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Is Minimally Invasive Hemodynamic Monitoring Ready for Cardiac Surgery?
By Ryan Young, MD and Hong Liu, MD
Accurate volumetric and cardiac assessment is essential to the perioperative management of patients undergoing cardiac surgery. Traditional means of evaluation have relied on the use of a pulmonary artery catheter which uses right sided pressures and flows to estimate the performance of systemic or left sided function. Many would consider this method as the “gold standard” for hemodynamic monitoring especially in the intensive care and cardiac surgery setting. However, its efficacy and performance have remained to be justified by means other than its wide use through time. Intraoperatively, what has been commonly utilized to examine cardiac function and hemodynamic parameters is transesophageal echocardiography (TEE). With this tool, direct observation of the left side of the heart with ultrasound permits evaluation of information such as left ventricular end systolic and end diastolic volumes which can be used to guide management and intervention to optimize a patient’s volume and cardiovascular status. However, it is highly invasive and associated with a set of significant complications. More recently, new advances have led to the development of minimally invasive techniques to monitor hemodynamic function and have been used to provide perioperative hemodynamic monitoring for cardiac surgery patients.
Several novel methods use dilution analysis to measure cardiac output. These include the LiDCO (LiDCO, London, UK) and PiCCO systems. The difference between these tools and PACs is that both LiDCO and PiCCO allow for dilution through the systemic or left sided circulation versus just the right heart.
Initially described by Linton et al1, lithium dilution was reported to have high correlation with PAC thermodilution cardiac output. This technique involves the administration of a bolus of isotonic lithium chloride (0.002-0.004 mmol/kg) into a central or peripheral vein. Detection of lithium is later measured with a lithium ion specific electrode which is attached to an arterial line. The plasma concentration of lithium as it varies over time is then incorporated into the derivation of cardiac output2. Advantages of lithium are that it is neither protein bound nor normally present in blood allowing for more accurate measurements at levels that are within the nontoxic range1.
In practice, LiDCO is commonly applied with a pulse contour or pulse power analysis which allows continuous monitoring of cardiac output. This system has been shown to be effective clinically at predicting volume responsiveness even when compared to TEE. Belloni et al3 compared the reliability of hemodynamic markers obtained via PAC, LiDCO, and TEE in predicting fluid responsiveness in patients undergoing off pump coronary artery bypass (OPCAB) surgery. Findings from this prospective study suggested that LiDCO/PCO was superior to PAC and TEE in identifying pathophysiologic states in which cardiac performance was improved by fluid administration.
The PiCCO (Pulsion Medical Systems, Munich, Germany) system uses transpulmonary thermodilution to calibrate continuous cardiac output monitoring. Initial measurements are attained when cold saline injected into a central vein causes temperature fluctuations detected by a thermistor tipped arterial catheter placed in an axillary or femoral artery, although some studies suggest that the usage of a radial artery catheter provides similar accuracy4. Cardiac output is calculated with the Steward-Hamilton equation5 and applied with the pulse contour method to monitor beat to beat variability in cardiac function. The basis of the calculations made in the pulse contour method rely on an algorithm derived by Wesseling et al. In this model, aortic flow pulsations are influenced by three elements: arterial compliance, systemic vascular resistance, and lastly aortic impedence which is obtained through transpulmonary thermodilution6. PiCCO has been shown to more accurately reflect left ventricular filling and volumes than the PAC in a number of studies7-9. In addition, de Castro et al10 reported on the high correlation (r= 0.885; bias 0.2) between stroke volume as determined by aortic Doppler and that derived from the PiCCO system in patients undergoing abdominal aortic surgery.
One unique aspect of the PiCCO tool is that through a transpulmonary double indicator dilution technique, the volume of intrathoracic blood can be estimated. Analysis of the dilution following the simultaneous injection of indocyanine green, which sequesters in the intravascular space, with cold saline, which equilibrates with the extravascular space, can be used to predict the amount of extravascular lung water11. This information may be used as an indicator of pulmonary edema which can reflect excessive volume loading.
Both PiCCO and LiDCO make measurements applying pulse contour analysis using external calibration, which usually occurs at intervals of several hours. One device has the ability for self-calibration based on patient specific data and is known as FloTrac Vigileo (Edwards Lifesciences, LLC). The distinction in this tool’s evaluation of the arterial waveform is its prediction of arterial compliance and resistance based on age, sex, height, weight, and body surface area. Its recalibration rate occurs from every 20 seconds to minutes, and more recent revisions of its operational algorithm have raised accuracy levels. In a meta-analysis validation study performed by Mayer et al12, FloTrac was concluded to have comparable accuracy to intermittent thermodilution method. Others have deemed the estimation of cardiac output by each of the above systems to be interchangeable with that from a PAC13.
The area in which pulse contour analysis has shown potential superiority over a PAC is the increased accuracy of predicting volume responsiveness in the intensive care units and operating room14. Those static values used in the past, such as PCWP and CVP, are poor reflections of preload status15 and they may now be superseded by dynamic parameters such as stroke volume variation (SVV) and pulse pressure variation (PPV). McGee delineated the physiology of SVV and how it can be an indicator of volume status16. During mechanical ventilation, positive pressure has the opposite but conceptually similar effect of pulsus paradoxus. With each phase of the ventilation cycle, inspiratory and expiratory, cardiac pressures and filling are altered causing variability in stroke volume from beat to beat. The degree of variability is related to the degree of preload insufficiency. Certain conditions are known to reduce the reliability of SVV, such as arrhythmias, IABP, severe aortic regurgitation, and extreme hemodynamic instability. However, when SVV and PPV can be measured accurately in cardiac surgery, it has been shown to have predictive capabilities used to improve cardiac performance with intravascular volume loading17-21.
Despite these encouraging findings, the validation of the above devices and technology remains a challenge. Most investigations compare the performance of the new pulse contour and thermodilution devices to each other and against what is now or has been common practice. But even these current standards are vulnerable to inaccuracy. Pulmonary thermodilution is not continuous and provides at best a time delayed indication of cardiac output which likely limits its utility in periods of hemodynamic instability. Even the estimation of left ventricular preload by TEE can be influenced by myocardial wall motion abnormalities and by nonideal positioning of the probe during quantitative algorithmic assessment.
Some investigators have analyzed the measurements of pulse contour devices at specified stages of OPCAB surgery. Halvorsen et al22 examined the concordance between pulmonary thermodilution, transthoracic thermodilution, and PiCCO at six different time points during surgery (induction of anesthesia, after pericardiothomy, after grafting on the anterior, posterior, and lateral walls, and after chest closure) in a prospective observational study. They found agreement between transpulmonary and transthoracic thermodilution but a discrepancy with pulse contour analysis, which was suspected to be due to time periods of hemodynamic instability.
Others, as referenced above, have studied the ability of pulse contour devices to predict fluid responsiveness in OPCAB surgery. Still others have documented the behavior of SVV as it is influenced by known changes in volume loading through altering physical positioning (between Trendelenberg and reverse Trendelenberg) in post CABG patients23. In addition, Sander et al24 suggested that global end diastolic volume, SVV, and PPV were more valuable than static parameters as indicators of changes in cardiac index under open chest conditions.
What is needed in the further validation of minimal invasive devices is additional investigation of outcomes measurements and goal-directed therapy using these new tools. Kapoor et al25 explored the effects of using FloTrac to guide early goal directed therapy (EGDT) in a small group of 30 patients undergoing CABG under CPB. Parameters such as cardiac index, stroke volume index, oxygen delivery index, SVV, and central venous oximetry were monitored with the FloTrac device and continuous central venous oxygen saturation. Volume loading and/or the altering of vasoactive agents were used to maintain theses parameters within a predetermined range. Despite inconclusive results, those within the EGDT group were observed to require shorter periods of ventilation, less days of ICU stay, and fewer days of inotropic support than the control group.
In conclusion, minimally invasive devices have rising promise in the effort to improve continuous hemodynamic monitoring during cardiac surgery. More recently, they have been used to replace the pulmonary artery catheters in certain cardiac surgical patient populations. There may not be one ideal method that preserves accuracy all the time, as each has its own set of limitations. But corroboration of new technology and innovation should justify the movement towards less invasive means of patient monitoring that would reduce morbidity, cost less, yield similar if not higher quality physiologic data, and improve patient outcomes.
Editor's Note: Drs Liu and Yang have no conflict of interest to report with any of the products in the above DIU.
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