The main indications for hemodynamic monitoring in critical care are the identification of the type of shock, guidance of therapeutic interventions, and the cardiopulmonary evaluation of the patient with respiratory failure. Hemodynamic monitoring techniques are classified as invasive, minimally invasive, and noninvasive. Invasive monitoring includes the pulmonary artery catheter and transpulmonary thermodilution techniques. Minimally invasive approaches include noncalibrated pulse wave analysis and esophageal Doppler. Noninvasive techniques comprise bioreactance and bioimpedance techniques, noninvasive pulse contour methods, and echocardiography. Although few data are available to demonstrate outcome benefit from hemodynamic monitoring, these techniques are widely used. Over the past two decades, there has been a marked trend in favor of less-invasive hemodynamic monitoring versus more traditional invasive techniques.
Echocardiography, which can be minimally invasive (transesophageal echo) or noninvasive (transthoracic echo), provides extensive hemodynamic information. Echocardiography has been recommended in the initial hours for the classification of shock. However, echocardiography outside of the operating room tends to be discontinuous and will not easily provide minute-to-minute response to fluid boluses, pressors, or inotropes during these dynamic changes. In addition, echocardiographic study evaluation requires significant bedside skills that may not be available around the clock.
The information provided by noninvasive techniques is often limited to cardiac output and stroke volume variations, whereas invasive techniques provide additional information such as intravascular pressures and cardiac volumes. In general, the more invasive the technique in critical illness, the more accurate the data accrued. Accordingly, the choice of the hemodynamic monitoring technique should not be guided only on invasiveness but should also take into account the accuracy of the technique and, more importantly, the potential interest of the additionally measured variables. The selection of a hemodynamic monitoring device should clearly be individualized based on the patient’s circumstances and the skills available.
Invasive or Noninvasive Arterial Pressure Monitoring?
Arterial pressure is a key determinant of organ perfusion and is routinely measured in critically ill patients, either noninvasively or invasively. Noninvasive blood pressure measurement may be inaccurate in critical illness, particularly in patients with shock, when accuracy of measurements is particularly important. As an example, an overestimation of 5 to 10 mm Hg will have minimal effect on patient management if real mean arterial pressure is 80 mm Hg, but it could have important consequences if mean arterial pressure is 55 mm Hg. Consequently, invasive arterial pressure monitoring is recommended in patients with circulatory failure.
Central Venous Pressure and Central Venous Oxygen Saturation
Central venous access is often required for the care of critically ill patients, especially when in shock, and the measurements of central venous pressure (CVP) and oxygen saturation can provide important information on the hemodynamic state.
However, CVP is an unreliable measure of cardiac function and volume status because of, among other things, changes in intrathoracic pressure. A very high CVP may reflect impaired cardiac function (biventricular or right heart), hypervolemia, or tamponade. A low CVP may suggest hypovolemia, but it can be misleading in patients with isolated left heart dysfunction. Importantly, the measured CVP is strongly affected by intrathoracic pressures; thus, it may overestimate the true CVP (transmural CVP) in patients undergoing mechanic ventilation, severely limiting the capacity of CVP to evaluate preload responsiveness and even cardiac function. Nevertheless, it does reflect the backpressure of the venous system and hence the driving force for tissue edema.
Measurement of central venous oxygen saturation (S cv O 2 ) provides information on the adequacy of oxygen transport and hence cardiac output. A low S cv O 2 , suggestive of excessive oxygen extraction per unit blood, may represent low or inadequate cardiac output, anemia, hypoxemia, agitation, or a combination of all of these factors.
In patients with septic shock, hemodynamic optimization based on these variables has been proposed to improve outcome. In a pivotal trial, the Rivers study, early goal-directed therapy (EGDT) based on CVP and S cv O 2 was associated with a marked reduction in mortality. Two more recent trials, ProCESS (Protocolized Care for Early Septic Shock) and ARISE (Australasian Resuscitation in Sepsis Evaluation), each including several thousand patients, failed to confirm these results. Several factors may explain this. At inclusion, S cv O 2 was close to 50% in both groups in the Rivers trial, whereas, by the time of inclusion, it was already reaching the goals (70%) in ProCESS and ARISE. This likely represents lead-time bias—patients had been preresuscitated before inclusion, and if the main variable to correct is already within target values, then minimal effect is to be expected from the intervention.
In the ARISE trial, 78% of the patients reached the S cv O 2 goal at inclusion, and this proportion increased to only 82%, illustrating that the studied interventions mostly failed to improve the monitored variable. Second, the recruitment rate was much lower in the ARISE and ProCESS trials (1 and 0.5 patients per center per month, respectively), whereas it was 8 patients/month in the single-center trial. This may have resulted in selection bias toward less severe patients, which might be reflected by the very low mortality in these trials. This does not alter the conclusions of these trials, but it does limit external validity. Finally, in ARISE and ProCESS, the control groups were essentially resuscitated by the time of inclusion as a direct consequence of care standards that evolved after the Rivers trial. This is reflected by the large amount of fluid administered and short delay to receiving appropriate antibiotics, two of the pillars of the Surviving Sepsis Guidelines. Implementation and compliance with these guidelines is associated with decreased mortality rates compared with more traditional standards of care. Of note, several “before-and-after” trials that seemed to confirm these data suggested an outcome advantage for EGDT, and a recent meta-analysis taking into account these observational trials and one of the recent multicentric randomized trials also suggested that EGDT may be associated with an improved outcome. Thus, the actual conclusions from these trials is that protocolized EGDT may not offer survival benefit in all patients with septic shock, but that hemodynamic optimization based on S cv O 2 may still be justified in the most severe patients presenting with an altered S cv O 2 .
Pulmonary Artery Catheter
The pulmonary artery catheter (PAC), although invasive, provides quasi-continuous information on cardiovascular status. The PAC measures three types of variables: intravascular/intrachamber pressures, cardiac output, and mixed-venous blood gases.
Measurements of pulmonary artery pressure are undoubtedly useful in cases of right ventricular dysfunction in which evaluation of the right ventricular afterload is crucial for diagnosis and therapeutics. With the exception of echocardiography, none of the noninvasive techniques can determine pulmonary artery pressure at the bedside. Measurements of pulmonary artery occlusion or “wedge” pressure (PAOP [or PAWP]) may aid in the diagnosis of left ventricular dysfunction and guiding fluid management. In addition, the PAOP provides information on lung hydrostatic pressure; thus, it may be used to characterize the risk of cardiogenic pulmonary edema. Cardiac output is measured intermittently by manual injection of a cold bolus or automatically using a semicontinuous system. Several cardiac output measurements are averaged, and rapid changes cannot be detected, even with the semicontinuous method. Thermodilution reliably measures cardiac output, except in severe tricuspid regurgitation or intracardiac shunt. At high cardiac output, the precision of semicontinuous cardiac output measurements is lower than that of classic thermodilution, but this has often minor consequences on patient management.
Measurement of cardiac output is useful for diagnosing the type of shock and evaluating the effect of therapies. Finally, each hemodynamic evaluation can be accompanied by measurement of mixed venous oxygen saturation (S v O 2 ), which enables the interpretation of the cardiac output by comparing oxygen transport with oxygen consumption. Although related, S v O 2 differs from S cv O 2 . S v O 2 represents the venous blood collected from all parts of the body, whereas S cv O 2 represents only the blood drained from the upper part of the body. As such, S v O 2 is a superior measure of global oxygen dynamics.
What is the impact of use of the PAC on outcome? Observational trials demonstrate that the use of the PAC allows more accurate determination of the hemodynamic state than clinic evaluation, is associated with significant changes in therapy, and may be associated with improved outcome. On the other hand, several randomized studies failed to demonstrate improved outcomes associated with its use in intensive care unit (ICU) patients with various medical conditions. Several factors may explain these findings. One argument is that the mechanic complication rate of PACs is so high that it outweighs its potential benefits. However, there were no statistically significant differences in such complications in those studies. In addition, perioperative studies of hemodynamic optimization with the PAC have demonstrated reduced perioperative complications and improved survival, suggesting that, in a broad context, the device is safe.
A more likely explanation is that many physicians fail to adequately interpret the data obtained in the complex situations that arise in critically ill patients, resulting in incorrect decisions. Most of the trials with the PAC in critical care patients did not use decision-support algorithms. Interestingly, the addition of echocardiographic data does not necessarily improve the interpretation of the data, suggesting that the physicians are more at fault than the monitoring device. Another factor is that the patients included in these trials were highly selected. In the Fluids and Catheters Treatment Trial, the number of patients not included, because they were already monitored by a PAC at the time of screening, was twice the number of patients randomized, suggesting that the most severely ill patients (in whom information was deemed more valuable) were not included. This high rate of exclusion may bias the results of these trials. Investigators did not include patients with cardiogenic shock because they were sicker, were being managed with a PAC for other reasons, and had significantly higher mortality rates than the patients included in the trial.
As a result of these negative trials and because of the wide availability of alternative techniques, the use of the PAC has decreased over time and has probably been abandoned altogether by many clinicians. However, it is worth arguing that the PAC remains a useful device in select patients in the ICU, and medical and nursing staff should be familiar with insertion, setup, and monitoring techniques.
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