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Less Invasive Hemodynamic Monitoring
By Andrew M. Luks, MD, Pulmonary and Critical Care Medicine, University of Washington, Seattle, is Associate Editor for Critical Care Alert.
Intensive care practitioners are constantly faced with questions about their patients' hemodynamic issues, including the volume status, fluid responsiveness, and the need for vasopressors or inotropic support. Because the clinical exam is often of limited utility in addressing these issues, intensivists typically rely on other tools to guide their assessment. For a long time the primary tool for this purpose was the pulmonary artery (PA) catheter, but for a variety of reasons, this device is being used less frequently and clinicians are relying on several less invasive tools for hemodynamic monitoring. The purpose of this special feature is to consider some of these less-invasive systems the PiCCO catheter, the FloTrac system, and the LiDCO Plus system in greater detail, reviewing the mechanisms by which the devices derive hemodynamic information, the necessary equipment, limitations of the systems, and the available data regarding their accuracy and utility in the ICU.
What Happened to the PA Catheter?
Before considering these less invasive systems, it is worthwhile to briefly review the issues with the PA catheter that have driven a search for alternatives. The device provides a lot of information including central venous pressure (CVP), PA pressure, pulmonary capillary wedge pressure (PCWP), cardiac output/index (CO/CI), systemic vascular resistance (SVR), and mixed venous oxygen saturation (SvO2), but it carries significant risks to the patient, including pneumothorax during line placement, arrhythmia, heart block, pulmonary artery rupture, and pulmonary artery infarction.
Aside from concerns about these risks, use of the PA catheter has been declining as a result of several problems. First, evidence suggests that two of the most commonly used pieces of information from the catheter, CVP and PCWP, do not adequately predict fluid responsiveness in hemodynamically unstable patients,1 and that more dynamic measures, stroke volume variation (SVV), and pulse pressure variation (PPV) information not available from a PA catheter are more useful in this regard.2 More importantly, multiple studies suggested that use of the catheter was associated with either worse outcomes or no improvement in outcomes in hemodynamically unstable patients, with the final nail in this coffin coming when the randomized, prospective Fluid and Catheter Treatment Trial (FACTT) demonstrated no difference in outcomes in acute lung injury patients managed with a PA catheter or CVP monitoring through a central venous line.3
As PA catheter use has declined in response to these concerns, a further insidious effect has taken place, further diminishing the device's utility. Less catheter use has translated into fewer trainees gaining experience with the device and fewer nurses being experienced in how to set up and maintain the system. As a result, each use of the PA catheter becomes an exercise in "re-learning" the tool, with high potential for misinterpreting data or generating misleading information.
Less Invasive Hemodynamic Monitoring Systems
Several less-invasive modalities have emerged for monitoring hemodynamic information and been put forth as alternatives to the PA catheter. While some sources refer to these systems as "non-invasive," the term "less invasive" is more appropriate as the systems still require invasive components including central venous and/or arterial catheters. In the space below, we consider three particular devices that are gaining attention: the LiDCO Plus, the PiCCO catheter, and the FloTrac system. Other less invasive modalities such as transesophageal echocardiography can also be used for hemodynamic monitoring, but will not be considered here because they require more technical expertise and cannot be easily employed in awake patients and, as a result, are less likely to be used as frequently as the systems considered below. The pressure recording analytical method (PRAM)4 also will not be considered as experience with the system and validation data are both very limited at this time.5
The Basics of the Three Systems
LiDCO Plus: The LiDCO Plus (LiDCO Ltd, Cambridge, U.K.) is comprised of two separate components, the PulseCO system, which uses arterial waveform analysis to perform continuous CO measurements as well as assessments of SSV and PPV, and the LiDCO system that uses the bolus indicator dilution method to measure CO for the purpose of calibrating the PulseCO system. Use of the system requires a peripheral arterial line and either peripheral or central venous access. Calibration for CO measurements involves injecting small amounts of lithium into the patient's venous access and measuring the decay in lithium concentration at the arterial line. Calibration is typically performed every 8 hours, with any significant hemodynamic change, or before any big change in management.6 The concentration of lithium (0.15-0.3 mmol for adult patients) is far below therapeutic levels and patients would have to exceed the device's maximum total dose many times before experiencing toxicity.6 Because of the small concentrations that are used for calibration, the device cannot be used in patients on pre-existing lithium therapy, as it would be impossible to detect the small changes in lithium concentration at the arterial line.6 It takes only a matter of minutes to connect the patient to the system and begin PulseCO measurements, but lithium calibration adds time to the system set up and often requires additional nursing support, particularly with providers not well versed in its use. Even if lithium calibration is not carried out, however, the PulseCO system can still be used for SVV and PPV assessments. A new LiDCO rapid system has been released, which does not require calibration. A complete review of the device is available elsewhere.6
FloTrac: The FloTrac system (Edwards Lifesciences, Irving, CA, USA) uses arterial pressure waveform analysis to provide continuous CO measurements as well as assessments of SVR and SVV. Unlike the LiDCO and PiCCO systems, the FloTrac system requires only an arterial line for operation and purportedly works regardless of the arterial line site. Some data suggest, however, that measurements may be affected by the line's location, particularly in hemodynamically unstable patients or those on vasopressors.7,8 Another important difference compared to the other systems is that the FloTrac system does not require external calibration. Instead, the system uses demographic data including height, weight, age, and sex in conjunction with arterial waveform analysis to perform its own internal calibration. This is supposed to limit set-up time, an important issue in hemodynamically unstable patients, but may represent a significant limitation if the data used to support the internal calibration were not drawn from an adequately broad population of test subjects, much the same way that utility of "normal" values in pulmonary function testing are limited by the test populations used to generate these values. A complete review of this device is available elsewhere.7
PiCCO: The pulse contour cardiac output system (PiCCO; Pulsion Medical Systems, Munich, Germany) also employs arterial waveform analysis, although the algorithm differs from those used by the other systems. It provides similar data as those systems, including PPV, SVV, and CO, but is also able to provide assessments of extravascular lung water and end-diastolic volume. A significant difference with the other systems is the fact that the PiCCO system requires central venous access and a special arterial line that is placed in a major artery (e.g., femoral or brachial) and extends to the central circulation. Peripheral arterial lines cannot be used. External calibration using the saline thermodilution technique is also necessary and may need to be performed as frequently as every hour in hemodynamically unstable patients.9 A complete review of the device is available elsewhere.10
Available Data Regarding the Utility of the Systems
The key issues with these systems are whether they provide accurate data and make a difference in patient outcomes. Space limitations do not permit a full assessment of the literature available for each system and this discussion will focus, instead, on broad issues in the data. In general, it appears that the PiCCO and LiDCO systems provide reasonably accurate data regarding CO, PPV, and SVV, although the accuracy is likely dependent on the frequency of calibration, particularly in hemodynamically unstable patients.5 Significant concerns have been expressed, however, about the accuracy of the FloTrac system, especially in hemodynamically unstable patients, although a recent software update may have led to improvements in this regard.5,7
A major limitation of the available data, however, is that most of the studies focus on the issue of accuracy and reliability of the systems (e.g., how well do the less-invasive CO readings correlate with invasive measurements using a PA catheter) and we continue to lack data showing that use of these systems in patient management is associated with improvement in patient outcomes such as mortality, length of ICU stay, or time on the ventilator. Given that the demonstrated lack of improvement in patient outcomes started the downward trend in use of the PA catheter, caution is likely warranted before widespread application of these less invasive systems.
Finally, in considering the utility of any new medication or device, it is important to ask whether the studies assessing utility or accuracy are relevant to one's patient population. This turns out to be a big issue with the less invasive monitoring systems as many of the studies have been done in animals or in various patient groups in the operating room (e.g., cardiopulmonary bypass patients), and fewer studies have been done in the setting most relevant to our practice, the ICU.
Important Limitations of the Less Invasive Systems
It is important for clinicians to be aware of several limitations in the applicability of these systems. Because the devices all rely on arterial waveform analysis to generate data, dampening of the arterial signals or significant dysrhythmias will adversely affect the accuracy of the data.7 For example, atrial fibrillation and the associated alterations in chamber filling time will affect PPV beyond that due to respiratory cycle-induced changes in intrathoracic volume. Other patient factors that affect the character of the arterial pressure waveform, such as aortic regurgitation or use of an intra-aortic balloon pumps, also preclude use.6 Significant atherosclerosis may also alter the accuracy of PPV measurements by altering arterial compliance and, as a result, pressure variations in response to a given change in stroke volume.10 This last issue is a tough one to deal with as we lack an easy bedside test to determine whether the patient has significant peripheral atherosclerotic disease and how that will affect measurements.
Another major issue with PPV measurements as well as SVV measurements two of the important pieces of data provided by these systems not available on a PA catheter is that they have only been validated for predicting fluid responsiveness in mechanically ventilated patients who are not initiating breaths on their own. As a result, the utility of the devices may be limited in our non-mechanically ventilated patients or those ventilated patients who are not adequately sedated or paralyzed and are making efforts to trigger the ventilator.10
Finally, aside from the general issues noted above that apply to each device, there are a few issues that are particular to the LiDCO Plus system. In addition to the fact that it cannot be used in patients on lithium therapy, measurements and calibration are adversely affected by the presence of non-depolarizing muscle relaxants. If such medications are necessary, they should only be given by bolus administration, rather than continuous infusion.6
On the surface, the less invasive monitoring systems appear to be an attractive tool for hemodynamic monitoring in the ICU. They involve less patient risk than the PA catheter and, with the exception of the PiCCO system, are generally fast and easy to set up and do not require special invasive lines. They also have the ability to provide data about SVV and PPV, better predictors of fluid responsiveness in mechanically ventilated patients than static measures of volume status such as the CVP or PCWP.2 The devil is in the details, however. Data exist demonstrating reasonable accuracy and reliability in cardiac output measurements for the LiDCO and PiCCO systems (although concerns still exist about the FloTrac device), but most of these data were not generated in the ICU and we still lack any evidence that using such tools to guide management actually improves outcomes. There are also important limitations with the systems, which, if not recognized at the time the systems are used could lead to erroneous data and misguided patient management.
The concern is that because the devices are easy to set up, nurses and other clinicians will quickly implement them at the bedside, start generating numerical data, and then act on that data before taking time to consider whether aspects of the patient's clinical situation make the data less reliable (e.g., the patient was on a LiDCO Plus while paralyzed for severe hypoxemic respiratory failure). Such details are often overlooked in the chaos surrounding the deteriorating patient.
Until further data are available demonstrating their accuracy and a positive effect on patient outcomes, nurses and intensivists should be measured in their use of these devices. They should not be used in all patients, and are probably best reserved for those with severe hemodynamic issues. Unfortunately, these also happen to be the patients for whom the FloTrac system is considered unreliable and the other systems need more frequent, time-consuming calibration. When these devices are implemented, clinicians should take the time to do the required calibrations, particularly before any big change in management or with any major alteration in patient condition, consider whether patient factors preclude use of the equipment, and always take time to ask whether the data make sense in light of what they are seeing at the bedside with their patient.