Year : 2010  |  Volume : 13  |  Issue : 1  |  Page : 28--33

Thoracic epidural analgesia in obese patients with body mass index of more than 30 kg/m 2 for off pump coronary artery bypass surgery


Munish Sharma, Yatin Mehta, Ravinder Sawhney, Mayank Vats, Naresh Trehan 
 Department of Anesthesiology and Critical Care & Cardiothoracic Surgery, Indraprastha Apollo Hospitals, New Delhi, India

Correspondence Address:
Mayank Vats
Apollo Hospital Delhi
India

Abstract

Perioperative Thoracic epidural analgesia (TEA) is an important part of a multimodal approach to improve analgesia and patient outcome after cardiac and thoracic surgery. This is particularly important for obese patients undergoing off pump coronary artery bypass surgery (OPCAB). We conducted a randomized clinical trial at tertiary care cardiac institute to compare the effect of TEA and conventional opioid based analgesia on perioperative lung functions and pain scores in obese patients undergoing OPCAB. Sixty obese patients with body mass index >30 kg/m 2 for elective OPCAB were randomized into two groups (n=30 each). Patients in both the groups received general anesthesia but in group 1, TEA was also administered. We performed spirometry as preoperative assessment and at six hours, 24 hours, second, third, fourth and fifth day after extubation, along with arterial blood gases analysis. Visual analogue scale at rest and on coughing was recorded to assess the degree of analgesia. The other parameters observed were: time to endotracheal extubation, oxygen withdrawal time and intensive care unit length of stay. On statistical analysis there was a significant difference in Vital Capacity at six hours, 24 hours, second and third day postextubation. Forced vital capacity and forced expiratory volume in one second followed the same pattern for first four postoperative days and peak expiratory flow rate remained statistically high till second postoperative day. ABG values and PaO 2 /FiO 2 ratio were statistically higher in the study group up to five days. Visual analogue scale at rest and on coughing was significantly lower till fourth and third postoperative day respectively. Tracheal extubation time, oxygen withdrawal time and ICU stay were significantly less in group 1. The use of TEA resulted in better analgesia, early tracheal extubation and shorter ICU stay and should be considered for obese patients undergoing OPCAB.



How to cite this article:
Sharma M, Mehta Y, Sawhney R, Vats M, Trehan N. Thoracic epidural analgesia in obese patients with body mass index of more than 30 kg/m 2 for off pump coronary artery bypass surgery.Ann Card Anaesth 2010;13:28-33


How to cite this URL:
Sharma M, Mehta Y, Sawhney R, Vats M, Trehan N. Thoracic epidural analgesia in obese patients with body mass index of more than 30 kg/m 2 for off pump coronary artery bypass surgery. Ann Card Anaesth [serial online] 2010 [cited 2019 Oct 15 ];13:28-33
Available from: http://www.annals.in/text.asp?2010/13/1/28/58831


Full Text

 Introduction



About one-third of the population of industrialized countries is at least 20% overweight. [1] Obesity predisposes to atelectasis and thus contributes to significant postoperative pulmonary morbidity by jeopardizing respiratory function. [2],[3] There is a significant negative correlation between perioperative spirometric tests and obesity. [4] Despite some controversies, many anesthesiologists consider perioperative epidural analgesia an important part of a multimodal approach to improve analgesia and patient outcome rather than relying solely on systemic opioid administration. [5],[6],[7] This may be particularly important for obese patients undergoing off pump coronary artery bypass surgery (OPCAB), although the benefits of thoracic epidural analgesia (TEA) in such patients has not yet been proved.

Significant impairment of pulmonary function is common after cardiac surgery and is multifactorial in origin. Median sternotomy leads to uncoordinated rib cage expansion and respiratory muscle weakness. [8] Basal atelactasis, which occurs after induction of general anesthesia, may persist into the postoperative period and is characterized by increased V/Q mismatch. [9] Impairment of lung functions becomes more pronounced in obese patients, who already have baseline restrictive pulmonary function defect. It has been shown that with increasing body mass index (BMI), functional residual capacity (FRC), respiratory system compliance and oxygenation index decrease exponentially. [10] Basal atelectasis with consequent increased intrapulmonary shunt may get pronounced in obese anesthetized patient as compared to non-obese anesthetized patient leading to diminished partial pressure of oxygen in the blood. [11]

TEA significantly attenuates postoperative pulmonary dysfunction that follows upper abdominal and thoracic surgeries. It has been shown that TEA improves the forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) following thoracotomy [12] and there is shorter ventilation time and better analgesia and ability to cough in patients undergoing coronary artery bypass graft (CABG). [13],[14] However, to the best of our knowledge, there are no reports on obese patients.

The use of TEA could be more beneficial in obese patients who are at risk of significant postoperative pulmonary complications.

We planned this study to assess the effect of TEA on pulmonary functions in obese (BMI>30 kg/m 2 ) patients after OPCAB.

 Materials and Methods



The present study was conducted after approval by the institutional ethical review board and obtaining informed written consent from all the patients. Sixty obese patients between 40-70 years age (BMI>30 kg/m 2 ) with physical status of ASA II and III scheduled for elective OPCAB were included in the study.

The exclusion criteria were as follows:

Emergency surgeryLeft ventricular ejection fraction (LVEF) ≤35%Chronic obstructive airway disease (to avoid the confounding effects on pulmonary function test)CoagulopathySepsisNeurological disorderCABG on cardiopulmonary bypass (CPB)Significant left main coronary artery disease

Patients were randomized into two groups of 30 each. The use of aspirin and other anticoagulants was discontinued seven days prior to surgery and all cardiac medications and antihypertensive agents were continued preoperatively. Patients in both the groups received premedication of oral Lorazepam and pantaprozole 40 mg pn the night prior to surgery. Baseline pulmonary functions were assessed by vitalograph 2120, a hand held spirometer (Vitalograph Limited, Buckingham, U.K.) before surgery.

Group 1 (GA+TEA)

On the day of surgery (at least two hours before heparinization), an epidural catheter was placed through a Tuohy's needle (Epidural mini pack 1, Smith Med. ASD Inc. Keene NH, USA) at C 7 -T 1 /T 1 -T 2 level using hanging drop technique via midline approach in sitting position. Intrathecal placement was ruled out by using 3 ml of 2% lignocaine without epinephrine as a test dose. We were prepared to postpone the surgery in case of encountering hemorrhagic tap.

Once on the operation table, the patient was administered a bolus dose of 8-10 ml of bupivacaine (0.25%), inducing sensory block till at least T4. After confirming the block by loss of sensation to cold and pin prick, general anesthesia was administered. Bupivacaine infusion (0.125%) with 1µg/ml fentanyl citrate) at the rate of 5 ml/hr was commenced and continued till 3 rd postoperative day for providing intra- and postoperative analgesia, We assessed the motor power in all four limbs three to four hourly as soon as the patients were awake and were ready to intervene as and when required.

Group 2 (GA only)

Patients did not receive epidural analgesia. In the postoperative period, these patients received Tramadol Hydrochloride 1-2 mg/kg intravenously as continuous infusion to prevent breakthrough pain.

Intraoperatively, standard monitoring was instituted along with invasive arterial blood pressure through femoral artery and thermodilution pulmonary artery catheter inserted via internal jugular vein. Patients in both the groups received general anesthesia as per hospital protocol with intravenous administration of propofol (1 -1.5 mg/kg), fentanyl lcitrate (2-3 µgm/kg). midaxolam (0.04 mg/kg)and vecuronium bromide (0.1mg/kg). Isoflurane (1-2 MAC). Propofol (1-1.5 mg/kg) for induction of anesthesia along with fentanyl citrate (2-3µgm/kg), midazolam (0.04mg/kg) and vecuronium bromide (0.1mg/kg) while isoflurane(1- 2 MAC) in air and oxygen mixture was used for maintenance of anesthesia. OPCAB via median sternotomy was performed in all patients. Surgical techniques were standard for both the groups. Mechanical stabilizing devices used were "Octopus" and "Starfish" (Medtronics Inc. Minneapolis, MN, USA). Anticoagulation was achieved with heparin sulphate (2mg/kg) administered at least two hours after epidural catheter insertion in group 1. Additional doses of analgesics (Fentanyl 1-2µg/kg bolus doses intravenously), neuromuscular blockers and sedatives (Midazolam 0.04mg/kg intravenously) administered intra-operatively were used at the discretion of the anesthesiologists and were also recorded. Postoperatively, patients were transferred to ICU and were electively mechanically ventilated. Once the patients were awake with adequate spontaneous ventilation and stable hemodynamic state, they were weaned off from the ventilator and trachea was extubated. Extubation criteria were: hemodynamic stability with mean arterial pressure greater than 60mmHg (without or with minimal inotropes and/or vasopressors), core temperature ≥36°C, spontaneous ventilation with PaO 2 >100 mm Hg on FiO 2≤0.4 and PaCO 2 1ml/kg/hr. Time taken for extubation was noted.

Perioperative and postoperative hemodynamic and oxygenation parameters along with inotropic and/or vasopressor requirements were recorded. Any untoward complications of TEA like paresis, hypotension, urinary retention (after removal of foley's catheter), respiratory depression, pruritis were noted every four hourly by a blinded observer in the ICU.

Pain assessment was done using 10cm Visual Analogue Scale (VAS) at rest and on coughing (10cms - maximum pain and 0 - no pain) by a blinded observer. Rescue analgesia (intravenous tramadol hydrochloride 1mg/kg) was administered whenever VAS was ≥5 at rest.

Pulmonary functions were recorded using vitalograph at six and 24 hours post extubation, and then on postoperative days- 2,3,4 and 5. PFTs were done with patient in sitting position and best of three attempts was recorded as judged by flow volume loops.

Hemoglobin, arterial blood gas (ABG) and PaO 2 /FiO 2 were measured at six hrs, 24 hours and subsequently once a day for the next three days. Postoperative incidence of myocardial infarction (if any) was recorded as documented by 12 lead EKG and CPK, CPK-MB levels. Pulmonary complications were observed by daily chest x-ray and were graded according to Richter , s Radiological score [15] (0- clear lung field; 1- plate like atelectasis or slight infiltration; 2- partial atelectasis; 3- lobar atelectasis; and 4- bilateral lobar atelectasis). Other complications, if any, related to epidural block i.e. respiratory depression, paraesthesia, motor blockade, pruritis, urinary retention etc. were also recorded every four-hourly by a blinded observer.

Statistical analysis

The values are presented as mean ± standard deviation. Categorical data was compared by using Chi Square test. Continuous data was compared by using student t-test. P-value less than 0.05 were considered significant (SPSS 12.0 statistical software).

 Results



The two groups were comparable with regard to demographics, BMI, left ventricular ejection fraction (LVEF), number of grafts and comorbid conditions. [Table 1].

[Table 2] shows that, preoperative values of vital capacity (VC), FVC, FEV 1 and PEFR were comparable in both the groups. In group 1, VC remained significantly high at 6hrs, 24hrs, 2 nd and 3 rd day postoperatively, however from 3 rd postoperative day onwards this difference became non-significant (versus Group 2). Similar trends were observed for VC, FVC and FEV 1. In both the groups the lowest values were recorded at 6hrs post extubation. However, FEV 1 /FVC ratio was comparable in both the groups postoperatively at all time points. All spirometric values were noted in relation to the predicted normal values in percentage.

ABG values were comparable in both groups preoperatively [Table 3]. In the postoperative period, statistically significant difference in PaCO 2 and PaO 2 /FiO 2 ratio were observed in group 1 at six hours and 24 hours postextubation (versus Group 2), which could be directly attributed to the beneficial effects of TEA on pain and consequent better ventilation and oxygenation matching although this is not clinically significant. After that, there was a progressive increase in PaO 2 /FiO 2 ratio in both the groups at all time points but the values were statistically higher in the study group.

VAS at rest [VAS(R)] was significantly lower in group 1 (versus group 2) at six hours, 24 hours, postoperative days -2,3 and 4. Similarly, VAS on coughing [VAS (OC)], was also significantly lower in group 1 till postoperative day 3, showing better pain relief in the study group. [Table 4]. Lower values of VAS(OC) are very important in postoperative period for clearance of respiratory secretions.

Richters scale was observed by a blinded radiologist in the postoperative period at six hours, 24 hours and thereafter everyday till postoperative day 5. Although at all time points the scale was lower in group 1, the differences were not statistically significant [Table 5]. Tracheal extubation time, oxygen withdrawal time and the length of ICU stay were significantly lower in the study group [Table 6].

 Discussion



Lung volumes in obese patients are reduced significantly in the postoperative period. Spirometry is considered as an accurate, reproducible and simple investigative tool that can be used easily in the immediate perioperative period. In order to increase the accuracy of spirometry, factors that potentially interfere with breathing such as pain, should be eliminated or minimized as much as possible. As previously reported by Sternberg et al. the lowest spirometric values are observed during the first assessment after extubation. [4] In our study, there was significant decrease in VC, FVC, FEV 1 and PEFR in initial postoperative period as compared to preoperative values but the values remained high and statistically significant in study group through first three postoperative days. This suggests a restrictive pattern of respiratory compromise in the immediate postoperative period, as previously described. [16] Postoperative impairment of spirometric measurements was probably not related to insufficient cooperation since all patients were alert and fully compliant within three to four hours of extubation. This immediate (at six hours) postextubation reduction of spirometric values observed in our study could have been caused by impaired respiratory mechanics, obesity and atelectasis aggravated by general anesthesia in the supine position, as well as by cardiac surgery. [10],[17],[18]

The effect of TEA on spirometric measurements is controversial. TEA at higher level has been reported to decrease spirometric measurements by blocking intercostal muscle innervation. [19] In contrast, others have not found that TEA has any influence on spirometry or lung dynamics. [20] In our study, we found better spirometric values in patients who received TEA, probably due to better analgesia as demonstrated by lower VAS values both at rest and coughing. We also used low concentration (0.125%) of bupivacaine which is unlikely to produce motor paralysis.

Parbrook et al. showed a direct relationship between reduced spirometric measurements and increased postoperative complication rates. [21] Despite the lack of evidence that TEA reduces in-hospital mortality, a recent large randomized trial showed a significant reduction in postoperative respiratory failure rates (23% versus 30%). [5] Other benefits of TEA includes earlier mobilization and more rapid recovery of bowel function, thus allowing oral nutrition. [22] Scott et al. show in their study that TEA significantly improves the quality of recovery after CABG. [23]

Spirometric measurements have been used to quantify postoperative pain. [24] Therefore it is crucial for a patient to be free of pain during spirometry. The shorter tracheal extubation time, oxygen withdrawal time and the ICU stay in the study group shows that TEA yields significant improvement in pulmonary function most likely due to better postoperative analgesia, as evidenced by statistically significant lower VAS at rest and on coughing in the study group. All these can contribute to early endotracheal extubation with subsequent cost reduction. We started weaning after assuring hemodynamic stability and excluding any postoperative bleeding by taking chest X-ray after four hours of surgery in both groups. This could be the reason for not finding clinically significant difference in extubation time.

The potential for a selection bias was minimized by not involving the anesthesiologists in this study who did the preoperative assessment of the patients. Additionally, postoperative spirometry was performed by trained doctors who were unaware of the study hypothesis and were not involved in this study.

 Conclusion



We conclude that obesity is an important risk factor for postoperative impairment of spirometric measurements in patients undergoing OPCAB and this was significantly greater in obese patients not receiving TEA. The severity of postoperative lung functions reduction was consistently reduced by TEA and restoration was significantly quicker in TEA group. Patients receiving TEA had significantly lower VAS, earlier extubation of trachea, O 2 withdrawal and significantly shorter ICU stay. There were no complications attributed to TEA. We strongly recommend the use of TEA as an adjunct to postoperative pain management in obese patients undergoing OPCAB.

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