During a discussion I was having about near-infrared spectroscopy (NIRS )-derived tissue oximetry, an expert surgeon who has written a lot about NIRS asked, “If we could have the perfect patient monitor, what would it look like? It would be non-invasive. It would be continuous. It would be reliable. It would be inexpensive, so you could use it repeatedly on anyone and everyone. It would tell us information that we couldn’t figure out on our own. And it would not only tell us that something was wrong, it would tell us WHAT was wrong ( and with sufficient advance warning) so we’d know what to do about it.”

All current monitoring devices – even the new, high-tech ones – fall short of this ideal. The monitors you will have in theater will provide you with raw data, but it will still be up to you to put various pieces of data together with the appearance of the patient to determine what is wrong with the patient and what needs to be done. Monitors are tools and looking magnifying glasses; the art of medicine is in the interpretation. There is no single lab, test, monitor, or device that can tell you what is wrong with a patient and what to do. No monitor can replace your eyes, hands, brain, and judgment. With that in mind, the follow-on lesson is that the establishment of monitors, particularly invasive ones, should never take priority over the interventions necessary to save a patient. The words, “We’re just going to get another central line and a-line in him, and then we’ll go to the OR,” should not come out of your mouth. Central lines or a-lines are almost never the intervention that saves a bleeding trauma patient’s life. Nor should monitoring delay the execution of life-saving interventions.

Although heart rate and blood pressure may be inadequate to gauge resuscitation, they are frequently sufficient as a triage tool to separate those that need immediate operation from those that do not. The character of the radial pulse and a simple GCS motor score can fairly reliably separate the “sick” from the “not sick.” This is particularly true if keen attention is paid to other nonmeasurable clinical signs (skin color, mental status, obvious physical signs of injury, etc.). Rapid point-of-care testing (INR, base deficit, lactate and Hgb) can provide confirmatory data. Invasive continuous monitoring is almost never required for this initial decision process. Typically, a simple pulse oximeter, electrocardiographic, and respiratory rate monitor are rapidly established and provide adequate continuous data for patients needing transfer from the trauma bay to the OR or CT scan (Fig. 35.1). All surgical units will have this basic monitoring capability. The other standard monitors available at the forward surgical team and combat support hospital include arterial lines and central venous pressure . More advanced continuous monitors, such as continuous central venous saturation, intracranial pressure, near-infrared spectroscopy-derived tissue oxygenation, and noninvasive cardiac output monitors, will only be available at selected facilities.

The ubiquitous pulse oximeter device consists of a light source and a detector that fits around the finger, toe, or earlobe. It passes visible (red) and infrared spectrum light through the tissues and, based on values programmed into the device, can detect the different amounts of red and infrared light that are absorbed or transmitted through oxygenated and deoxygenated hemoglobin in the blood vessels. The percentage of oxygenated blood can be calculated automatically by the device. The pulse rate is also displayed.

Several problems or conditions can cause the pulse oximeter to provide faulty readings. Patients in profound shock, elderly patients (particularly those with vascular disease), and patients with very cold extremities may have poor perfusion of their digits, and the oxygen saturation may be falsely decreased and/or pulse rate may not be detected. Please remember to develop a prejudice in the trauma bay that the reason the oximeter is not working is more often related to SHOCK and hypoperfusion than to defective equipment. The authors have all too often watched precious time wasted as multiple probes/monitors are placed on the patient in shock in an effort to troubleshoot equipment rather than troubleshoot the patient in shock . It is critically important that you look at not only the number being displayed on the monitor but also the waveform. You will frequently see false values of 95–100% saturation displayed when there is no discernible waveform.

For the combat environment, the relevant things to remember are that the pulse oximeter may be unreliable in casualties in profound shock – and in these cases, the value of the pulse oximetry should not be decisive anyway. The fact that the pulse oximeter cannot detect a pulse IS a decisive finding. Casualties may also have profound hypothermia, even in the hot desert; others may have cold weather exposure as well. Finally, casualties are sometimes trapped in burning vehicles or buildings and may have a carbon monoxide exposure. All of these scenarios may affect the accuracy and reliability of the pulse oximetry reading .

Electrocardiographic monitoring is also standard for combat casualties and is available in most medical units, all surgical units, and aeromedical units. There are essentially no downsides to continuous monitoring of electrocardiographic tracings, and most trauma bay personnel get used to rapidly applying these to casualties. Unfortunately, the data obtained from these tracings are somewhat limited, at least from the standpoint of rapid decision-making in trauma. The cardiac telemetry monitors do not provide much additional information aside from heart rate, particularly in young, otherwise, healthy casualties. There are instances where older foreign national patients, contractors, and even soldiers may suffer cardiac ischemia, and these monitors can be useful in that setting. The monitors are also useful for easy, continuous monitoring in the intensive care unit and are standard in ICUs throughout the world. One key point is if your patient suddenly becomes asystolic, check that the leads have not fallen off or the cable has become disconnected before you crack the chest open.

Continuous monitoring of central venous pressure (CVP) is available in most surgical units, but the utility to reliably determine volume status and monitor resuscitation is limited. The administration of fluids prior to establishing hemorrhage control remains of very questionable value. The administration of sedation, analgesia, medications, antibiotics, and sufficient sedation to initiate anesthesia for surgery may often be accomplished via the intraosseous route. If adequate large-bore peripheral IV access has already been established, central line placement is not a valid reason to delay taking the patient to the OR. Central lines can be placed by anesthesia personnel while the surgeons operate. Once the central line has been established in the appropriate setting, the best monitoring data it will provide is through intermittent assessments of the central venous oxygen saturation.

One of the biggest fallacies in critical care that continues to be taught and practiced is that central venous pressure is an accurate reflection of ventricular preload. While this looks reasonable on diagrams, the reality is that this is an “estimate of an assumption about a surrogate.” The lack of correlation between CVP and volume status has been proven in multiple studies, even among healthy volunteers where the relationship between end-diastolic volume and CVP should be the most reliable . While it may have some use at the extremes of measurement (complete volume collapse or massive volume overload), CVP IS NOT AN ACCURATE OR RELIABLE REFLECTION OF INTRAVASCULAR VOLUME! This is so important that it bears repeating. CVP IS NOT AN ACCURATE OR RELIABLE REFLECTION OF INTRAVASCULAR VOLUME (got it?). Changes in CVP can just as easily reflect changes in patient position, monitor position, respiratory pressures, and most importantly ventricular compliance. Intubating a patient and placing them on positive pressure ventilation to include PEEP will always elevate CVP while simultaneously impeding central venous return and cardiac filling volume. This truism explains the often observed paradox of the freshly intubated trauma patient who demonstrates an elevated CVP and concomitant hypotension. The higher the airway pressure, the higher the CVP and the lower the venous return. This patient needs fluids, not Lasix. The compromise of venous return effected by airway pressures may also explain why trauma patients placed on APRV early in their resuscitation (prior to adequate volume restoration) display more hypotension than patients on conventional ventilation.

The measurement of CVP is also highly user and interpreter dependent; we have found that drastic CVP changes more often reflect a nursing change of shift or recalibration of transducers than they do fluid shifts. You will be much better off using some of the alternative measures for assessing volume status outlined in this chapter and in Chap. 6 (Ultrasound in Combat Trauma).

The term mixed venous oxygen saturation (SvO₂) refers to the hemoglobin saturation of blood in the proximal pulmonary artery. Central venous oxygen saturation (CVO₂) refers to the hemoglobin saturation in the superior vena cava (or other central vein). They are not identical because of the consideration of the return of highly desaturated blood at the level of the coronary sinus. When oxygen supply provided by the cardiopulmonary system is insufficient to meet the bodily/cellular oxygen demand, there is a resultant increase in oxygen extraction at the tissue level. This state of increased extraction is manifested by a decrease in the venous oxygen content. Monitoring of venous oxygen content therefore serves as a shorthand of the equity between oxygen supply and oxygen demand. Shock is, in its most basic understanding, a mismatch of oxygen supply and demand.

Traditionally the monitoring of venous oxygen saturation has required the placement of a pulmonary artery catheter in order to obtain sample(s) from the pulmonary artery (SvO₂). The logic of this approach recognizes the variability and sampling error that may be introduced by the contribution of cardiac return via the coronary sinus. The contribution of the effluent of the coronary sinus is significant and may (or may not) be accounted for by a central venous line. This anatomic and physiologic consideration is the basis for the traditional use of SvO₂ as the correct measurement of venous oxygen saturation (requiring full admixture of venous blood in the right ventricle) for truly representative sampling of venous oxygen content.

A number of recent studies have attempted to identify a correlation between mixed venous saturation (pulmonary artery catheter) and central venous saturation (CVP line). These studies have reached alternative conclusions regarding the absolute correlation between individual observations of SvO₂ and CVO₂. However, what can be concluded from these studies is that although individual observations may vary, the trend in SvO₂ closely approximates the trend in CVO₂ and may be used as an accurate surrogate (Fig. 35.2). The author recommends the use of CVO₂ (via central venous sampling) as a means of tracking the direction and trend of an ongoing resuscitation. Most combat casualties with significant injuries will have a central line placed in anticipation of needs for resuscitation and transport by the CCATT team. The availability of CVP lines in combat casualties affords the opportunity for frequent and simple sampling of the trend in CVO₂ as a means of monitoring the progress of an ongoing resuscitation. Limited studies in the setting of hypovolemic shock (animals and humans) have demonstrated the utility of monitoring venous saturation as a marker of the severity of shock as well as its subsequent resolution during resuscitation. To date, there has not been a prospective study examining the efficacy of venous saturation as an endpoint of resuscitation in trauma patients. A study completed in medical patients did demonstrate that the use of CVO₂ is a valid endpoint of resuscitation capable of altering mortality and superior to standard goals. In the setting of trauma patients, positive trends in the resolution of venous desaturation have been demonstrated to be associated with improved survival. It is the authors’ belief that monitoring of venous saturation (SvO₂ or CVO₂) is much more reliable and meaningful than the oft-quoted standards of heart rate, blood pressure, and urinary output. This is especially true in the young and otherwise healthy combat casualty. In addition to helping guide your resuscitation, this data can also prevent the other cardinal sin: over-resuscitation. If you have an isolated abnormality (tachycardia, low blood pressure , borderline urine output) but your CVO₂ and other markers (hematocrit, lactate) are reassuring, then you do not have to chase the abnormality with more volume resuscitation.