Australasian Anaesthesia 2011 - Australian and New Zealand ...

Cardiac output monitoring in non-cardiac surgery: how and why? 183182 Australasian Anaesthesia 2011access that the pulmonary artery catheter provides is not given by these less invasive technologies, it is likely thatFluid bolusVasopressor Maintenance fluidthey have comparable performance to thermodilution in measurement of cardiac output. 10,11 & replace lossesFigure 4. The changes in CO 2 variables during partial CO 2 rebreathingThe percentage errors shown by this meta-analysis represent the combined effects of a number of sources oferror of agreement with thermodilution. There are systematic sources of error for some methods, for example, thetendency of thermodilution to overestimate low cardiac output values due to thermal decay during slow right heartBaseline Partialpassage. 13-15 There is inter-patient variability, reflecting a wide variety of sources of error, and there is intra-patientvariability.period rebreathingIn general, intra-patient variability is smaller, but most important. Most technologies are able to track changesor trends in cardiac output, at least qualitatively, and arguably this is the most important function of a monitor. 16,17A device which gives misleading data on the direction of change of cardiac output may be of more harm thanbenefit in clinical practice. Statistical methods have been discussed recently for assessing “concordance” betweena method and thermodilution in measuring changes in cardiac output. 18,19 The ability of devices to track suddenTime –>dramatic changes in cardiac output is hard to study in the clinical setting, and relies on animal studies and occasionalcase reports of critical events. Some methods, including thermodilution itself, have not performed well in thesereports. 20-22USE IN MANAGEMENT OF PATIENTSA growing number of studies have used these devices to influence patient management during surgery. Most ofthese studies have used the data to guide fluid administration and use of vasopressors. A few have included theuse of inotropes to improve cardiac output as well. Where fluid and drug administration is guided by specificmeasures of preload, cardiac output or afterload, the term “goal directed” management has been coined.CO = cardiac output, S = solubility coefficient of CO 2 in blood, = change in CO 2 elimination, =Dynamic indices such as SVV appear to be more reliable than static measurements of preload such as centralvenous pressure. 23 Along with systolic pressure and pulse pressure variation, 24 SVV appears to be a useful indexof intravascular volume depletion in patients ventilated at tidal volumes of 500 mL or more and who are in a regularcardiac rhythm. Kungys et al showed that SVV increase correlated with a reduction in cardiac index and transoesophagealechocardiography-based left ventricular end-diastolic volume in patients during acute normovolaemichaemodilution involving removal of 15% of estimated blood volume and subsequent replacement with colloid. 25The reliability of PPV or SVV in patients with severe ventricular dysfunction is still unclear, however.For both indices of preload and cardiac output measurement, changes in a given patient appear to be moremeaningful than does any isolated value. For this reason, fluid responsiveness has been central to most algorithmsusing these devices to guide management. Increases in stroke volume (SV) and cardiac output of 10% or moredecrease in PPV or SVV in response to fluid challenge are considered significant. If preload is judged to be adequateon the basis of a reduced response to fluid administration, hypotension is then treated with vasopressors, (and lowcardiac output is treated with inotropes in more aggressive protocols). A simple approach, targeting blood pressure,is given in Figure 5.rise in end-tidal CO 2 partial pressureFigure 5. A simple goal directed haemodynamic strategy targeting blood pressureACCURACY AND PRECISION:All of these technologies have been examined in numerous studies by comparison with other more invasive standardmethods, most commonly thermodilution. The accuracy (overall bias) and precision (scatter of measurement) ofMAP < 80 mmHg ?these methods is generally measured by making simultaneous paired measurements and calculating the meandifference (bias) between the measurements by the two methods and the standard deviation of the difference(precision), as described by Bland and Altman. 7 95% of measurements lay within 2 standard deviations either sideof the mean bias, and these are called the upper and lower limits of agreement. When divided by the mean cardiacoutput in the study, 2 standard deviations are known as the “percentage error”.The usefulness of this comparison depends on the accuracy and precision of thermodilution itself relative tothe true cardiac output. Older studies in the critical care setting which used the oxygen Fick method as the standardsuggested that the precision of thermodilution was within ± 20% of the true value. 8,9 However newer invasive toolsYesNohave become available, such as indwelling ultrasonic transit time flow probes which can be applied to the aorticSVV > 13% ?root or pulmonary trunk intraoperatively. Animal and human studies using these devices show that thermodilutionFTc < 350 msec?has poorer precision than this during haemodynamic instability and cardiac surgery and that it is not significantlySV |^> 10%?better than that of the less invasive methods outlined above 10,11A meta-analysis of published studies over 10 years combined pooled weighted data collected during surgeryand critical care and showed that these methods all had similar precision of agreement with bolus thermodilution,with percentage errors of between 40-45%. 12 Assuming that thermodilution itself has a similar level of precision,the percentage error of all these methods is likely to be around ± 30% relative to the true cardiac output. This iswider than the acceptable limits recommended by previous authors, 9 but likely to be comparable to the real precisionof thermodilution during cardiac surgery. 10 While much of the direct filling pressure data and enhanced vascularYesNo