Conference Lectures
REGIONAL ANAESTHESIA IN OBESE
-AN UPDATE
DR.ANAND BANGERA
PROFESSOR AND HEAD,
K.S.HEGDE MEDICAL ACADEMY
MANGALORE, KARNATAKA.
The incidence of morbid obesity has tripled over the past three decades throughout the world. WHO projects that by 2015,2.3 billion people will be overweight and 700 million will be obese.Obesity and particularly morbid obesity (BMI>40 kg/m2) is associated with an increased incidenceof medical comorbidities, including type 2 diabetes,hypertension, obstructive sleep apnoea, cardio-pulmonary disease, venous thromboembolism and psychosocial disease.In adults, the body mass index (defined as ratio of body weight in kilogram and squared body height in meter) has been used as thecomparative measure to define overweight (BMI = 25–29 kg/m²), obese (BMI = 30–39 kg/m2) and morbidly obese (BMI =40 kg/m²).
Classification of obesity
Classification |
BMI(kg/m2) |
|
|
Principal cut-off points |
Additional cut-off points |
Underweight |
<18.50 |
<18.50 |
Severe thinness |
<16.00 |
<16.00 |
Moderate thinness |
16.00 - 16.99 |
16.00 - 16.99 |
Mild thinness |
17.00 - 18.49 |
17.00 - 18.49 |
Normal range |
18.50 - 24.99 |
18.50 - 22.99 |
23.00 - 24.99 |
||
Overweight |
≥25.00 |
≥25.00 |
Pre-obese |
25.00 - 29.99 |
25.00 - 27.49 |
27.50 - 29.99 |
||
Obese |
≥30.00 |
≥30.00 |
Obese class I |
30.00 - 34.99 |
30.00 - 32.49 |
32.50 - 34.99 |
||
Obese class II |
35.00 - 39.99 |
35.00 - 37.49 |
37.50 - 39.99 |
||
Obese class III |
≥40.00 |
≥40.00 |
Source: Adapted from WHO, 1995, WHO, 2000 and WHO 2004
Obese patients pose specific problems to anaesthesiologists including the decision about selecting the anaesthetic technique that carries the least risk for the patient.The use of regional anaesthetic techniques for obese patients is increasing in popularity.A regional anaesthetic allows minimal airway manipulation, avoidance of anaesthetic drugs with cardiopulmonary depression, and reduced postoperative nausea and vomiting (PONV) as well as greater postoperative pain control. Regional anaesthesia is generally believed to be associated with a great rate of unsuccessful blocks and for this reason anaesthesiologists often give preference to general anaesthesia. Obese surgical patients often have difficult airways, may be at increased risk for acid aspiration, and frequently have obstructive sleep apnoea (OSA) with increased sensitivity to opioid analgesics.
Drug dosage adjustment in performing regional anaesthesia in obese patients
Factors affecting drug pharmacokinetics in obesity:
Volume of distribution
Decreased fraction of total body water
Increased adipose tissue
Increased lean body mass
Altered tissue protein binding
Increased blood volume and cardiac output
Increased concentration free fatty acids, cholesterol, ALPHA 1 acid glycoprotein
Organomegaly
Plasma protein binding
Adsorption of lipophilic drugs to lipoproteins so increased free drug available
Plasma albumin unchanged
Increased ALPHA 1-acid glycoprotein
Drug clearance
Increased renal blood flow
Increased GFR
Increased tubular secretion
Decreased hepatic blood flow in congestive cardiac failure.
The appropriate dose of local anaesthetics for any regional block is important.
Unfortunately, data on anaesthetic dosing for regional blocks on obese patients are lacking, with most information relating to neuraxial (epidural and spinal) anaesthesia.
Although there is controversy the majority of studies suggest that obese patients need less local anaesthetics than non obese patients undergoing identical anaesthetics. Several studies of obese patients and non-obese patients receiving the same dose and volume of bupivacaine for spinal anaesthesia demonstrated a direct positive correlation between the height of the sensory block with increasing BMI.Most of the information we have on local anaesthetic dosing for obese patients comes from obstetrical patients.At term, pregnant patients are similar to obese patients since their anatomic land marks may be obscured, their lung volumes are reduced, and the aorta and inferior vena cava can be compressed causing engorgement of the epidural veins with a reduction in the epidural space. Likewise, obese patients have a smaller cerebrospinal fluid (CSF) volume than non-obese patients, and these changes are exaggerated in obese parturient. It has been suggested that the smaller CSF volume in obese gravid patients is a major contributing factor for their lower spinal anaesthetic dose requirements. Obese parturients have significantly decreased epidural labour analgesia requirements than patients with a BMI < 30kg/m2. With identical epidural anaesthetic dosing, higher sensory levels for surgical anaesthesia are achieved in obese compared to normal weight obstrectical patients. Applying these findings to obese non-pregnant surgical patients supports the clinical evidence that the local anaesthetic dose for neuraxial blocks should be reduced in morbidity obese patients.
Obese patients have normally smaller cerebrospinal fluid volumes and the reduced epidural space resulting in higher spread of local anaesthetic. There is the wide variability in dosage and sensitivity to spinal anaesthetics especially if comorbidity such as pre-eclampsia coexists. Many authors recommend the low dose and the low concentration of local anaesthetic for neuraxial block in obese patients. The others did not find evidence for different doses of hyperbaric bupivacaine if single shot spinal anaesthesia is used for caesarean delivery in obese and normal weight patients.
Difficulties in performing regional anaesthesia:
The major challenges of regional anaesthesia in obese patients are usually technical and relate to identifying appropriate landmarks, positioning the patient prior to and after performing the block, and using needles of sufficient length. Since information on regional anaesthesia in extreme obesity comes from the obstrectic anaesthesia experience, relevance for obese non-obstrectical patients must be inferred. Not surprisingly, epidural catheter placement is more difficult in obese parturients. Fluoroscopy and ultrasonography have been used to facilitate spinal and epidural needle placement in obese and morbidly obese patients.
The anthropometric changes associated with obesity can make performance of peripheral nerve blockade technically difficult.BMI of more than 25 kg/m2 is an independent risk factor forblock failure.Paravertebral and continuous epidural, continuous supraclavicular, andsuperficial cervical plexus blocks had the highest failure rates. Failed blocks often required supplementation withgeneral anaesthesia.There are two basic challenges that should be overcome to get successful block in obese patients: technical and logistic difficulties attributed to the unpredicted anatomy of the obese body and appropriate adjustment of local anaesthetic dosage.Establishing neuraxial blockade in the morbidly obese patient can also bechallenging. In the obese patient, there may be difficulty in palpating bony landmarks or even identifying the midline, and the presence of fat pockets may result in false-positive loss of resistance during needle placement. Drug distribution may also be altered. Hood and Dewan 13described an initial success rate of only 42% for placing epidural catheters in obese patients compared with an initial 94% success rate in nonobese controls. Obese patients require less local anaesthetic in theirepidural and subarachnoid spaces in order to achieve the same level of block when compared with nonobese controls. After 3ml of 0.5% bupivacaine was injected into the subarachnoid space at the L3–4 interspace, Taivainen et al6demonstrated a higher cephalad spread in obese vs. nonobese individuals.
Ultrasound-guided techniques have been promoted to aid epidural catheter placement. Even with the use of ultrasound, there have been reports of accidental dural puncture during attempted epidural placement in morbidly obese patients. Improved success rates using ultrasound-guided regional anaesthesia in the obese population have also been reported for peripheral nerve blocks. Sufficient proficiency with ultrasound is needed before consistent success can be achieved. However, the use of ultrasound for performing neuraxial blockade remains controversial. Familiarity with spinal anatomy, pharmacology of local anaesthetics and experience in performing neuraxial blocks are sufficient to perform a successful neuraxial block in morbidly obese patients. Increased difficulty of performing neuraxial blocks in obese patients must be taken into consideration.There are significant practical difficulties in siting epidural catheters in the morbidly obese. These include the lack of palpable bony landmarks, the depth of the space (extra long needles may be required), and ‘false’ loss-of-resistance in fatty tissues.Longer spinal and epidural needles may be necessary, and landmarks may be concealed by excess body tissue.Weight and position influence the actual distance from the skin to the epidural space. When changing from the sitting position to the lateral or supine positions following catheter placement, an epidural catheter fixed to skin can be pulled from the epidural space, the greatestchanges (>1cm) observed in patients with a BMI > 30kg/m2. For larger patients a, multi orificed epidural catheter should be advanced minimum of 4cm into the epidural space in anticipation of this outward migration.
There are even fewer studies of peripheral nerve blocks in obese patients, and most consider only technical aspects and success rates. One study reported a slightly greater failure rate of supraclavicular block in obese (BMI > 30kg/m2) patients, but no differences in rate or types of complications compared with normal weight patients. Once again, the use of ultrasound devices to identify the nerve to be blocked has greatly increased the success rate of peripheral nerve blocks in obese patients.Although needle placement was successful in the majority of morbidly patients, block failure necessitating re-performance of the block,further local anaesthetic supplementation, or complete abandonment of the block for general anaesthesia, occurs slightly more frequently among morbidly obese patients.
The actual incidence of block complications, including those associated with acute local anaesthetic overdose or toxicity (e.g. metallic taste, peri-oral paresthesia, and tinnitus), central nervous system toxicity (e.g. excitation and seizures), and cardiac problems (e.g. hypotension, arrhythmias, cardiac arrest) in obese patients is unknown. Likewise, there are no data on the frequency of other complications of regional anaesthesia, such as hematoma at the site of the block, unintentional epidural and intrathecal spread and pneumothorax. In fact, although complications would seem to be more frequent in obese patients due to technical challenges with needle placement and lack of data on the appropriate amount of local anaesthetic required, the actual incidence of complications is low.
Obesity and parturients:
Obesity has been identified as a significant risk factor for anaesthesia related morbidity/mortality in obstetrics. Obesity compounds most of the physiological changes in pregnancy. The obese parturients are at increased risk of having either concurrent medical problems or superimposed antenatal diseases including preeclampsia and gestational diabetes. These patients need thorough preoperative assessment, meticulous preparation and preparation for difficult airway as incidence of failed intubation is many times higher in these patients. Regional anaesthesia whenever possible is the best choice in this group of patients.
Most reports and studies about performing regional anaesthesia in obese patients come from specific subpopulation of obstetric patients. In majority of these, especially earlier published articles, we can find that regional anaesthesia can be performed without major difficulties and complications. Horikowa et al14. reported no significant differences in time for spinal tap and the incidence of hypotension between the obese and control group in 90 parturients. In retrospective analysis of 1,461 consecutive caesarean sections Schultzek et al15. recommended spinal anaesthesia even for obese parturients without reported difficulties. Saravanakumar et al16. also advocated a more liberalised use of regional techniques as means to further reduce anaesthesia-related maternal mortality in the obese population. Moreover, they recommended prophylactic placement of an epidural catheter when not contraindicated in labouring morbidly obese women.
Technical problems associated with regional anaesthesia in the morbidly obese parturients include difficulties inpatient positioning, identification of anatomical land marks and more frequent dislodgement of epidural catheters. The sitting position is more preferred because the line joining the occiput or the prominence of C7 and the gluteal cleft can be used to approximate the position of the midline in sitting position, which allows the fat of the back to settle laterally and symmetrically and improves the identification of the midline. Verbal communication with the patient can be used to identify the midline. Patient can help guide to the midline by telling if she senses pressure from needle advancement toright or left. Though epidural anaesthesia offers several advantages likethe ability to titrate the dose to achieve the desired level ofanalgesia, ability to extend the block for prolonged surgery, adecreased incidence and slow speed of developing hypotensionand utilisation for postoperative analgesia. The reasonfor opting spinal anaesthesia was concern about obtaining an inadequate sensory block with the epidural anaesthesia.Generally, a T4–S5 sensory block is needed for adequateanaesthesia during caesarean section. Because of the difficultyassociated with blocking sacral nerve roots, epidural blockmay be inadequate in up to 25% of patients.8 Also; there is anincreased risk of intravenous placement of epidural catheterdue to engorged epidural veins and decrease in epidural spacein addition to dislodgement of epidural catheter.
Concerns on pulmonary function:
Obesity is associated with perioperative hypoxia and an increased risk of postoperative pulmonary complications, including pneumoniaand respiratory failure. Opioid analgesia can be dangerous in some obesepatients, especially those with obstructive sleep apnea orobesity hypoventilation syndromes. Epidural anesthesia in obese patients undergoing thoracic and upper abdominal surgery decreases opioid requirements and reduces postoperative pulmonary complications. When combined with a general anesthetic,epidural anesthesia may result in earlier time to tracheal extubation than with a balanced anesthetic alone.Lung volumes in obese patients are reduced significantly in the postoperative period. There is a linear inverse correlation after premedication between vital capacity and BMI and a curvilinear correlation between FRC and BMI.Abdominal wall muscles play the major role in forced expiration.
These may become less effective in the presence of a good epidural block.
Obesity and ambulatory surgery:
Obese surgical patients are at an increased risk for difficult airways, cardiopulmonary dysfunction, acid aspiration, and even death. These concerns have forced some to consider morbid obesity a contraindication for ambulatory surgery. Guidelines include the need for careful selection of patients appropriate for day surgery, the presence of skilled surgeons and anesthesiologists, prophylaxis against deep venous thrombosis and PONV, and adequate postoperative pain control. Regional anesthesia for obese patients in the ambulatory settings does offer several theoretical advantages over general anesthesia. These advantages include reductionin the need for airway intervention, fewer drugs with less cardiopulmonary depression, decreased need for opioid and other sedatives, and decreased PONV. The use of regional anesthesia has reduced postanesthesia care unit (PACU) and overall hospital length of stay. Although regional anesthesia is a viable option for theobese patient undergoing outpatient surgery, the difficulty in performing these blocks must also be considered.Furthermore, the type of anesthetic performed should never supersede a thorough history of the patients’comorbidities and physical examination when determining appropriateness for outpatient surgery.
Contrary to neuroaxial blocks, the authors consistently,from early publications up to nowadays, report about different degree of failure of peripheral nerve blocks in obese patients. Nielsen et al17. prospectively collected data including 9,038 blocks performed on 6,920 patients in a single ambulatory surgery centre. Their investigation showed that obesity was associated with higher block failure and complication rates in surgical regional anaesthesia in the ambulatory setting. Nonetheless,the rate of successful blocks and overall satisfactionremained high in patients with increased BMI. Therefore,the authors concluded that overweight and obesepatients should not be excluded from regional anaesthesia procedures in the ambulatory setting.
Conclusion
Regional anaesthesia is becoming increasingly popular for obese and morbidly obese patients. The potential benefits of regional anaesthesia in obese patients are substantial and have increased the interest in these techniques for obese patients undergoing ambulatory surgery. Difficulties of performing regional techniques must be considered. Despite the fact that a successful regional anaesthetic allows minimal manipulation of the airway, it does not free the patient from the potential for airway compromise. Ultrasonography should be used for guidance of peripheral nerve blockade; however, its use for neuraxial blockade remains controversial. So obesity is not a contraindication for the use of regional anaesthesia when performed by an experienced anaesthesiologist familiar with morbidly obese surgical patients. An active approach and regular performance of blocks can improve our skills and change perspectives of our clinical practice. Many anaesthetists consider perioperative epidural anaesthesia (EDA) an important part of a multimodal approach to improving patient outcome and analgesia rather than relying solely on systemic opioid administration. EDA seems particularly attractive in obese patients undergoing major abdominal surgery. So when compared with general anaesthesia a properly performed regional anaesthetic technique can eliminate the need for airway intervention, minimize cardiopulmonary depression , provide excellent postoperative analgesia , decrease intra- and postoperative requirements , minimize postoperative nausea and vomiting (PONV), and result in a shorter postoperative anaesthesia care unit (PACU) and hospital stay.
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