Accurate blood pressure measurement is essential for the perioperative care of all patients, including those who are obese. Anesthetic care involves preoperative evaluation and optimization of the patient, followed by intraoperative delivery of anesthesia and monitoring of oxygenation, ventilation, and hemodynamic parameters. This close monitoring must continue postoperatively until complete recovery from anesthesia. Accordingly, accurate blood pressure measurement is considered essential throughout the perioperative period—however, the accuracy of standard blood pressure monitoring equipment is known to be compromised by obesity. Because adults who are overweight or obese comprise two-thirds of the population in Western countries, a significant number of patients are likely to have inadequate blood pressure monitoring during their perioperative care.
This article will discuss the relevance of blood pressure management in the care of patients with obesity, the currently available methods for perioperative monitoring of blood pressure and opportunities that exist to improve the perioperative blood pressure care in patients with obesity undergoing surgical procedures.
THE OBESITY EPIDEMIC
The prevalence of obesity is increasing worldwide, and without significant public policy change, the prevalence is expected to continue rising.1 In high-income countries, the body mass index (BMI) has shown the greatest increases since 1980 in the United States, Australia, New Zealand, and the United Kingdom.2 Bariatric surgery is recommended by the International Federation for the Surgery of Obesity and Metabolic Disorders3 to reduce the complications of obesity in patients with a BMI of >40 or >35 kg/m2 with coexisting comorbidities. Anesthesiologists in medium- and high-income countries can expect to be caring for an increasing proportion of obese individuals for both bariatric weight loss procedures and nonbariatric surgery.
THE RELATIONSHIP BETWEEN OBESITY AND HYPERTENSION
Individuals with obesity are ≤6 times more likely to have coexistent hypertension than lean individuals.4 There are many ways of measuring obesity, however BMI, classified by the World Health Organization,5 is the most commonly used in clinical practice. The highest World Health Organization classification is class III obesity (BMI ≥40 kg/m2), and there is no uniformly accepted classification of body mass indices >40 kg/m2. Data from Australia1 and the United States6 demonstrate that the prevalence of adults with a BMI >40 kg/m2 has steadily increased since 1980. The increase in morbidity and mortality associated with increasing BMI, including that associated with hypertension, is well documented.7 Hypertension, part of the metabolic syndrome, is 1 of 5 variables included in the Obesity Surgery Mortality Risk Score (Table 1)8 and is also included in perioperative surgical risk calculators such as the American College of Surgery National Surgical Quality Improvement Program Surgical Risk calculator, which is not specific to bariatric surgery.9
The importance of hypertension as a modifiable cardiovascular risk factor was highlighted in the 2017 American Heart Association Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults.10 This document lowers the previously defined level at which blood pressure is considered elevated to a systolic of 120–129 mm Hg (Table 2). Hypertension in obesity is most likely a consequence of increased sympathetic activation11 and the proinflammatory state induced by enlarged fat cells. These cells secrete an excess of proinflammatory peptides, while anti-inflammatory substances are reduced.7 However, the presence of hypertension may also indicate the presence of obstructive sleep apnea, another obesity-related condition, arising due to excess pharyngeal fat. Along with type 2 diabetes mellitus and hypercholesterolemia, hypertension presents a modifiable risk factor, the treatment of which can reduce the subsequent development of cardiovascular and cerebrovascular disease.12 Weight loss (including that secondary to bariatric surgery),11 increased physical activity, dietary modification, and pharmacological treatments have all been shown to reduce hypertension.10 The safe implementation of the various pharmacological treatments available requires an accurate method of blood pressure monitoring and medication dose titration.10
METHODS OF BLOOD PRESSURE MEASUREMENT
Noninvasive measurement methods are used to monitor the blood pressure of outpatients and general ward inpatients. Riva-Rocci13 reported the use of a mercury sphygmomanometer in 1896, a technique in which the systolic pressure only was determined by palpation of the radial pulse. Identification of the Korotkoff14 sounds in 1905 permitted identification of the diastolic pressure. While blood pressure is still commonly measured by auscultation, intermittent automated oscillotonometric devices are also widespread in clinics and hospitals. The first widely used device was the Dinamap (Applied Medical Research, Tampa, FL), which was introduced in 1976 and used a microprocessor to calculate the mean arterial pressure according to changes in the arm-cuff pressure caused by the pulsating brachial artery below.15 Later models, similar to those used in current practice, incorporated an algorithm and provided the systolic and diastolic pressures.16 Whether using auscultatory or automated techniques, the use of an appropriately sized arm cuff influences the accuracy of the measurement results. Automated oscillotonometric devices are commonly used by anesthesiologists in the preoperative, intraoperative, and postoperative periods.
While generally considered a safe monitoring device, intermittent noninvasive blood pressure monitoring using an arm cuff has been reported to cause a number of complications, including radial nerve palsy,17–19 a tourniquet effect,20 allergic contact dermatitis,21 and humeral fracture.22 While some complications occurred under anesthesia, a radial nerve palsy occurred in a laboring woman,18 and the humeral fracture occurred in an awake patient22 with bone pathology. The tourniquet effect, with resultant soft tissue injury and neuropraxia, occurred under anesthesia in the prone position, when the automated cuff was not cycling.20 These examples highlight the fact that under some circumstances, noninvasive blood pressure cuffs may cause significant morbidity.
The Importance of Arm-Cuff Size.
Even in the early days of technology development, the issue of appropriate cuff size was recognized.23 The original cuff used by Riva-Rocci13 was only 5 cm in width. Since then, the importance of using an arm cuff of appropriate size has been well discussed in the literature24–28 and emphasized in clinical guidelines.10,29,30 Recommended blood pressure cuff bladder sizes based on arm circumference are provided by the American Heart Association Scientific Statement by Pickering et al29 (Table 3). The circumference is the mid-arm circumference (MAC), and it is measured at the midpoint of the arm, half-way between the acromion process and the olecranon.31 The recommended ratio of cuff length to width is 2:1,29 although 1 study suggested that a cuff width of 46% of the MAC is the ideal ratio.32
It has been demonstrated that undercuffing (choosing a cuff that is too small) results in erroneously high blood pressure readings,25,27,33 while choosing a cuff that is too big will result in erroneously low readings.33,34 Manning et al25 found that miscuffing resulted in erroneous readings in the order of 8.5 mm Hg for systolic readings and 4.6 mm Hg for diastolic readings. In 1240 patients (weight range 46–244 kg, MAC 18–60 cm), Maxwell et al33 found a high correlation between MAC and all measures of obesity. They also found that as the MAC increased, the difference in readings obtained from different-sized cuffs was greater. Applying the wrong-sized cuff to patient with obesity risks overdiagnosing hypertension if the cuff is too small or missing the diagnosis of hypertension if the cuff is too large (Table 4).
It is important to note that the measurements in Table 3 refer to the bladder dimensions, which are enclosed in an outer cloth case and therefore not obvious to clinicians. From Table 3, it can be seen that the recommended bladder width does not change for the large or thigh-sized cuffs, so the optimal ratios of 46% are not achieved. For individuals of arm circumference 35–44 cm, the optimal bladder width would be 16.1–20.2 cm. It has been identified that as the arm circumference rises to >35, the length of the arm is commonly insufficient to fit a cuff with a bladder of that width.29 US data from 2007 to 2010 demonstrated that according to the recommended sizes29 and based on the MAC, 1.9% of women and 2.8% of men required thigh-sized cuffs.35
The Problem of Arm Shape in Obesity.
When it comes to the practical task of measuring blood pressure in individuals with obesity, the difficulties of large arm circumferences are compounded by the shape of the arm, especially when the circumference is increased by significant amounts of adipose tissue (Figure 1). These issues have been identified in the literature27,28,34,36 and are well known to clinicians caring for individuals with obesity.24 Bonso et al37 described a “trono-conical” arm shape that was defined by a set of standard measurements: the proximal arm diameter D1 (below the axilla), the distal arm diameter (above the antecubital fossa), and the arm length L. From these measurements, they calculated the conicity index using the equation: conicity index = 100 × (D1 − D2)/L. All the subjects in their sample of 142 showed some degree of conicity in their arm shape. In a subsequent study, Palatini et al34 demonstrated that MAC and arm length could predict arm conicity in both man and woman. The effect on blood pressure measurement on cone-shaped arms is exaggerated as the conicity of the arm increases, and the inflated rectangular cuff expands irregularly against the distal part of the arm, compromising the accuracy of blood pressure measurements.37 Recently, clinicians’ observations have been supported by biomechanical models,38 showing that the layer of subcutaneous fat in patients with obesity reduces the transmission of pressure from the inflated blood pressure cuff to the brachial artery. This would increase the required cuff pressure to occlude an artery with a given intraarterial pressure, compared with the pressure required in a nonobese arm, thus overestimating the systolic and diastolic pressures.
Clinicians faced with the practical difficulties of applying arm cuffs to large arms are known to place the cuff on the lower arm or the leg, as described by the patient in Figure 1.28 These are unvalidated techniques and likely to be inaccurate.27,29 While placing the cuff on the forearm and palpating for the return of the radial pulse may give a “trend” of change in systolic pressure,28 this is inadequate for continuous monitoring in the intraoperative and postoperative periods.
Alternatives to Noninvasive Measurements With a Traditional Arm Cuff
Given the significant problem of applying rectangular arm blood pressure cuffs in patients with obesity and the resulting inaccuracies of measurement, it would seem useful to explore methods of blood pressure measurement that overcome this issue. Devices that take measurements from the wrist or finger would avoid the variability in upper arm circumference and shape that results from deposits of adipose tissue. Currently available options discussed in the literature include the use of specialty arm cuffs or microprocessor technology,39 wrist or finger cuffs utilizing oscillotonometric techniques,40 wrist cuffs utilizing radial artery applanation tonometry,41 and finger cuffs utilizing the photoplethysmography and volume-clamp method of Penaz.42 While many studies have explored the accuracy of such options, in practice, they are not available widely, if at all, for the use of health care providers.
Obesity-Specific Arm Cuffs.
Rather than trying to conform to arms of different shapes, the Visomat Comfort 20/40 device was designed to use a single cuff on arms ranging from 23 to 43 cm circumference.39 The microprocessor estimated the arm circumference during inflation, and that value was used to adjust the recorded blood pressure. This device was designed for home blood pressure monitoring and requires further testing in patients with arm circumference >44 cm. A single rigid conical cuff tested by Bonso et al37 on a range of arm circumferences from 22 to 45 cm passed all 3 phases of testing in adults. The majority of oscillotonometric devices that utilize cuffs around the wrist or finger are not generally not recommended due to inaccuracy.29,43,44 One device that showed promise was a conical wrist cuff designed by GE Healthcare (Chicago, IL) specifically for patients with obesity, which was tested in subjects with an arm circumference >40 cm and showed good agreement with invasive radial arterial monitoring.40
Invasive Blood Pressure Monitoring.
The use of invasive intraarterial monitoring is usually restricted to the perioperative environment or intensive care ward due to the requirement of trained staff to insert the line and monitor its use. The use of invasive monitoring avoids the problems of arm circumference and shape and is the gold standard in accuracy for peripheral blood pressure measurement. Typically, this approach involves the insertion of an intraarterial catheter into the radial artery. This catheter is connected to a pressure transducer that must be appropriately leveled and zeroed to provide accurate beat-to-beat measurements of systolic, diastolic, and mean blood pressure. The main limitation of invasive monitoring is the risk of uncommon but potentially serious complications, such as site hematoma or infections, systemic sepsis, arterial thromboses, and emboli. Intraoperatively, the benefits include the ability to take blood samples, as well as reliable, accurate, and real-time blood pressure measurements, particularly at times of potential instability such as induction and tracheal intubation, inflation of a pneumoperitoneum, changes in patient position, major hemorrhage, and tracheal extubation.
While invasive monitoring is accurate, is reliable, and avoids the problem of arm shape and size, the staffing requirements and potential complications make it unsuited to the intermittent monitoring required in the outpatient preoperative environment. Intraoperatively, the use of invasive monitoring is appropriate when indicated and is feasible in this environment; however, use in the postoperative period will depend on the patient’s final disposition. For patients with obesity requiring intensive care or high-dependency care, continuation of invasive monitoring is likely to be possible. The majority of patients with obesity will not be cared for in these environments and will require some other form of blood pressure monitoring.
Continuous Noninvasive Blood Pressure Measurement.
Three commonly available devices can provide continuous blood pressure measurement using noninvasive techniques. ClearSight (Edwards Lifesciences, Irvine, CA) and continuous noninvasive arterial pressure (CNAP, CNSystems Medizintechnik AG, Graz, Austria) both use finger cuffs and apply the volume-clamp method of Penaz.42 The T-Line TL-200 (Tensys Medical, San Diego, CA) uses applanation tonometry using a wrist sensor over the radial artery. These 3 devices avoid the dependence on an appropriately fitting arm cuff, but their performance in patients with obesity has not been well assessed.
ClearSight and CNAP both use finger cuffs. The ClearSight cuffs come in small, medium, and large sizes and are disposable—a single cuff can be used for ≤8 hours or 2 cuffs can be used simultaneously for ≤72 hours. CNAP comes with a reusable double-finger cuff that can be used for ≤24 hours. Both devices use photoplethysmography to measure the change in blood volume of the finger and convert the changes to display a continuous waveform. Both use proprietary software algorithms.45,46 The ClearSight device is shown in Figure 2.
Because of its critical care applications, ClearSight has been assessed largely in the cardiothoracic anesthesia47–49 and noncardiac anesthesia50,51 settings. However, it has also been assessed in pregnant patients52 and in an outpatient environment.53 There is good agreement between ClearSight continuous noninvasive blood pressure measurements and both invasive47,48 and noninvasive50 measurements. Unfortunately, none of these studies has been undertaken in participants with class III obesity (maximum weight of 130 kg in the study by Martina et al47 and maximum BMI 29 ± 6.7 in the study by Balzer et al50). Thus, its performance in patients with class III obesity is unclear.
CNSystems describe the indications for CNAP as “monitoring the impact of a medical procedure on blood pressure” and “blood pressure monitoring of ill or circulatory unstable patients.” Validation studies have demonstrated good agreement with intraoperative invasive blood pressure monitoring54,55 but less agreement at times of hemodynamic instability.54 The main difference between ClearSight and CNAP is that the CNAP device includes an arm cuff that may be used (as 1 of 3 methods) to calibrate the device. That cuff is intended for a maximum MAC of 40 cm. CNAP has been studied in patients undergoing weight loss surgery (maximum BMI 7556 and 68 kg/m257) and showed good trending ability, but the absolute values obtained by CNAP were not interchangeable with those from invasive monitoring. In addition, Tobias et al56 noted the limitations of size of both the arm cuff and finger cuffs of CNAP in patients with obesity, and the accuracy of the device was compromised when the arm cuff was placed on the forearm for calibration.
Potential limitations of photoplethysmography using a finger cuff include inaccuracy in awake patients58 and during low-perfusion states. The available finger cuff sizes are larger with CNAP than ClearSight, but these may be inadequate for some patients with obesity.56
The T-Line TL-200 measures pressure changes at the radial artery and derives values for systolic, mean, and diastolic pressures. The technique needs the wrist sensor to be optimally positioned over the radial artery and requires the presence of radial pulses. It has been studied intraoperatively during a range of operations59,60 and showed good agreement with values obtained from radial arterial catheters. However, the mean BMI in these studies was 3159 and 28 kg/m2,60 and it is unclear whether obesity may affect the accuracy of the device. This method may also be compromised in the awake or uncooperative patient.
Despite CNAP and ClearSight also providing additional hemodynamic parameters such as stroke volume, stroke volume variation, cardiac output, and systemic vascular resistance, these devices have not replaced invasive arterial monitoring in perioperative care. This may be due to the frequent need for intraoperative blood sampling, the familiarity of anesthesiologists with the well-known and accepted technique of arterial cannulation, or a lack of confidence in newer noninvasive technologies. As shown here, in patients with obesity, new devices are often tested in healthy populations but not tested in the patient groups in which they may be of the most use.61 If designed specifically for patients with obesity and validated in a population with a large MAC, these devices could be suitable to fill the gap in accurate, noninvasive postoperative monitoring for patients with arms of large circumference or short length. To warrant regular clinical use in select individuals, the benefits of accurate blood pressure monitoring in this population would need to outweigh the significant overall monetary cost of the devices and consumables.
Validation of Blood Pressure Monitoring Devices for Use in Patients With Obesity
In 2002, the European Society for Hypertension (ESH)62 simplified the protocol for validation of blood pressure monitoring devices. This decision was based on information from the US Association for the Advancement of Medical Instrumentation (AAMI) protocol,63 published in 1987, and the British Hypertension Society (BHS) protocol, published in 1990.64 The most recent revision of the ESH protocol was published in 2010,65 and an updated version of the AAMI protocol was published in 2013.66 It is not uncommon for validation studies to apply criteria from >1 study.52,67,68 The ESH protocol sets out precisely the sample size, equipment, environment, protocol, and statistical analysis required to validate a noninvasive blood pressure monitoring device.65 There are some important considerations in how these different protocols serve the population of patients with obesity who require accurate noninvasive blood pressure measurements. Table 5 compares the salient features of these 3 protocols.
Which Reference Standard?
The most recent ESH protocol65 and the BHS protocol69 identify only the mercury sphygmomanometer as the reference standard, whereas the AAMI protocol published in 201366 also provides for the use of invasive monitoring as the reference standard. The ESH and BHS protocols are based on the presumption that noninvasive monitoring is accurate in patients with obesity, whereas good evidence indicates that it is not.25,33,34 When considering the validation of novel devices in patients with obesity, it is unclear what the appropriate reference standard should be—a potentially inaccurate measurement derived from a poorly fitting arm cuff or an invasive method. There are limitations associated with the use of an invasive method as the reference standard; such studies would be restricted to patients undergoing specific procedures (eg, operative procedures or cardiac investigations).
Subject Selectionfor Validation.
We have presented evidence to demonstrate the importance of MAC and cuff size, as well as the influence of these on the accuracy of blood pressure readings. The 3 available protocols have different requirements relating to the inclusion of subjects with large MAC. The ESH protocol simply states that “the bladder must be of sufficient length to encircle 80%–100% of the arm circumference.”65 BHS states that subjects should not be selected according to MAC, as it is presumed that by selecting on the basis of blood pressure readings, a range of MAC will be provided.69 It is plain that a device may be validated according to this protocol, without having been tested on subjects with a MAC >44 cm. The AAMI protocol is the most specific, stating that a certain fraction of the subjects should meet certain MAC specifications66 based on the intended use of the cuff.
The 2016 Position Statement of the European Society of Hypertension Working Group on Blood Pressure Monitoring and Cardiovascular Variability highlighted the importance of validating blood pressure devices across a large range of cuff sizes.43 It raises the point that an appropriate range of cuff sizes is rarely available in clinical practice, particularly those designed for individuals with obesity. It recommends that separate validation studies be undertaken specifically in populations of patients with large arms and that tronoconical and wide-range cuffs be developed and assessed to cater to patients with obesity. The World Hypertension League has also acknowledged the difficulty of having appropriate cuff sizes available44 and also called for arm cuffs to be labeled with the arm-size range in which they have been evaluated for accuracy.44
Accurate Blood Pressure Measurement Is Vital in Perioperative Care
As part of comprehensive perioperative care and risk assessment, anesthesiologists providing care for patients with obesity require accurate measurement of blood pressure in the preadmission clinic (for elective surgery) or ward (for emergency admissions). The accurate identification of hypertension allows appropriate surgical risk calculation, an evaluation of the effectiveness of current medications, or the institution of appropriate pharmacological treatment. Intraoperatively, if faced with patient arm dimensions that cause difficulty with noninvasive measurement of blood pressure, anesthesiologists are fortunate to be able to institute invasive monitoring for the intraoperative and immediate postoperative period. In the recovery unit or on the general ward, the detection of hypertension in the postoperative period is important for the evaluation of postoperative pain, hypoventilation with carbon dioxide retention, inadequate reversal of neuromuscular blockade, and minimization of early bleeding risk. Equally as important, the accurate detection of hypotension is essential for the diagnosis of hemorrhage, sepsis, or postoperative myocardial infarction. The quality care of patients with obesity will be compromised if low or high blood pressure readings are attributed to a poorly fitting cuff rather than an underlying pathology that requires treatment. Rather than accepting the limitations of our current equipment in the caring for patients with obesity, we must pursue purpose-made devices for patients with obesity and ensure specific validation in this population.
The anesthetic care of patients with obesity is common; it is continuing to increase in frequency, and accurate perioperative blood pressure measurement is essential. The difficulties of applying standard arm cuffs in patients with obesity and subsequent inaccurate readings are well documented in the literature. In response to these problems, some advances in cuff technology have been made; however, this technology has not become widely available to health care providers. In our role as perioperative physicians, anesthesiologists must accept that this is an area in which our care is potentially compromised and demand better for our patients. Cuffs and devices designed and validated for individuals with a large arm circumference must be made available for use outside the critical care environment when accurate invasive monitoring is not possible. Only then can we provide equality in perioperative care across patient groups with and without obesity.
Name: Victoria A. Eley, PhD.
Contribution: This author helped with the concept, literature review, and writing of article.
Name: Rebecca Christensen, PhD.
Contribution: This author helped write the article.
Name: Louis Guy, MBBS.
Contribution: This author helped write the article.
Name: Benjamin Dodd, MSc.
Contribution: This author helped write the article.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
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