Noninvasive Monitoring of Stroke Volume and Fluid Responsiveness
Hypotension during an operation, commonly defined as sustained mean arterial pressure less than 55mmHg or systolic blood pressure less than 80mmHg is a known risk factor for postoperative mortality, myocardial infarct, cerebrovascular accident, demand ischemia, acute kidney injury, and reperfusion injury.5,8 In fact, mortality increases by 1.036 times per minute of systolic pressure less than 80 mmHg according to a recent study.8 ASA guidelines for basic anesthetic monitoring currently require arterial blood pressure monitoring at least every five minutes unless extenuating circumstances dictate otherwise and are properly documented in the patient record.2 This has been shown to be insufficient in the case of moderate and high-risk surgeries in the critically ill, in cases with sustained intraoperative hemodynamic instability, and in many high risk surgeries regardless of patient ASA classification. It has recently been demonstrated that continuous blood pressure monitoring via either invasive or non-invasive means decreases the duration and severity of intraoperative hypotension, which has led to the increased utilization of continuous non-invasive hemodynamic monitoring (CNHM).8 Although invasive monitoring with a pulmonary arterial catheter and/or arterial catheter remain the gold standard, these newer devices have proven quite useful given their incorporation of technology that quickly and accurately assesses cardiac parameters such as cardiac index (CI) and stroke volume variation (SVV) continuously without direct vascular access. Such information, when gathered and interpreted correctly in the appropriate clinical setting, can prove invaluable in directing an anesthesiologist toward the proper intervention for intraoperative hypotension, be it fluid challenge or administration of pharmacologic support via inotropes or vasopressors.
Regarding the concept of fluid responsiveness and fluid challenge, the theoretical benefit to giving a fluid bolus in the setting of hypotension is augmentation of cardiac output via increased venous return to the heart. Given that anesthetized patients are typically hypovolemic due to a combination of NPO status, blood loss, insensible losses, and the fact that the venous circulation has high capacitance, being a fluid responder is a normal physiologic response. Fluid responders typically demonstrate augmentation of stroke volume by 10-15% with an appropriate fluid challenge.3 In the case a fluid challenge does not increase cardiac output, the effects of expanding intravascular volume are likely counterproductive. This is because extra-vascular lung water (EVLW) and tissue edema increase due to increased cardiac filling pressures as volume responsiveness decreases, an effect that becomes accentuated in the setting of pre-existing endothelial damage, as seen in sepsis and ARDS. We now know the increase in natriuretic peptides seen in volume overloaded individuals, leads to cleavage of the endothelial glycocalyx, predisposing the pulmonary vasculature to diffusion of water, increasing EVLW and the likelihood of perioperative mortality.6 Thus, ideally the anesthesiologist should use a low risk means of assessing volume responsiveness prior to administering fluid challenge to patients at high risk of volume overload. When given based on clinical assessment alone, it has been found that only 50% those receiving a fluid challenge will be volume responders. Goal-directed therapy, an essential component of the majority of enhanced recovery after surgery (ERAS) protocols, takes this dynamic into account, suggesting that fluid challenge decisions should be made in the context of a likely volume deficit when expected to improve hemodynamics in the absence of associated risk.3 Non-invasive hemodynamic monitors provide clinicians with the best chance at making the appropriate decision and minimizing risk.
In 1959, Hughes and Magovern described the use of central venous pressure as a surrogate for volume status and responsiveness, a concept which was utilized in practice for some time but has since been invalidated. By 1970, the pulmonary artery catheter was introduced by Swan and Ganz, providing better insight into fluid status by describing use of the pulmonary artery occlusion pressure (PAOP) based on its relationship to the left heart. Fluid responsiveness remained challenging however as it was shown later that PA pressures were poorly predictive of responsiveness. It was not until the 2000s, that pulse pressure variation (PPV) and stroke volume variation (SVV) were shown to correlate with fluid responsiveness, with a value >13% being considered a significant threshold. However, later studies demonstrated that <3% of patients in a critical setting actually met validity criteria for use of PPV as a surrogate. Current evidence suggests that patients with significant SVV or PVV should have their volume status correlated via other tests such as passive leg raise, which is generally undesirable intraoperatively for a number of reasons. In the operative setting, fluid challenge can be used as a test and response correlated with use of minimally invasive hemodynamic monitors if available.6,8 The gold standard for assessment of stroke volume and fluid responsiveness in the anesthetized patient has subsequently become the use of a pulmonary artery catheter or concurrent use of central venous catheter and arterial line for a PiCCO (pulse index continuous cardiac output) monitor, however in the past few decades, noninvasive means have been developed and expanded in their accuracy and utility.4,8 This was an impactful breakthrough, as invasive methods are associated with several notable risks related to both the procedural placement of the device, its operation, and removal. In addition, these devices are limited to use within a critical care or operating room setting to severely ill patients and/or high-risk surgeries as the risk of its use must be weighed against potential benefit and utility of the data it provides. There are several options currently in use, however the Edwards LifeSciences ClearSight is perhaps the most commonly used and widely studied, thus we will describe it for the purpose of this review.
The ClearSight is a finger cuff monitor with built-in photoelectric plethysmography similar to that used in continuous pulse oximetry, however it incorporates physiocal and volume clamp methods to the design for calculation of its some of its hemodynamic parameters. The internal algorithm converts finger cuff pressure to a brachial pressure, which is then processed using the pulse contour method to provide a cardiac output. It has been shown to be useful in more than 85% of operating room and critical care patients, however in cases of extremely poor peripheral flow (i.e., vasculopathy, high dose vasopressor use, presence of VAD, etc.) the device may not function properly.1 In general, device setup is very simple, can be performed by nursing staff, and takes about five minutes. While the data obtained using ClearSight is not completely interchangeable with invasive means due to a slightly higher standard deviation, there is conflicting data regarding this finding. It demonstrated high concordance with invasive means regarding its ability to track trends and directional change, and data was within 10% agreement limits >90% of the time. In a recent review, the noninvasive CNAP agreement with invasive arterial monitoring was reported as 94% for systolic pressure and 99% for mean arterial pressure. Multiple studies agree on the noninferiority of noninvasive continuous monitors like ClearSight and CNAP when compared to other noninvasive means and compared to invasive means, and agree their use is appropriate in patients at risk for hemodynamic instability where close monitoring is needed but invasive means pose unreasonable risk.8
While the ClearSight is perhaps the most popular, there are various methods of noninvasive hemodynamic monitoring available including: CNAP, T-Line, DMP-Life, etc., all of which use varied means of providing hemodynamic data. As a result, they cannot be assumed to be equivalent in terms of accuracy, precision, resolution, or choice of parameters. While invasive arterial monitoring and placement of a pulmonary arterial catheter remain the gold standard for assessment of advanced cardiac data, noninvasive means have been developed and deserve careful consideration, especially in high-risk patients such as those who are coagulopathic, vasculopathic, septic, immunocompromised, or prone to malignant arrhythmia. They provide comparable data at less cost, less risk, and can be utilized with increased duration. The use of continuous noninvasive monitoring can be expected to expand with time, likewise their accuracy, reliability, functionality should improve with subsequent iterations as new technology is developed and incorporated.8 While invasive monitors will always have a place in the critical and emergent settings, noninvasive devices should continue to phase-in as we continue to find settings in which advanced hemodynamic data are needed without exposing the patient to the risks of establishing central or arterial access.
References
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2. ASA Committee on Standards and Practice Parameters (CSPP). Standards for basic anesthetic monitoring. https://www.asahq.org/standards-and-guidelines/standards-for-basic-anesthetic-monitoring Web site. https://www.asahq.org/standards-and-guidelines/standards-for-basic-anesthetic-monitoring. Updated 2015. Accessed Mar 15, 2020.
3. Kendrick JB, Kaye AD, Tong Y, et al. Goal-directed fluid therapy in the perioperative setting. J Anaesthesiol Clin Pharmacol. 2019;35(Suppl 1):S29-S34. Accessed Mar 16, 2020. doi: 10.4103/joacp.JOACP_26_18.
4. Litton E, Morgan M. The PiCCO monitor: A review. Anaesth Intensive Care. 2012;40(3):393-409. Accessed Mar 16, 2020. doi: 10.1177/0310057X1204000304.
5. Mahehshwari K, Kanna S, Bajracharya G, Makarova N. A randomized trial of continuous noninvasive blood pressure monitoring during noncardiac surgery. Anesthesia and Analgesia. 2014;127(2):424-431.
6. Marik PE, Lemson J. Fluid responsiveness: An evolution of our understanding. Br J Anaesth. 2014;112(4):617-620. Accessed Mar 16, 2020. doi: 10.1093/bja/aet590.
7. Martina JR, Westerhof BE, van Goudoever J, et al. Noninvasive continuous arterial blood pressure monitoring with nexfin®. Anesthesiology. 2012;116(5):1092-1103. Accessed Mar 16, 2020. doi: 10.1097/ALN.0b013e31824f94ed.
8. Pour-Ghaz I, Manolukas T, Foray N, et al. Accuracy of non-invasive and minimally invasive hemodynamic monitoring: Where do we stand? Ann Transl Med. 2019;7(17):421. Accessed Mar 16, 2020. doi: 10.21037/atm.2019.07.06.