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1Department of Anesthesiology and Perioperative Medicine, University of California, Los Angeles, Los Angeles, CA, United States

1Department of Anesthesiology and Perioperative Medicine, University of California, Los Angeles, Los Angeles, CA, United States

Surgical volume grows exponentially each year. It is estimated that over 350 million surgeries are performed each year worldwide.1 The sheer number of high-risk patients and their concomitantly complex surgical procedures are, in part, accountable for the multiplying and expanding deployment of health-care resources.2

High-risk patients and high-risk surgeries are two variables that contribute to the increase in postoperative complications.3 While ambulatory surgical centers are the fastest growing providers, more than 51 million inpatients procedures are performed annually in nonfederal hospitals in the US and in-patient surgery centers, both with ever-increasing caseloads of sicker and older patients. This is extremely relevant because 5.7 million Americans are admitted to an intensive care unit in one year, while more than 60 millions undergo surgery annually.1,4,5 Intraoperative mortality is rare due to significant improvements in surgical techniques, anesthesia management, and intraoperative monitoring, but sadly global postoperative mortality remains the third leading cause of death in the US.6,7 Recent studies have demonstrated that while the incidence of postoperative major complications after major surgery is similar between hospitals (~20%), the postoperative mortality following major complications from one hospital to the other can be up to 2.5-fold higher.8 This suggests that reducing variations in mortality following major surgery will require strategies to improve the ability of hospitals with high-mortality rates to manage major postoperative complications and decrease “failure-to-rescue”8–11 numbers, but also the need to further optimize intraoperative management in order to decrease the global incidence of postoperative complications. Postoperative morbidity affects, both short and long-term outcomes, leading to increased hospital stay and health care expenditure.12,13 Intraoperative hemodynamic optimization can have a significant impact on short- and long-term sequelae. Specifically, hemodynamic monitoring and more advanced functional monitoring (depth of anesthesia monitoring, predictive monitoring, and non-invasive monitoring) allow for larger patient populations to be monitored and optimized continuously with improved postoperative patients’ outcomes.

In this focused narrative review, we explore how new and non-invasive monitoring modalities have the potential to improve postoperative outcome of surgical patients.

Blood pressure has become one of the most important vital signs evaluated in the perioperative setting. The ASA monitoring standards recommend that blood pressure should be monitored every 5 minutes during the intraoperative period. Similar recommendations were incorporated in the World Health Organization’s Guidelines for safe surgery in 2009.14 Hypertension and hypotension can impair function of vital organs such as brain, heart and kidneys. Several studies have demonstrated associations between low mean arterial pressure (MAP), organ injury, as well as, 30-day mortality.15 How best to characterize hypotension remains unclear. There is no universal definition of hypotension and no consensus can be reached regarding the optimal range for intraoperative blood pressure.16 Some studies show improvement in postoperative outcomes when an individualized blood pressure management strategy is used during surgery that is tailored to the individual patient (reduction from preoperative value).17 Others argue that associations based on relative MAP are no stronger than those based on absolute thresholds (intraoperative MAP).18 Also, not only the depth of hypotension but also the cumulative time spent in hypotension can be associated with worse outcomes.16,19,20 Intraoperative hypotension should be a dynamic phenomenon depending on various factors, rather than dichotomizing blood pressures based on arbitrarily chosen thresholds.18

Due to the potential devastating consequences of intraoperative hemodynamic disturbances, advanced monitoring tools are used in many high-risk surgical patients without well-defined goals to prevent or mitigate devastating consequences. In recent years there has been a trend to maintain MAP higher than 65 mmHg to avoid adverse outcomes.16 During general surgery, MAP values which decreased more than 30% from baseline blood pressure were associated with increased postoperative ischemic stroke.19 In patients undergoing cardiopulmonary bypass, MAP values are an important therapeutic hemodynamic target that has the potential to reduce the incidence of stroke.21 Higher MAPs during cardiopulmonary bypass can be achieved in a technically safe manner and effectively improve outcomes after surgery.22 But does one size serve all? What type of monitoring is adequate? Should cardiac output (CO) be invasively monitored in all high-risk patient or high-risk surgeries?

Accurate monitoring and optimization of cardiac output is a central component of perioperative management in patients undergoing cardiothoracic and vascular surgery. CO is the product of heart rate (HR) and stroke volume (SV) and the main determinant of organ perfusion. Of note, CO is a determinant of MAP, where MAP can be calculated from:

CO is central to perfusion in every organ, including the brain. In the normal adult, cerebral perfusion pressure (CPP) is variable, ranging between 70 and 90 mmHg with cerebral blood flow (CBF) constant:

Acute changes in MAP can have consequences in CBF. An imbalance in CBF can lead to changes in cerebral metabolic rate of oxygen (CMRO2) and ultimately ischemia:

Changes in MAP values are common during the intraoperative period and of multifactorial origin (anesthesia induction, vasoactive drug administration, surgical stimulation or bleeding). Oscillometric techniques allow for the measurement of a reasonably accurate MAP (in normal blood pressure ranges) and the possibility of having an automated tool to determine a patient’s blood pressure at preset intervals.23 With intermittent blood pressure monitoring, short detrimental periods of hypotension can be overlooked leading to changes in vital organ’s perfusion.24 Another disadvantage is overestimation of low and underestimation of high values, added with the possibility of artifact values due to an individual patient’s morphology. The direct measure of blood pressure via arterial cannulation is regarded as the clinical reference method. Cannulation of an artery can be time-consuming, needs to be performed by a trained operator and is associated with potential complications such as infection, thrombus formation, embolism, lesion of nerves or vessels, and ischemia.25 Although the risk of arterial cannulation is not very high,26 this type of technique is generally reserved to high-risk patients presenting with major comorbidities or for high-risk surgeries. Arterial lines are also often inserted not only for blood pressure monitoring but also to facilitate blood sampling. Until recently, accurate hemodynamic monitoring was only possible by using invasive or labor-extensive techniques (echocardiogram) that are not always applicable in the operating room.

By 2026, one can predict that 75% to 95% of all vascular lesions requiring treatment will undergo an endovascular procedure.27 These non-invasive procedures will require accompanying non-invasive monitoring. As we move toward less and less invasive surgical techniques, researchers developed continuous non-invasive monitoring tools and other types of adjuvant monitoring such as electroencephalography (EEG) and cerebral oximetry.

Several monitoring guidelines have been proposed which presumably lower risks.28,29 Depth of anesthesia has also been proposed to rationalize drug use and avoid side effects from high anesthetic effects.30 In patients undergoing major non-cardiac as well as cardiac surgery, the implementation of hemodynamic and EEG monitoring was associated with improvement in postoperative outcome.29,31

Available non-invasive monitoring techniques can be found in figure 1.

Available non-invasive monitoring techniques.

New non-invasive continuous blood pressure technologies combine the advantages of both oscillometric non-invasive cuffs and arterial catheters. Indeed, a panoply of monitoring techniques have flooded the field and are in desperate need of validation studies. Unlike the occlusive technique used in standard oscillometric pressure cuffs, continuous non-invasive arterial pressure methods are based on non-occlusive pressure transduction over the vessel wall that rely on two major principles: volume clamp/vascular unloading or applanation tonometry. Other techniques to determine CO non-invasively are also available. By analyzing changes in thoracic bioimpedance and reactance during one cardiac cycle, it is possible to estimate SV. This method measures the impedance or reactance between a pair of electrodes places on the chest wall or on the tracheal tube.32 Using a dedicated rebreathing loop on mechanically ventilated patients, it is possible to measure CO by calculating carbon dioxide metabolism with partial rebreathing technology.

All hemodynamic systems have unique characteristics in term of accuracy, precision, validity, stability, and reliability.32 Numerous validation studies have been performed under various conditions. A recent meta-analysis concluded that the inaccuracy and imprecision of continuous non-invasive arterial pressure monitoring devices were larger than what was defined as acceptable by the Association for the Advancement of Medical Instrumentation.33 The authors cautioned the use of these new technologies until a more rigorous methodology and presentation of method-comparison studies are produced, allowing physicians a better vantage to assess the dependability and optimal use value of these devices.34

In order to effectively optimize hemodynamic interventions for a patient, it is important that these changes are identified as early as possible. Continuous non-invasive arterial pressure monitoring has the potential to decrease the duration of intraoperative hypotension and hypertension compared to conventional intermittent blood pressure (cuff) monitoring.24 Using continuous non-invasive arterial pressure devices Chen et al demonstrated that for every hour of surgery this technique has the potential to identify an average of 14 minutes of treatable hypotensive and hypertensive time.24 Several studies show the potential to decrease the moments of hemodynamic disturbance and increase blood pressure stability using these devices in different patient/surgical populations.35–37 These techniques offer the possibility to positively impact clinical outcomes, allowing for a more accurate and continuous picture of the patient’s hemodynamic status and/or decreasing the risks associated with invasive devices.24 Of note, continuous non-invasive arterial pressure devices are sensitive to patient movement, and in case of severe vasoconstriction, peripheral vascular disease or distorted fingers due to arthritis: in other words, it may be difficult to obtain a valid waveform using finger cuffs.

The benefit of continuous non-invasive arterial pressure devices is not only in detecting changes in blood pressure. Given the reconstruction of the arterial curve, a beat-to-beat analysis of hemodynamic variables and/or their induced fluctuations are inevitably part of the displayed information. Dynamic variables of fluid responsiveness, such as pulse pressure variation (PPV), have been shown to accurately predict the response of a fluid challenge in mechanically ventilated patients.38 While traditionally PPV was measured using invasive techniques, it has been extensively studied using continuous non-invasive arterial pressure devices. Non-invasive assessment of PPV seems valuable, reliable, and accurate in predicting fluid responsiveness.39–41 It can allow for effective fluid management by guiding volemic status and coordinating the use of fluid and vasoactive drugs.

Blood pressure and flow are global hemodynamic indicators and possess limited information about end-organ perfusion and tissue metabolic well-being. The simultaneous use of continuous non-invasive arterial pressure devices can complement these global indicators with the use of EEG and brain oximetry devices, all without adding harm to the patients.28 Transcranial doppler is a non-invasive technique that measures local and regional CBF, but just like cardiac echocardiogram, is labor-extensive and not always applicable in the operating room.

EEG is a test that attaches sensors to the scalp in order to provide information regarding the electrical activity of the outermost layers of the brain. The waveforms recorded reflect the cortical electrical activity of the patient. The standard polysomnography study uses 20 electrodes placed on the head, with difficult and time-consuming setup to be practical in the operating room. Leaps in increased computer power and smaller size were critical technologies that aided in the development of processed EEG modalities. Machines such as the Bispectral index (BIS™),42 SedLine®,43 GE Datex-Ohmeda Entropy™,44 Narcotrend®-Compact M,45 Cerebral State Monitor (Danmeter) and Neurosense (Neurowave) are well-studied EEG-based monitors that, when coupled with standard clinical practice, offered quicker setup and access to EEG waveforms and spectrograms during the intraoperative period.46 Combined with hemodynamic monitoring, these monitors can provide information regarding depth of anesthesia and provide a window into mechanisms of consciousness and awareness. The cogency of relating general anesthesia to an EEG pattern depicted by spindles and delta waves is strongly suggestive of an unconscious state.47 Using EEG, it is possible to directly measure the effect of drugs administered, individualize anesthetic dosing, and ultimately be used to improve outcomes.30,31,48–50

The complexity and sensitivity of the EEG signal led manufacturers to devise an EEG-derived index from proprietary algorithms. These indices are generated from the EEG waveform but provide little neurophysiological information. Further, they might not accurately embody the pragmatic clinical condition due to artifacts from surrounding electrical devices, inter-individual variability of baseline EEG characteristics, delay times between EEG procurement, and display. To date, there is no rigorous evaluation of the processed values these monitor systems provide. Importantly, future technologies, and iterations thereof, will need to provide a clearer picture of specific and complex clinical conditions, including hypothermia, hypoglycemia, dementia, cortical atrophy, advanced age, seizures, carotid clamping, and cerebral ischemia. Raw EEG varies widely from patient to patient, and unprocessed EEG patterns and spectrograms shift when differing concentrations of anesthetic are administered among patient cohorts, including age-related conditions.46

Cardiovascular monitoring alone might not be able to recognize local tissue ischemia, particularly cerebral hypoperfusion. While EEG might help in adequately titrate anesthetic administration, when faced with situations of low tissue perfusion, the wave form might be distorted and of little value.

The addition of near-infrared spectroscopy (NIRS) technology offers an accurate measurement of tissue oxygenation (regional oxygen saturation, rSO2) in the brain: evaluating the electromagnetic spectrum where photons are capable of penetrating deeper tissues and also capturing changes to oxygenated and deoxygenated hemoglobin. Approximately 75% of CBF is venous. NIRS oximetry gives a measure of predominately venous oxygenation, which reflects oxygen extraction. Transcutaneous NIRS is relatively non-invasive and safe, using NIR light (700 – 900 nm) where absorption from skin and bone is low.

NIRS monitors are available from several companies: Nonin Medical Inc., Covidien, Medtronics, and Masimo. Depending on the device, the NIR light issues from the probe itself, while in others it comes from within the monitor module. These monitors are quick to detect imbalances between cerebral oxygen supply and demand, specifically in high-risk individuals with deficient cerebral oxygen delivery, including vascular and cardiothoracic patients.51

Monitoring and maintenance of rSO2 (within 10 to 20% of baseline) safe guards the maintenance of oxygen supply and tissue oxygenation, thereby reducing complications.52 NIRS-based cerebral oximetry is used increasingly in cardiothoracic and vascular anesthesia, providing clinicians with timely and pertinent information. Prolonged intra-operative cerebral desaturations are associated with adverse neurologic outcomes and prolonged hospital stay after cardiac surgery.53 Interventions guided by rSO2 are associated with the improvement of outcomes.53–56

The ASA physical status classification system, created in 1941, is a system for assessing the fitness of patients before surgery.3 Most cardiothoracic and vascular patients fall into the ASA 3 and 4 categories, usually associated with increased postoperative complications and mortality rates.3 Perioperative outcomes are complex and related to several circumstances: patient-related factors as well as surgical-related factors. When appropriate hemodynamic management is individualized, using appropriate tools and timing, clinician decision making and efficacious modifications can positively influence outcomes.

Continuous non-invasive monitoring tools, described above, may potentially improve patient care. On one hand they can replace invasive techniques and improve monitoring in low-risk patients, or alternatively, be employed in high-risk patients undergoing less-invasive or lower-risk surgeries (figure 2).

Choice of adequate hemodynamic monitoring equipment according to patient and procedure risk.

ASA, American Society of Anesthesiologists physical status.

The literature is very consistent in showing the detrimental outcomes of intraoperative hypotension. Periods of hypotension as short as a few minutes can adversely affect organ function.57 Non-invasive monitoring has been studied and validated in the cardiothoracic and vascular surgery settings.34,58–66

Perioperative stroke is a catastrophic complication of surgery. 1.3-2.2% of coronary artery bypass grafting procedures are complicated by stroke. Albeit at lower rates, stroke after noncarotid vascular surgery is also a too common reality and no clear readily modifiable risk factors have been determined.67 Sun et al. found that CPB MAP < 64 for more than 10 minutes was associated with perioperative stroke, along with other risk factors.21 The study’s findings suggest that MAP may be an important intraoperative therapeutic hemodynamic target to reduce the incidence of stroke in patients undergoing cardiopulmonary bypass.21

Cognitive decline diagnosed up to 30 days after the procedure (delayed neurocognitive recovery) and up to 12 months (postoperative neurocognitive disorder) is a common reality, particularly after cardiothoracic and vascular surgery.68 Also, the occurrence of postoperative delirium69,70 during hospitalization doubles a patient’s risk of post-discharge institutionalization and increases risk of dementia 10-fold.71 The incidence of delirium after cardiac surgery can reach up to 50% and is a strong independent predictor of mortality up to 10 years postoperatively.72–74 Patients who develop delirium have greater decline in a composite measure of cognition and in visuoconstruction and processing speed domains at 1 month.73

Among precipitating factors, depth of anesthesia, presence of electrical burst suppression patterns on EEG,30,31,48–50 cerebral oxygen desaturation,75 and hypotension76 or blood pressure fluctuation77 have been suggested to increase the incidence of perioperative cognitive disorders. The use of intraoperative brain monitoring, through processed EEG devices, allows for assessment of brain electrical activity. By failing to provide adequate CBF to affected areas (by intermittent changes of the hemodynamic parameters during anesthesia), the potential for even further neuroinflammation and brain tissue damage exists. Postoperative cognitive outcome was significantly better in patients with intraoperative cerebral oximetry monitoring. Prolonged rSO2 desaturation is a predictor of cognitive decline and should be avoided.20

Acute kidney injury is a common perioperative complication for patients undergoing both cardiac and vascular surgery. It has been suggested that surgical patients receiving perioperative hemodynamic optimization are at decreased risk of renal impairment.78 High-risk patients, such as vascular and cardiac patients, are at an increased risk of developing AKI and can benefit from perioperative optimization78.

There is an increase in acute kidney injury in non-cardiac patients when MAP values are below 60-65 or more than 20% from baseline for extend periods of time.18,79 In addition, more and more of this patient population (elderly with multiple cormorbidities) who receive complex cardiovascular surgery, which predispose them toward the development of acute kidney injury and even end stage renal disease.80

After cardiac injury, acute kidney injury incidence can reach up to 36.1%,81 with 1–5% needing dialysis, with mortality rates approaching 80%.82 The pathogenesis of acute kidney injury after cardiac surgery is multifactorial. Hemodynamic (diminished renal blood flow, loss of pulsatile flow), inflammatory, and nephrotoxic factors are responsible and overlap one another in leading to kidney injury.83 In low-risk procedures that require contrast agents, continuously monitor non-invasively blood pressure and calculate PVI can estimate patient’s volemia and potentially minimize the risk of contrast nephropathy.

The use of continuous non-invasive arterial pressure devices for intraoperative hemodynamic care offer improvements for patient care and should be considered a central feature of current and future Enhanced Recovery programs.84 Individualized and tailored blood pressure monitoring might be used to advance the positive outcomes for a wide spectrum of patients, from low- to high-risk. Intermittent oscillometric blood pressure measurements are usually sufficient for stable and low-risk patients. Patients who are at risk for hemodynamic instability should be monitored with continuous blood pressure measurement. Whether continuous non-invasive arterial pressure monitoring can improve patient outcomes, in certain patient collectives or clinical settings (perioperative medicine, emergency medicine), is the subject of current clinical research.85

Current technologies enable a much wider application of continuous monitoring, leading to the decrease of blood pressure fluctuation and length of hypotensive periods. Sophisticated analysis of the arterial blood pressure curve allows clinicians to monitor both blood pressure, blood flow, and guide volemic status. Future advances in cardiothoracic and vascular surgery and vascular disease treatment will depend on advances in other fields as well. As always, the development in medical care and practice parallel the major trends in the advancement of technologies like artificial intelligence, nanotechnology, and computing power. We are fortunate to have experienced vascular practice in this era, where the pace of development has been both astonishing and ground breaking.

For patients undergoing surgical procedures, the appropriate method of blood pressure monitoring needs to be identified by use of a perioperative cardiovascular risk stratification system. There is a growing body of evidence that shows how continuous monitoring can be beneficial in terms of blood pressure stability. Whether continuous non-invasive blood pressure monitoring associated with advanced functional monitoring (EEG monitoring, cerebral oximetry, predictive monitoring) can improve outcomes in certain patient populations, within the clinical settings, it remains and important subject for further research.

Further, technical advancements may help to overcome the practical limitations of current CO monitoring technologies with the refinement of mathematical assumptions and improved algorithms. Cutting edge technologies may lead to better hypotension recognition, or even predictive software, improving the ability to maintain hemodynamic stability and the preservation of vital organs.

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