen content values and thereby calculate oxygen consumption by rearrangement of the Fick equation. However, values of oxygen consumption derived from these measurements generally underestimate direct oxygen consumption measured by mass spectrometry, metabolic cart, or water-sealed spirometry.^[581] Oxygen delivery may also be calculated from these PAC-derived values.

ḊO₂ = · CaO₂ (5)

where ḊO₂ = oxygen delivery
= cardiac output (dL/min)
CaO₂ = oxygen content of arterial blood (mL O₂ /100 mL blood)

Many physicians have used these derived measures of oxygen consumption and oxygen delivery to guide treatment of critically ill patients. Unfortunately, as noted in the earlier discussions of pulmonary artery catheterization, this "goal-oriented therapy" has not been shown to result in improved patient outcomes in most instances. Few clinical trials have directly assessed the value of oximetry PAC monitoring versus standard PAC monitoring. A small 226-patient trial comparing CVP, PAC, and oximetry PAC monitoring for cardiac surgery failed to show any benefits for the oximetry PAC and noted increased costs associated with this technique.^[549] More recently, in a large Veterans Affairs observational trial of 3265 cardiac surgical patients, 49% of the patients received continuous mixed venous oximetry PACs, and use of this catheter was associated with increased cost but no better outcome than observed in the standard PAC group.^[686]

In summary, continuous mixed venous oximetry remains an appealing, widely used monitoring technology because of the ability of this technique to provide continuous information about the global oxygen supply-and-demand balance in the body. These catheters are more expensive than standard PACs, and clinical studies to date have not yet defined appropriate clinical indications for their use.^{[559] [671] [675] [686]}