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1 Introduction

Much of the evidence for the anesthetic management of morbidly obese (MO) patients is adapted from the management of normal-weight subjects. The profound changes in body composition and organ function in extreme obesity require strict perioperative monitoring. This chapter will discuss which monitors we consider essential for MO patients. As in normal-weight patients, the routine intraoperative monitors (blood pressure, ECG, pulse oximetry, temperature) are the same as those recommended by current anesthesia guidelines for all patients. However, the pathophysiology associated with obesity requires additional monitoring. Although only limited evidence is available, the rationale and existing factual data supporting the use of these monitors are reviewed.

2 Is Monitoring of Intraoperative Hypnosis Necessary?

Dosing anesthetics based on total body weight (TBW) may not be appropriate in obese patients as this could result in circulatory depression and prolonged recovery. The repercussions of excessive hypnosis should not be underestimated particularly in patients with underlying comorbidities. In normal-weight high-risk patients an association between a bispectral index (BIS) value <40 for >5 min and increased risk of both myocardial infarction and stroke has been described [1]. On the other hand inadequate depth of anesthesia may result in intraoperative awareness [2]. Studies on awareness have been conducted on normal-weight patients and indicate an 82% risk reduction using BIS in a high risk population.

The use of hypnosis monitoring devices has been advocated for MO patients undergoing bariatric surgery in whom early and uneventful postoperative recovery is a priority [3, 4]. In obese patients anesthetic depth monitoring is even more useful than in normal-weight patients since it allows establishment of an optimal dosing algorithm. Indeed overdosing of anesthetic agents during general anesthesia in obese patients is a very common scenario. Several studies demonstrate a reduction in consumption of different intravenous and volatile anesthetics using BIS monitoring [5].

Moreover, BIS-guided anesthesia proved superior to target-controlled propofol infusion in MO patients [6]. Attempting to apply a weight adjustment formula for the ‘Marsh’ pharmacokinetic model of propofol target controlled infusion (TCI) resulted in a clinically unacceptable performance bias. Therefore no weight adjustment formula should be used to correct the input weight in TCI systems. This problem appears to be partially corrected when no weight adjustment is used (i.e. relying on TBW). Yet this approach often results in an overestimation of real plasma concentrations in MO patients. We believe it is therefore advisable to titrate predicted plasma propofol concentrations to targeted processed-EEG values.

Obesity and in particular morbid obesity have been identified as a risk factor for perioperative respiratory events. A more accurate intraoperative titration of hypnotic agents is not only associated with improved awakening and extubation times, but positively influences the post-anesthesia recovery profile and spontaneous respiration of obese patients [3].

Although BIS is not the only commercially available device that can be used to assess adequate depth of anesthesia, it is considered the standard to which other systems of hypnosis monitoring are compared. Entropy and acoustic evoked potentials have been also described as guides to anesthesia depth. To date their use has not been extensively reported in the obese population.

Therefore, we believe that obese patients who are deemed at higher risk due to their concomitant underlying pathologies and in those for whom a favorable post-anesthesia recovery is a priority, will benefit from intraoperative depth of anesthesia monitoring. However, at the present time there is no evidence from randomized, controlled trials of a reduction in intraoperative or postoperative mortality using these devices.

3 Arterial Line or Non-Invasive Blood Pressure Monitoring?

Since obese patients may be at increased risk for pulmonary and cardiovascular complications during anesthesia, accurate hemodynamic monitoring might improve postoperative outcome in these patients. Oscillometric blood pressure measurements with a repeatedly inflated cuff are inaccurate on large upper-arm circumferences, and in many cases proper placement of a pressure cuff over the upper arm in a MO patient is not even feasible. Measurement via a radial artery catheter is the gold standard for blood pressure monitoring. However, placement of a radial artery catheter is often technically difficult in obese patients. Furthermore, arterial cannulation is not devoid of complications and can be costly. This has fueled a debate on whether invasive arterial pressure monitoring is an essential intraoperative monitor in obese patients. Bariatric surgery is usually performed today with limited blood loss. Thus, many anesthesiologists find an arterial line too invasive for this type of surgery.

The choice of intraoperative monitoring should also take into consideration postoperative management needs and potential complications. Invasive arterial cannulation enables beat-to-beat monitoring as well as access for blood sampling. Arterial catheters can be difficult to place in awake or anesthetized MO patients. Furthermore, arterial monitoring may lead to significant complications including vascular trauma, distal embolization, thrombosis and ischemia of distal extremities, bleeding at the site of insertion, patient discomfort, and nosocomial infection.

Despite the minimal complications and low costs associated with non-invasive blood pressure (NIBP) monitors, oscillometric measurement in the obese patient has several limitations. Their accuracy has also been called into question. The most commonly encountered difficulty is use of an improper size oscillometric upper arm cuff. Any reliable measurement NIBP is strictly dependent on the choice of a correct-sized cuff [7]. It is particularly important to place the center of the bladder over the brachial artery pulse. Overestimation of blood pressure when using a cuff that is too small on an obese arm can lead to an incorrect diagnosis of hypertension causing unnecessary concern or therapy.

Often, an arm cuff has to be mounted elsewhere due to a very large arm circumference combined with short upper arm length. Indeed such geometry may preclude ability to place an adult thigh cuff on the upper arm. An alternative approach is to place the cuff on the forearm and auscultate the radial artery while holding it at the level of the heart [8]. The accuracy of these methods has not been validated, but they provide a general estimate of the systolic blood pressure. Forearm measurement may overestimate both systolic and diastolic blood pressure [9].

Continuous NIBP monitoring at the wrist can be accomplished with commercially available devices. Wrist pressure was compared to upper arm oscillometric and invasive arterial blood pressure measurements in extremely obese patients (mean body mass index [BMI] 66.7 ± 13.8 kg/m2) [10]. The wrist device was more comfortable than an oscillometric cuff placed on the arm. The two NIBP systems gave comparable measurements. Although the average accuracy was good, individual mean wrist and standard NIBP often differed considerably from invasive arterial line measurements. The study confirmed the well-documented differences between oscillometric and invasive arterial blood pressure measurements. NIBP values fell within the range of ±15 mmHg of the concomitant invasively acquired values [10]. These results suggest that a wrist pressure monitor can be used as an alternative to NIBP but should not be substituted for an arterial catheter in super-obese patients. This investigation was performed on patients under stable perioperative conditions and so differences between invasive and NIBP measurements may be even greater in unstable patients.

Anesthesiologists should be aware of the limitations of NIBP monitoring in obese patients and the potential complications of arterial line placement. The gold standard for blood pressure monitoring remains direct arterial blood pressure measurement. Invasive monitoring is not essential for all procedures but should always be considered based on degree of obesity, pre-existing comorbidities, type of surgery, planned anesthesia, anticipated intraoperative and postoperative management (including the need for frequent blood gas and blood samples). The differences between NIBP and invasive pressure readings may increase in unstable patients.

4 Is There a Role for Central Venous/Pulmonary Artery Catheters?

Central venous catheters are generally used during surgery to measure hemodynamic variables, for delivery of medications, and for nutritional support. Central venous lines also allow easy and rapid blood sampling. In bariatric surgery, a central venous catheter may be necessary if there is difficulty with peripheral venous access. Such need should not be underestimated and can be anticipated during the preoperative anesthesia assessment. Central line placement can be scheduled and correct catheter position verified before surgery.

Rather than for monitoring purposes, central venous lines are usually placed to assure a reliable intravenous access for administration of medications and for high volume infusions. The medical literature on hemodynamic monitoring via central venous or pulmonary artery catheters in obese patients does not exist since there have been no randomized trials that have specifically considered this population.

In MO patients, placement of a central venous catheter may be difficult due to poorly identifiable neck landmarks. Anatomic variability of internal jugular veins is frequent in obese patients and the common diminished vein diameter (<10 mm) may cause difficulty with placement [11]. Catheters are often positioned after induction of anesthesia, but obtaining a chest X-ray to check for correct placement and the absence of pneumothorax is usually delayed until the end of surgery. Ultrasound guidance increases the success of placement and decreases the incidence of complications [11, 12].

There are no strict recommendations or sufficient evidence to consider central venous or pulmonary artery catheter placement in MO patients as an essential intraoperative monitoring tool. The need for advanced hemodynamic monitoring can be considered prior to scheduled surgery, and these catheters should be placed based on the degree of obesity, severity of associated comorbidities, and the need for a viable venous access. The anesthesiologist should weigh the risk/benefit ratio.

5 Thromboelastography

Obesity is a risk factor for postoperative deep vein thrombosis (DVT) and pulmonary embolism (PE), and postoperative DVT and PE are the most important causes of morbidity and mortality in bariatric patients [13, 14]. Despite the overall consensus to use any of several methods of pharmacological prophylaxis to prevent thromboembolic complications, the reported rates of postoperative DVT and PE range from 1 to 15% and approximately 50% of deaths occurring in bariatric patients are attributed to a PE [15].

Decreased fibrinolysis and increased fibrinogen concentration occur in obesity and may increase the risk of thromboembolic events. Studies from a non-surgical obese population have demonstrated increased plasminogen activator inhibitor (PAI) activity. A suggested explanation is augmented PAI release from the increased adipose tissue. Moreover, markers of both inherited and acquired thrombophilic state, including D-dimer, fibrinogen, factor VIII, factor IX, factor XI, and lupus anticoagulant elevation have been reported in obesity [13, 14].

Surgical stress may exacerbate an underlying prothrombotic predisposition. Indeed laparoscopic surgery causes variable serum hypercoagulability. Other data suggests that dependent positioning in combination with the pneumoperitoneum decreases venous flow from the lower extremities, possibly increasing the risk of developing DVT [15]. Despite the large number of prophylactic regimens, ranging from low-molecular weight heparin (LMWH) and intermittent compression devices to inferior vena cava filters, an optimal prophylaxis regimen is unknown. Measurements of laboratory indices of fibrinolysis such as PAI activity are impractical in the operating room. Commonly used blood tests are often nonspecific, whereas the dosage of all plasma factors involved in coagulation and platelet activity is expensive and mostly not useful to stratify the clinical risk of hypercoagulability.

Thromboelastography (TEG) can be used to monitor hemostasis as a whole dynamic process instead of revealing information of isolated conventional coagulation screens. Visco-elastic properties are analyzed by allowing whole blood to clot under a low shear environment resembling sluggish venous flow. Clot formation is graphically represented over time and, jointly with numerical parameters, describes the entire process from clot formation to fibrinolysis. Although visco-elastic measures of coagulation do not assess the role of the endothelium in the clotting process, the interaction of fibrin, fibrinogen, and platelets that is displayed is more reflective of in vivo conditions than the routinely performed laboratory tests that assess only time to fibrin formation. TEG is performed at actual patient temperature without applying a post hoc correction algorithm. In addition, the effect of heparin and LMWH can be easily subtracted by conducting the test in a heparinase enriched tube. Several parameters concur to describe the coagulation process such as the rate of initial fibrin formation, time to clot firmness, rate of clot growth, maximal strength of the clot depending on platelet function and interaction with fibrin (maximal amplitude [MA]).

This test is easily accessible to anesthetists and may be more practical for perioperative monitoring than isolated laboratory indexes of coagulation and fibrinolysis. Although the entire analysis may require as much as 30 min, point-of-care testing allows meaningful preliminary information within 8 min. TEG is increasingly used for decision making within blood product utilization protocols and has guided appropriate anti-fibrinolytic therapy in diverse clinical scenarios [16]. Overall TEG offers the possibility to monitor the coagulation status in a rapid, repeatable, and inexpensive manner.

A prospective study investigated the use of TEG to assess coagulation in MO and normal-weight patients intraoperatively [16]. The MO group demonstrated accelerated fibrin formation, flbrinogen-platelet interaction, and platelet function compared with normal-weight controls but no difference in fibrinolysis. The lack of any difference in the percentage fibrinolysis in the obese group suggests an imbalance predisposing to accelerated clot formation. These clinical data support the in vitro laboratory evidence of increased PAI activity and decreased fibrinolysis in this population. These differences were not reflected by routine laboratory measurements of coagulation (PT, PTT and platelet count). A more recent study evaluated whether TEG parameters were able to detect a hypercoagulability state in obese patients undergoing laparoscopic bariatric surgery and to monitor the effect of the perioperative antithrombotic prophylaxis. Again obese patients displayed a coagulation status of hypercoagulability mostly due to an increase of platelet activity and clot stability.

A perioperative increase in the MA parameter, as shown in studies on obese surgical patients, has been previously associated with a significantly higher incidence of thrombotic complications. Postoperative myocardial infarction was more frequent in patients with an increased MA value (>68 mm) compared to patients with normal MA, and in a multivariate analysis, increased MA was effective in predicting postoperative myocardial infarction [17]. Since the MA parameter is the best prognostic index of hypercoagulability associated with thrombotic complications, obese patients screened intraoperatively with TEG that show an increase in MA could benefit from either a higher dose of LMWH or the administration of antiplatelet drugs. Once again growing evidence points in the direction of individualized therapy based on direct monitoring and titration of drug effect in this specific case of intraoperative evaluation of thrombosis prophylaxis by TEG in order to minimize intraoperative risks and postoperative complications.

TEG is not the only commercially available device that evaluates whole blood coagulation. Although less studied on obese patients, Rotem and Sonoclot are examples of alternative systems that analyze the important interactions between all soluble hemostatic components plus platelet function that are essential to the clinical evaluation of hemostasis.

Operating room and postoperative visco-elastic measurement of coagulation in obese have demonstrated a hypercoagulability state not matched by a comparable increase in fibrinolysis. This suggests that routine coagulation tests may not reflect appropriately the in vivo coagulation state of MO patients. Monitoring and treatment of obese patients with increased risk of thromboembolic complications should be strongly encouraged since prophylaxis is recommended as part of routine anesthetic preoperative assessment. A standard prophylactic regimen of LMWH despite mitigating the hypercoagulable state failed to exert a significant protection based on what are currently considered strong endpoints (i.e. preventing the increase of MA).

6 Is Neuromuscular Blockade Monitoring Essential?

Monitoring neuromuscular transmission during anesthesia is essential when using neuromuscular blocking agents. Neuromuscular function can be evaluated by a nerve stimulator. In brief, an accelerator transducer attached to the distal interphalangeal joint of the thumb allows objective quantification of the response at the adductor pollicis to train-of-four (TOF) stimulations. The ulnar nerve is stimulated via surface electrodes at 15 s intervals. TOF ratio is defined as the ratio of the fourth response to the first response. TOF is the most commonly used method to assess onset of action, duration, and recovery from neuromuscular blocking agents. TOF monitoring is useful to titrate the dose of various agents, determine when to repeat administration, and to identify any residual muscle relaxation during recovery from anesthesia. Recovery from neuromuscular blockade is defined as a TOF ratio >0.9.

Obese patients are particularly susceptible to respiratory complications in the postoperative period, including airway obstruction, hypoventilation, hypercapnia, and hypoxia. Postoperative residual paralysis is one of the major etiologies for increased risk of these critical respiratory events. Even minimal degrees of neuromuscular blockade (i.e. TOF ratio <0.9) can result in functional impairment of the muscles of the pharynx and esophagus resulting in misdirected swallowing and even aspiration. Normal pharyngeal function is restored when a TOF ratio >0.9 is achieved [18–20]. Complete recovery of neuromuscular function is mandatory to avoid postoperative residual weakness. Unfortunately, laryngeal and pharyngeal muscles are among the last to recover after non-depolarizing muscle relaxants. Reliance on clinical signs and symptoms to determine reversal of neuromuscular function is not effective and only monitoring neuromuscular function can avoid residual curarization [21]. The best strategy for dosing neuromuscular blocking agents and to avoid residual paralysis is monitor-guided administration of both supplemental relaxant doses of and reversal agents with a twitch monitor.

Neuromuscular monitoring by TOF is a simple non-invasive inexpensive means for assessing neuromuscular function. Since MO patients are prone to postoperative complications including postoperative residual weakness, we feel that intraoperative monitoring of neuromuscular function is essential in MO patients.

7 Conclusions

Monitoring is the key to fill the gap between limited evidence and patient safety. Hence additional monitoring of such as intraoperative depth of anesthesia should be considered in patients deemed at higher risk due comorbidities and whenever a favorable post-anesthesia recovery is a priority. Yet there are no randomized, controlled trials assessing a reduction in intraoperative or postoperative mortality when monitoring intraoperative depth of anesthesia in obese patients.

The same applies to invasive blood pressure monitoring that is not essential for all procedures but should always be considered based on degree of obesity, patient assessment and planned perioperative management. The need for advanced hemodynamic monitoring should be guided by the very same principles with the addition of the requirement for a viable venous access.

Monitoring and treatment by visco-elastic measurement of coagulation of obese patients with increased risk of thromboembolic complications should be strongly encouraged since prophylaxis is recommended as part of routine anesthetic preoperative assessment and LMWH may fail to exert a significant protection. By the same token, neuromuscular monitoring by TOF should be strongly considered since MO patients are prone to postoperative complications including postoperative residual weakness.