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Dr. Dueck is Clinical Professor of Anesthesiology, University of California, San Diego, and Veterans Affairs Medical Center, San Diego.
Correspondence to: Ron Dueck, MD, 3350 La Jolla Village Dr, San Diego, CA 92161-5085; e-mail: rdueck{at}ucsd.edu
The value of noninvasive cardiac output (Qt) monitoring lies first and foremost in its convenient, low-risk application to a variety of clinical settings. The noninvasive thoracic electrical bioimpedance monitor is well suited to hospital bedside, physician’s office, emergency department, ICU, and operating room settings. Thus, Shoemaker et al, in this issue of CHEST (see page 528), were able to utilize impedance cardiography (ICG) for early comprehensive cardiovascular assessment and trending of acute trauma victims.
The low risk and convenience of noninvasive Qt monitoring allowed Shoemaker et al to test the hypothesis that early, continuous cardiovascular assessment would help identify patients whose injuries were so severe that they were not likely to survive. This included severe blood and/or fluid volume deficits induced by trauma, which did not respond readily to expeditious volume resuscitation and vasopressor therapy. Monitoring was initiated during the early, critical management period, at a time when important diagnostic procedures were proceeding in the emergency or radiology (CT scan) departments. Their findings identified four cardiorespiratory variables that served as statistically significant measures of treatment outcomes: Qt, BP, pulse oximetry, and transcutaneous PO2 (PtcO2).
The characteristic of these four variables that most reliably predicted a favorable outcome was high normal or supernormal values during the first 12-h monitoring period. This observation is consistent with earlier reports showing reduced morbidity and mortality in high-risk surgical patients who have high oxygen delivery during their perioperative course.1 2 Presumably, this feature derives from both high catecholamine levels associated with the stress induced by trauma, and from the release of cytokines associated with tissue injury leading to the systemic inflammatory response syndrome (SIRS) in both trauma and elective surgical patients.3 4 The need for both high normal Qt and PtcO2 was probably related to the high metabolic demand induced by the inflammatory response. The clinical picture of SIRS is identified by either high or low temperature, high heart rate, high respiratory rate, and high WBC count.5 Clearly, this syndrome represents an elevated metabolic state with the demand for a high Qt. An obligatory high Qt may not be sustainable in the presence of hypovolemia, acute anemia, pre-existing impaired cardiac function, acute myocardial injury, or coronary ischemia. Thus a fall in PtcO2 could also be interpreted as too high a metabolic demand for a patient’s cardiovascular reserve.
Too high a metabolic demand may compromise other critical organs. Acute lung injury from hypotension, blunt trauma, and massive fluid resuscitation can drastically reduce respiratory reserve. Thus, the finding that both oxygen saturation (by pulse oximetry) and PtcO2 were significantly reduced in nonsurvivors suggests gas exchange impairment may very well have been a vital component of survival versus nonsurvival. Indeed, studies4 5 show that failure to resolve SIRS within 24 to 48 h is associated with development of sepsis, multiple organ dysfunction, and a high mortality rate. The known susceptibility to sepsis after multiple trauma and the difficulty in differentiating SIRS from sepsis may be another indication for noninvasive monitoring.6
The comprehensive monitoring requirement suggests that another new approach to noninvasive Qt monitoring, using CO2 elimination for applying the Fick principle (NICO; Novametrix Medical Systems; Wallingford, CT), may be well suited to trauma victims who need intubation and mechanically assisted ventilation.7 This monitor offers the added feature of measuring CO2 production on a breath-by-breath basis, providing a continuous estimate of metabolic demand. In addition, the precision breath-by-breath volumetric analysis provides continuous tidal volume flow/volume and compliance assessment. This should facilitate adjusting ventilatory parameters to optimize compliance with positive end-expiratory pressure (PEEP) and tidal volume settings, while minimizing unnecessary auto-PEEP and Qt depression.8
The article by Shoemaker et al does not suggest that early cardiorespiratory assessment with a pulmonary artery (PA) catheter is contraindicated. Indeed, they observed that thermal dilution Qt measurements obtained in a number of subjects showed good correlation with ICG Qt values. These authors reflect that late introduction of PA catheter monitoring may not improve outcome in critically ill patients. Instead, early comprehensive cardiovascular assessment may be so critical for major trauma victims as to make early utilization of a more convenient method of monitoring, compared to the invasive PA catheter, of even greater value in this patient population.
The reasons for choosing ICG were as follows: (1) it is the least invasive method currently available for determining Qt, and (2) the application procedures for ICG are as simple as placing electrodes on the chest.9 These two features make it a feasible technology for Qt monitoring in an emergency department setting. Improvements in signal processing and algorithms with the BioZ ICG device (Cardiodynamics International; San Diego, CA) have yielded favorable reliability and reproducibility findings in lower-body negative pressure suit and in congestive heart failure patient studies.10 11 12 Conversely, a meta-analysis of ICG by Raaijmakers et al13 found a pooled r2 value of 0.67 for the period from January 1966 to April 1997. They found no change in r2 for the technical changes in ICG design introduced during this period. However, the correlation was higher in repeated-measures designs than in single-measurement conditions, where r2 was 0.53.13 Thus, the repeated-measures (continuous monitoring) design of Shoemaker et al may well have benefited from trending analysis.
The focus on early, aggressive hemodynamic resuscitation of major trauma victims is obviously justified.4 Pursuing a goal of super-high normal Qt and/or PtcO2 with volume challenges without measuring cardiac filling pressures, however, increases the likelihood of fluid overload. Not only is this a risk for patients with compromised cardiac function, but specifically for major trauma victims who are inherently at increased risk of developing ARDS. This necessitates judicious use of noninvasive monitoring for ensuring adequate fluid volume resuscitation, together with a low threshold for converting to or adding PA catheter and/or echocardiographic assessment.14 The Pulmonary Artery Catheter Consensus Statement15 identified trauma victims as showing improved outcomes with PA catheter use, although this was based on nonrandomized studies and expert opinion. Echocardiography is clearly valuable, especially if there is possible injury to the heart and/or great vessels, analogous to the concepts presented by Weil.16 These two modalities both apply to patients who remain hemodynamically unstable in spite of early resuscitative measures and noninvasive hemodynamic monitoring. Patients who fail to develop the anticipated stress response by showing a high normal Qt and/or PtcO2, and/or have seriously impaired lung or renal function, will probably need more invasive measurements.
In conclusion, the value of more comprehensive cardiovascular monitoring, especially with new noninvasive devices for estimating cardiac output, has become increasingly well documented. The cardiovascular system is too complex for assessment with something as unimodal as systemic BP. Combining comprehensive noninvasive cardiovascular with respiratory monitoring is essential for early management of major trauma victims who are so susceptible to circulatory decompensation, sepsis, and ARDS.
References
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