Pulmonary artery occlusion pressure (PAOP): With the balloon inflated, pressure at the tip of the catheter reflects the static back pressure of the pulmonary veins. The balloon must not remain inflated for > 30 sec to prevent pulmonary infarction. Normally, PAOP approximates left atrial pressure, which in turn approximates left ventricular end-diastolic pressure (LVEDP), which itself reflects left ventricular end-diastolic volume (LVEDV). The LVEDV represents preload, which is the actual target parameter. Many factors cause PAOP to reflect LVEDV inaccurately. These factors include mitral stenosis, high levels of positive end-expiratory pressure (> 10 cm H₂O), and changes in left ventricular compliance (eg, due to MI, pericardial effusion, or increased afterload). Technical difficulties result from excessive balloon inflation, improper catheter position, alveolar pressure exceeding pulmonary venous pressure, or severe pulmonary hypertension (which may make the balloon difficult to wedge).

Elevated PAOP occurs in left-sided heart failure. Decreased PAOP occurs in hypovolemia or decreased preload.

Mixed venous oxygenation: Mixed venous blood comprises blood from the superior and inferior vena cava that has passed through the right heart to the pulmonary artery. The blood may be sampled from the distal port of the PAC, but some catheters have embedded fiberoptic sensors that directly measure O₂ saturation. Causes of low mixed venous O₂ content (SmvO₂) include anemia, pulmonary disease, carboxyhemoglobin, low cardiac output, and increased tissue metabolic needs. The ratio of SaO₂ to (SaO₂ minus SmvO₂) determines the adequacy of O₂ delivery. The ideal ratio is 4:1, whereas 2:1 is the minimum acceptable ratio to maintain aerobic metabolic needs.

Cardiac output: Cardiac output (CO) is measured either by intermittent bolus injection of ice water or, in new catheters, continuous warm thermodilution. The cardiac index divides the CO by body surface area to correct for patient size (see Table 3)

Table 3

Normal Values for Cardiac Index and Related Measurements Measurement Units ± SD O2 uptake 143 ± 14.3 mL/min/m2 Arteriovenous O2 difference 4.1 ± 0.6 dL Cardiac index 3.5 ± 0.7 L/min/m2 Stroke index 46 ± 8.1 mL/beat/m2 Total systemic resistance 1130 ± 178 dynes-sec-cm−5 Total pulmonary resistance 205 ± 51 dynes-sec-cm−5 Pulmonary arteriolar resistance 67 ± 23 dynes-sec-cm−5 SD = standard deviation. Adapted from Barratt-Boyes BG, Wood EH: Cardiac output and related measurements and pressure values in the right heart and associated vessels, together with an analysis of the hemodynamic response to the inhalation of high oxygen mixtures in healthy subjects. Journal of Laboratory and Clinical Medicine 51:72–90, 1958.

Other variables can be calculated from the CO. These include systemic and pulmonary vascular resistance and right and left ventricular stroke work (RVSW, LVSW).

Complications and precautions: PACs may be difficult to insert. Cardiac arrhythmias are the most common complication. Pulmonary infarction secondary to overinflated or permanently wedged balloons, pulmonary artery perforation, intracardiac perforation, valvular injury, and endocarditis may occur. Rarely, the catheter may curl into a knot within the right ventricle (especially in patients with heart failure, cardiomyopathy, or increased pulmonary pressure).

Pulmonary artery rupture occurs in < 0.1% of PAC insertions. This catastrophic complication is often fatal and occurs immediately upon wedging the catheter—either initially or on subsequent occlusion pressure check. Because of this, many physicians prefer to monitor pulmonary artery diastolic pressures rather than occlusion pressures.

Noninvasive Cardiac Output

To avoid the complications of PACs, other methods of determining CO are being developed.

Thoracic bioimpedance systems use topical electrodes on the anterior chest and neck to measure electrical impedance of the thorax. This value varies with beat-to-beat changes in thoracic blood volume and hence can estimate CO. The system is harmless and provides values quickly (within 2 to 5 min); however, the technique is very sensitive to alteration of the electrode contact with the patient. Thoracic bioimpedance is more valuable in recognizing changes in a given patient than in precisely measuring CO.

The esophageal Doppler monitor (EDM) device is a soft 6-mm catheter that is passed nasopharyngeally into the esophagus and positioned behind the heart. A Doppler flow probe at its tip allows continuous monitoring of CO and stroke volume. Unlike the invasive PAC, the EDM does not cause pneumothorax, arrhythmia, or infection. An EDM may actually be more accurate than a PAC in patients with cardiac valvular lesions, septal defects, arrhythmias, or pulmonary hypertension. However, the EDM may lose its waveform with only a slight positional change and produce dampened, inaccurate readings.

Consequently, while thoracic bioimpedance and EDM are potentially useful, neither is yet as reliable as a PAC.

Intracranial Pressure Monitoring

Intracranial pressure (ICP) monitoring is standard for patients with severe closed head injury. These devices are used to optimize cerebral perfusion pressure (mean arterial pressure minus intracranial pressure). Typically, the cerebral perfusion pressure should be kept > 70 mm Hg.

Several types of ICP monitors are available. The most useful method places a catheter through the skull into a cerebral ventricle (“ventriculostomy” catheter). This device is preferred because the catheter can also drain CSF and hence decrease ICP. However, the ventriculostomy is also the most invasive method, has the highest infection rate, and is the most difficult to place. Occasionally, the ventriculostomy becomes occluded due to severe brain edema.

Other types of intracranial devices include an intraparenchymal monitor and an epidural bolt. Of these, the intraparenchymal monitor is more commonly used. All ICP devices should generally be changed or removed after 5 to 7 days due to the risk of infection.

Other Types of Monitoring

Gastric tonometry determines the gastric intramucosal pH (pH_i) and tissue CO₂ through a special nasogastric tube with a semi-permeable liquid- or gas-filled balloon at the tip. Both a decreased pH_i and an elevated tissue CO₂ to arterial CO₂ ratio occur with decreased splanchnic perfusion and therefore are markers of systemic hypoperfusion. The liquid-filled balloon requires intermittent sampling. The gas-filled balloon has an infrared sensor at the tip that provides a constant readout; however, this model has not proved very reliable. Although able to diagnose hypoperfusion, the readings are slow to normalize after successful resuscitation.

Sublingual capnometry uses a similar correlation between elevated sublingual Pco₂ and systemic hypoperfusion to monitor shock states using a noninvasive sensor placed under the tongue. This is easier to use than gastric tonometry and responds quickly to perfusion changes with resuscitation.

Tissue spectroscopy uses a noninvasive near infrared (NIR) sensor generally placed on the skin above the target tissue to monitor mitochondrial cytochrome a,a redox states, which reflect tissue perfusion. NIR may help diagnose acute compartment syndromes (eg, in trauma) or ischemia after free tissue transfer, and may be helpful in postoperative monitoring of lower extremity vascular bypass grafts. NIR monitoring of small-bowel pH may be used to gauge the adequacy of resuscitation.