Clinical Resarch Artrcale
 

OPCAB in Acute Coronary Syndrome: Predictors of
Intra-aortic Balloon Pump use


Amit Chaudhary1, Shantanu Pande1, Surendra K Agarwal1, Udgeath Dhir1, Satyendra Tewari2.
Department of Cardiovascular and Thoracic Surgery1 and Cardiology
Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, UP, India.

Abstract

Acute coronary syndrome (ACS) patients undergoing OPCAB often require intra-aortic balloon pump support. Indications and timing of IABP insertion are controversial. A study was done to determine criteria for predicting the use of IABP in these patients. Method: 46 patients were operated for ACS within one week of event between January 2004 and July 2006 and categorized into, UA (n=25), with ongoing pain after late admission to emergency room (>6 hours) and AMI (n=21) with acute myocardial infarction within 7 days of event. There was no statistical difference in the demographic data of two groups. UA group had higher incidence of mitral regurgitation preoperatively (p=0.004) which improved postoperatively (p=0.1). IABP was used preoperatively more in AMI group (10 vs. 6, p=0.03). Multivariate analysis revealed pressure ratio of mean pulmonary and systemic arteries as a predictor of IABP use (p=0.01, odds ratio 19.4, 95% CI 1.9-190). There is a significant positive correlation (r=0.519) between IABP use and ratio of mean pulmonary and systemic arterial pressures (p=0.004) with cutoff value at 0.40 Conclusion: Ratio of mean pulmonary and systemic artery of > 0.40 indicates preoperative use of IABP to complete the operation safely.

Keywords: Intra aortic balloon pump; Acute Myocardial Infarction; OPCAB
INTRODUCTION

Off pump coronary artery bypass grafting (OPCAB) for myocardial revascularization is thought to be a better strategy for high-risk patients including those with acute coronary syndrome (ACS).1,2,3 Patients presenting with ACS in form of unstable angina (UA), non-ST-segment elevation myocardial infarction (non-STEMI) and ST-segment myocardial infarction (STEMI) have higher peri-operative mortality as compared to those with chronic stable angina4,5 and it may advisable to delay the surgical procedure wherever possible.  However, continuing chest pain and hemodynamic instability form indications for emergency surgical therapy. The mortality in this subset ranges from 1.6-32% depending on preoperative hemodynamic condition if surgery is done using conventional on pump surgery on arrested heart.4,5 Use of intra aortic balloon pump (IABP) is one method to support the heart to improve outcome in these high risk patients.6  The indication and timing of IABP insertion is controversial.7  Some reports suggest elective preoperative IABP in these patients especially during OPCAB8 while others use it during or after surgery to treat low cardiac output.  Even after adjustments, there is almost three fold variation in IABP use across centers.9 IABP is not a benign intervention.  It is an intensive case

based treatment and therefore expensive.  In addition there is 11-33% rate of vascular complications including limb ischemia.10 On the other hand, unusual delay in use of IABP may result in poor outcome.  These factors reflect a lack of consensus in appropriate use of IABP in OPCAB patients.  Aim of this study is to determine objective criteria for predicting the use of IABP in patients with ACS undergoing OPCAB and to determine short-term results in these patients.

Patients & Methods:

All the patients operated at our institution for ACS, within one week of event, from January-2004 to July-2006 were included in the study.  Patients operated for complication of myocardial infarction (MI) like ventricular septal defect and left ventricular aneurysm were excluded from the study as they were operated electively using cardiopulmonary bypass (CPB) and cardioplegic arrest. There were 46 patients (43 men, mean age 59.6 + 9.1 years), which comprised 7.36% of total 625 patients undergoing CABG in our institution during the same period.  The patients were divided into two groups: Group1, patients with ongoing pain and presentation to the emergency room > 6 hours after onset of pain (UA group) and
Correspondence: Dr. Shantanu Pande, Associate Professor, Department of Cardiovascular and Thoracic Surgery, Lucknow, 226014 UP, India
E mail: [email protected]
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Group2, patients with MI within 1 week of presentation and having Post angina (AMI Group; diagnosis of MI based on conventional electrocardiographic and enzyme criteria). Need for surgery was confirmed by presence of regional wall motion abnormality on echocardiography and coronary artery occlusion not amenable to percutaneous intervention (PCI) on coronary angiography (left main disease, complicated lesions and multiple lesions).  Surgery was indicated only if PCI was not possible or after failed PCI.
The same surgical team performed all the operations.  We have started using OPCAB as a primary strategy for CABG for all patients since January-2003 and the intent in this group of patients was also to perform OPCAB.
The decision to use intra aortic balloon pump (IABP) was reached during induction, based on heamodynamic parameters namely ventricular tachycardia, ventricular fibrillation, systolic arterial blood pressure of less than 70 mm Hg and peak pulmonary artery pressure of more than 60 mm Hg, measured by Swan Ganz catheter (Edwards Lifesciences LLC, Irvine CA, USA). The decision to use CPB was based on presence of hemodynamic instability, evidenced by change in cardiac rhythm, ventricular fibrillation or systolic arterial blood pressure of <70 mm Hg requiring inotropic support even when the cardiac position was returned to normal.
All patients underwent general anaesthesia induced with propofol, midazolam  and a moderate dose of fentanyl. Anaesthesia was maintained with inhalational agents (isoflurane) and fentanyl. Body temperature during OPCAB was maintained by adjusting room temperature and patient warming mattress. Systemic pressure was maintained above 100 mm Hg for adequate coronary perfusion by use of patient positioning, intravenous fluids and inotropes as indicated. Heparin (1 mg per Kg body weight) was given just before dividing the distal end of the internal mammary graft. The skeletenization of the artery was done in all cases.

A midline sternotomy was performed routinely on all patients. Octopus® 3 stabalizer(Medtronic Inc. Minneapolis, MN, USA) was used in all cases for stabilization during distal anastamosis.  Hemostasis following arteriotomy was achieved by pressing the artery proximal to arteriotomy site with the back of tissue forceps by the assistant and then intracoronary shunt (Chase medical, Richardson, Tx, USA) of appropriate size was inserted to restore the coronary circulation to avoid regional wall ischaemia during all distal grafting. Intracoronary shunt was removed before tying the last knot of the anastamosis. First graft in all the patients was left internal mammary artery (LIMA) to left anterior descending artery (LAD) after placing the sponge roll between posterior aspect of left ventricle and pericardium. All distal arterial anastamosis

were completed using 8/0 polypropylene (detail) suture and venous anastamosis were done with 7/0 polypropylene suture. After distal anastamosis of first graft (LIMA to LAD), all proximal anastamosis for reversed saphenous vein graft were done using 6/0 polypropylene suture on partial claming of ascending aorta. The second distal anastamosis was done to posterior descending artery or right coronary artery followed by diagonal or obtuse marginal artery grafting. For posterior and lateral grafts, positioning of the left ventricle was done by placing two posterior pericardial sutures, one between inferior vena-cava and left inferior pulmonary vein and the other one cm. above the left inferior pulmonary vein.
In patient requiring conversion to CPB, the aortic and two stage single venous cannulation was used and operation was performed under normothermia with empty beating heart. Pulmonary artery venting was done in all cases. Rest of the operation was completed using octopus 3 stabilizer and intracoronary shunt as described above.
IABP insertion, if indicated, was done from groin in all the cases. The balloon 8 french with 40 cc balloon (Arrow International, Everett, MA, USA) was inserted percutaneously using sheathless technique.
Statistical analysis: Data are expressed as median and range. Mann-Whitney U test was used for comparision between two groups for continuous variables and Chi-square test with Fisher’s exact test was used for categorical variables. A p value of <0.05 was taken as statiscally significant. Multivariate logistic regression analysis was performed to predict use of IABP by various risk factors. The odds ratio (OR) and 95% confidence intervals (CI) are given. Pearson correlation was done to verify correlation between two variables. All analysis were performed using SPSS version 10.0 software (Chicago, IL)
Result:

46 patients were included in this study.  The AMI group & UA group had 21 and 25 patients respectively. The preoperative patient characteristics and clinical details are given in table 1.  Except for the need of heparin infusion (13 in AMI group. Vs. 6 in UA group; P = 0.02) and dopamine infusion (5 in AMI group. vs. none in UA group. P=0.03), the two groups did not differ significantly.  Table 2 gives the details of preoperative echocardiography in these patients.  Left ventricular dimensions and function did not differ significantly in two groups but the incidence of grade 2 or more mitral regurgitation (MR) was higher in UA group (8 vs. none in AMI group; P= 0.004).  Table 3 shows the details of surgery postoperative outcome in the two groups.  The use of IABP and inotrope was higher in AMI group and these
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patients also had higher ICU stay as compared to UA group.  But the total hospital stay was similar in two groups.  CPB support was required in 2 patients in each group.  The indications for CPB use in AMI group were hemodynamic instability and low cardiac output and it was initiated at the start of surgery.  In UA group, the CPB was initiated during surgery while performing lateral grafts due to hemodynamic instability persisting even after the heart was returned to normal position.  Both these patients had LIMA to LAD anastomosis completed before CPB was required. IABP was also inserted in all the patients requiring CPB.

Two out of 46 (4.34%) patients died (on day 5 and 7 respectively) after surgery and both these cases were in UA group.  The cause of death in both was infection and septicemia leading to multi organ failure.  There were no IABP related complications.  All the surviving 44 patients were followed-up for a median of 14 months (8 – 24 months). 42 patients are free of angina at last follow-up. Two patients had hospitalization. One was admitted following left ventricular failure with severe uncontrolled arterial hypertension and the other with chronic stable angina CCS II. Angiography was done for these 2 cases at 6 and 8 months follow-up respectively. One patient had a blocked graft to postero-lateral vessel and the other had a blocked

Table 1. Preoperative patient and clinical characteristic

 

AMI
N=21
Median  (range)

UA
N=25
Median (range)

P value

Age (years)

61 (34-74)

58 (48-77)

0.5

BSA (m2)

1.62 (1.4-2.0)

1.63 (1.32-2.0)

0.6

AOE (CCS)

2 (1-3)

2 (2-3)

0.7

DOE (NYHA class)

3 (1-4)

2 (2-3)

0.2

S. creatnine (mg/dL)

1.1 (0.7-2.1)

1.1 (0.8-2.7)

0.8

Diabetes Mellitus (n)

6

11

0.7

Hypertension (n)

10

12

0.1

Smoking (n)

9

7

0.3

Heparin infusion (n)

13

6

0.02

Dopamine infusion (n)

5

0

0.03

GTN infusion (n)

14

11

0.3

AMI= acute myocardial infarction, UA= unstable angina, BSA= body surface area, AOE= angina on exertion, CCS= Canadian cardiology society score, DOE= dyspnoea on exertion, NYHA= New York heart association, GTN= glycerine trinitrate

 

Table 2. Echocardiograhic details

 

AMI (n=21)
Median (range)

UA (n=25)
Median (range)

 

Preoperative

Postoperative

P

Preoperative

Postoperative

P

LVEDD(mm)

50(33-58)

47.5(35-65)

0.6

49(35-67)

54(43-74)

0.2

LVESD(mm)

32(15-46)

32(15-67)

0.7

34(22-51)

40(20-64)

0.2

LVEF(%)

50(20-70)

50(20-70)

0.2

40(15-76)

50(20-61)

0.5

MR +

5

5

 

5

1

 

MR ++

0

0

 

8

7

 

MR +++

0

0

 

0

1

 

AMI= acute myocardial infarction, UA= unstable angina, LVEDD= left ventricular end-diastolic dimension, LVESD= left ventricular end-systolic dimension, LVEF= left ventricular ejection fraction, MR= mitral regurgitation

 

Table 3. Operative details and post operative outcome

 

AMI (n=21)

UA (n=25)

P value

CPB use

4

0

0.001

CPB in min.

99.5(31-180)

Nil

 

Time from AMI to surgery (in hrs.)

96(24-300)

Nil

 

Grafts (n)

2 (1-5)

3 (1-4)

0.2

IABP use

10

6

0.03

Pressure ratio

0.28(0.13-0.50)

0.37(0.2-0.61)

0.12

Ventilation (in hrs)

26.5(4-166)

8(3-18)

0.04

Inotrope (in hrs)

49.7(0-120)

13(0-60)

0.001

Preoperative MR (n)

5

13

0.004

Postoperative MR (n)

5

9

0.07

ICU stay (in hrs)

115(56-791)

65(26-91)

0.002

Total stay (in days)

13(0-28)

14(9-23)

0.21

AMI= acute myocardial infarction, UA= unstable angina, CPB= cardiopulmonary bypass, IABP= intra-aortic balloon pump, MR= mitral regurgitation, Pressure ratio= ratio of mean pressure of pulmonary artery and radial artery, ICU= intensive care unit

 

Table 4. Analysis on use of IABP


Factors

IABP n=16
Median (range)

No IABP n=30
Median (range)

P value

BSA

1.6 (1.4-1.8)

1.62 (1.45-2.02)

0.08

Preop S. creatnine(mg%)

1.1 (0.8-2.7)

1.0 (0.7-1.3)

0.25

Preop LVESD(mm)

38 (26-64)

31.5 (15-55)

0.31

Preop LVEDD(mm)

50 (40-74)

47 (33-68)

0.31

Preop EF (%)

40 (20-60)

55 (20-75)

0.04

Preop Mild MR (n)

5

5

 

Preop Mod MR (n)

2

6

 

AMI (n)

8

13

 

UA (n)

8

17

 

CPB use (n)

2

2

 

No. of grafts

3 (2-4)

3 (1-5)

0.50

Pressure ratio

0.36 (0.16-0.61)

0.25 (0.13-0.35)

0.008

ICU stay (hrs)

120 (72-216)

72 (48-791)

0.02

Ventilation time (hrs)

14 (5-68)

8 (3-166)

0.006

Inotrope duration(hrs)

50 (0-120)

1 (0-96)

0.008

Total stay (days)

13 (0-28)

12 (8-24)

0.74

IABP= intra-aortic balloon pump, BSA= body surface area, Preop= preoperative, LVESD= left ventricular end-systolic dimension, LVEDD= left ventricular end-diastolic dimension, EF= ejection fraction, MR= mitral regurgitation, AMI= acute myocardial infarction, UA= unstable

graft to posterior descending artery. In both instances a reverse saphenous vein grafts was used. Both these patients are doing well under optimal medical treatment at 10 and 22 months follow-up respectively.

After analysis of groups based on the use of IABP (Table 4), patients having poor preoperative left ventricular ejection fraction (55 ± 14.6 vs. 40 ± 14.3, p=0.04) and higher ratio of mean pulmonary and systemic arterial pressure (0.25 ± 0.06 vs. 0.36 ± 0.13, p=0.008) were associated with higher need of preoperative IABP use. In patients where IABP was used had longer ventilation time, longer duration of inotropic support, and longer ICU stay. Total hospital stay was similar regardless of IABP use.
There was highly significant positive correlation (r=0.519) between IABP use and ratio of mean pulmonary and systemic arterial pressures (p=0.004). The cutoff value is 0.40 angina, CPB= cardiopulmonary bypass, Pressure ratio= ratio of mean pressure of pulmonary artery and radial artery, ICU= intensive care unit.

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Multivariate analysis of predictors of IABP use
The factors included in the analysis were LV dimension and function, time since AMI, CPB use, preoperative inotropic use, preoperative mitral regurgitation, size of the target coronary arteries and ratio of mean pulmonary and systemic arterial pressures. The only significant predictor was preoperative mean pulmonary and systemic arterial pressure (p=0.016, odds ratio 3.32, 95% CI 1.24-8.84). With ratio of more than 40% the positive prediction for use of IABP was 73.3% and with ratio less than 40% the prediction of no use of IABP was 64.3% with total prediction in 69% cases. Similarly the use of CPB was correctly predicted in 31% cases with preoperative IABP and there was correct prediction of no requirement of CPB in 90% cases without preoperative IABP (p = 0.08, odds ratio 4.09). Pressure ratio predicted the use of IABP (p=0.01, odds ratio 19.4, 95% CI 1.9-190).
Discussion:

This study included 13.6% patients with ACS out of 625 patients undergoing isolated CABG during same time period. The reported incidence of ACS with or without presence of shock is 3 – 24% in different series11, 12. There was 34.7% incidence of use of IABP in this study, which is more than the incidence of 11.7% reported in a large Portuguese registry of ACS13. The controversy about indications of IABP use may be responsible for the apparent practice variation14. Wennberg has stated that “variation occurs because the profession lacks consensus on the correct way to practice medicine” and clinical research should target those intervention for which there is greatest variation15. Christenson’s trial 16 demonstrated a survival benefit with preoperative IABP insertion for coronary artery bypass grafting surgery in patients with any two of the following: ejection fraction < 40%, left main stenosis > 70%, redo surgery or preoperative intravenous nitroglycerine. In contrary Baskett et al has demonstrated the use of preoperative IABP was consistently associated with higher mortality17. With these contrasting views, we evaluated the prediction for need for use of IABP in high-risk surgical group rather than preoperative insertion as a protocol. The IABP was used in 16 of 46 cases studied. The indication followed for use of IABP in this study was hemodynamic parameters rather than disease profile and poor preoperative left ventricular function as used in other studies. The most important factor predicting the use of IABP was ratio between mean pulmonary and systemic arterial pressure preoperatively. This ratio has been extensively investigated in cardiac surgical patient to predict hemodynamic complications after cardiac surgery and is not influenced by the induction of anaesthesia, and tends to correlate with the severity of left ventricular diastolic profile18.

This ratio is a dynamic parameter that keeps changing with the change in the end diastolic pressure of the left ventricle and can thus fairly predict the future hemodynamic compromise. The rising ratio of mean pulmonary and systemic arterial pressure can thus predict the need for IABP with grafting being performed in all the position. Though it has been shown that delayed IABP insertion is associated with higher mortality19, the right time of using the IABP can be predicted by the rise in this parameter and thus hemodynamic compromise and occurrence of unfavorable outcome can be avoided. The use of preoperative IABP in high risk off pump coronary artery bypass20 can thus be replaced by easily available simple analysis of a hemodynamic parameter.
In this study mitral regurgitation preoperatively was observed more in unstable angina group probably due to evolving ischaemia. This factor also predicted the use of IABP in multivariate analysis but had a very wide range of confidence interval. This signifies other confounding variables which may be affecting the status of mitral regurgitation during induction, operation and postoperatively. The prediction on this single factor is therefore unreliable.
This study has not used the classical indications for preoperative insertion of IABP in high risk patients, still the conversion rates to conventional on pump coronary artery bypass grafting was low (2/46)21. Since there is considerable risk of lower limb ischaemia with IABP insertion9,22 this reliable and easily available criteria helps in decision making. In cases where IABP was used required longer ventilation time, inotropic support and ICU stay, though the hospital stay was similar. These cases were sicker subset of the cohort. There was 7.7% incidence of angina noticed in this study which was observed in 17.6% cases in the Portuguese registry of ACS patients.
Conclusion: Intra aortic balloon pump insertion in high-risk group can be safely predicted by using the criteria of ratio between mean pulmonary and systemic arterial pressure of more than 0.40. This ratio alone can replace the classical indications of preoperative IABP use in high-risk coronary artery bypass grafting patients.

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