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PRECISION NEUROCRITICAL CARE: MINIMALLY INVASIVE HEMODYNAMIC MONITORS By Fawaz Al-Mufti, MD The authors have no actual or potential conflict of interest in relation to the topics discussed in this column. This article may discuss non-FDA approved devices and “off-label” uses. The NCS and Currents do not endorse any particular device. Hemodynamic monitoring is a keystone of the management of critically ill neurologic patients. Adequate brain resuscitation cannot be done efficiently without addressing the hemodynamic status during early phase of brain injury. Hence, hemodynamic optimization is integral for delivery of oxygen and ensuring optimal cerebral metabolic rate of oxygen consumption in brain injury patients. Markers of end-organ hypoperfusion, can be divided into indices of global hypoperfusion (hypotension, tachycardia, oliguria, delayed capillary refill, clouded sensorium, elevated blood lactate, low mixed venous O2 saturation) and indices of regional hypoperfusion (myocardial ischemia, decreased urine output, increased blood urea nitrogen and creatinine, elevated transaminases, increased lactate dehydrogenase, increased bilirubin). These indices reflect inadequate oxygen delivery and should prompt interventions in order to achieve optimal MAP and cardiac output. The Swan–Ganz catheter and similarly designed thermodilution catheters have served as valuable tools for calculating cardiac output. A number of studies, however, demonstrated little or no change in clinical outcome and this has resulted in their declining use. Despite this, thermodilution catheters are still considered the gold standard for cardiac output measurement, against which all new monitors are measured. Minimally invasive cardiac output monitors have varying degrees of ‘invasiveness’ with some being totally non-invasive and others only marginally less invasive than a pulmonary artery catheter (PAC). Herein we will discuss two examples of cardiac output monitors used in the Neuro-ICU, the FloTrac/Vigileo (Edwards Lifesciences, Irvine, CA) and PICCO (Phillips Healthcare, Andover, MA) devices. Commercially available, minimally invasive, continuous cardiac output monitoring systems available in United States such as the FloTrac/Vigileo (Edwards Lifesciences, Irvine, CA) and PICCO (Phillips Healthcare, Andover, MA) devices, both of which relate the contour of the arterial pressure waveform to stroke volume and systemic vascular resistance in order to determine the cardiac output and produce a continuous reading. Additionally, they can display a range of user-determined physiological variables, including cardiac output, cardiac index, and stroke volume variation. Stroke volume variation (SVV) is the difference between maximum and minimum stroke volumes over the respiratory cycle and is caused by changes in preload with alterations in intra-thoracic pressure. SVV can be used as an indicator of fluid responsiveness. Patients with an SVV of <10% are unlikely to be fluid responsive, whereas those with an SVV of 15 percent or greater are likely to benefit from fluid resuscitation (Table 1). As SSV may vary during different phases of respiration (due to preload variation during inspiration and expiration), SVV is considered more reliable in patients who are being sedated and mechanically ventilated on a set rate without any added spontaneous breaths. The PICCO system further calculates extravascular lung water (EVLW) and global end-diastolic volumes based on a transpulmonary thermodilution curve. PICCO (PULSE INDEX CONTINUOUS CARDIAC OUTPUT) DEVICE The PiCCO system (Pulsion Medical Systems, Munich, Germany) uses a thermistor-tipped arterial line in a proximal artery to measure the aortic trace waveform morphology. An algorithm is used to determine the cardiac output by integrating the area under the curve of the arterial pressure versus time trace. A central venous catheter is used to calibrate the system using a transpulmonary thermodilution technique in order to calibrate the pulse contour PiCCO monitor. The thermodilution equation is used to calculate the cardiac output. Other variables can also be measured, including global end-diastolic volume as a measure of preload and EVLW as a measure of pulmonary edema. There are several disadvantages of the PiCCO device and these include the need for recalibration with changes in position, therapy or condition to account for compliance of vascular bed. Additionally, in obese patients, EVLW is underestimated as related to weight of patient and in patients with severe aortic valve regurgitation readings may be inaccurate due to thermodilution wash out. Finally, it should be mentioned that EVLW is only measured in parts of the lung that are perfused (underestimated post-pneumonectomy). 31


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