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Eur J Cardiothorac Surg 2006;29:941-947
© 2006 Elsevier Science NL

Morbidity and mortality following acute conversion from off-pump to on-pump coronary surgery

Bristol Heart Institute, University of Bristol, Bristol Royal Infirmary, Bristol BS2 8HW, UK

Received 19 December 2005; received in revised form 8 March 2006; accepted 10 March 2006.

^_* Corresponding author. Tel.: +44 117 928 3145; fax: +44 117 929 9737. (Email: g.d.angelini{at}bristol.ac.uk).

	Abstract

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

 
Objective: Many studies have described reduced morbidity in hospital and equivalent midterm outcomes with off-pump coronary artery bypass (OPCAB) surgery compared to conventional CABG (CABG-CPB). However, OPCAB is sometimes converted acutely to CABG-CPB. We describe the risk of acute conversion and compare patients’ outcomes for acutely converted OPCAB with unconverted OPCAB and CABG-CPB. Methods: Consecutive acute conversions, i.e. OPCAB patients in whom CPB was instituted urgently for hemodynamic or electrical instability, cardiac arrest or uncontrolled bleeding, were compared with propensity-matched unconverted OPCAB and CABG-CPB patients. Relative risks of death and complications in hospital, and subsequent survival were estimated. Results: The risk of acute conversion between 1996 and 2004 was 1.1% (27/2492): 5.1% in the first 2 years, 2.2% in the third year and 0.8% subsequently. Odds ratios for death in hospital compared to unconverted OPCAB and CABG-CPB were 4.4 (95% confidence interval (CI) 0.67–29.1) and 4.7 (95% CI 1.03–21.1), respectively, and ranged from 0 to 4.5 for serious complications. Converted patients had an increased hazard of death for 3 years after surgery compared to unconverted OPCAB (hazard ratio 3.21, 95% CI 1.20–8.59) and CABG-CPB patients (hazard ratio 3.23, 95% CI 1.41–7.39). Conclusions: Experienced OPCAB surgeons have a low risk of acute conversion. Acutely converted patients have a moderately increased risk of death and serious complications in hospital. These risks are difficult to quantify precisely because conversion is rare.

Key Words: CABG • OPCAB • Conversion • Mortality • Morbidity

	1. Introduction

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

 
Both observational studies [1–5] and randomized controlled trials [6–8] have described reduced morbidity in hospital and equivalent midterm outcomes [9,10] with OPCAB surgery compared to conventional CABG (CABG-CPB). In some centres, OPCAB is now the preferred technique for isolated coronary artery bypass (CABG) operations [1,11]. Evidence is accruing that OPCAB surgery has benefits compared to CABG [4,12–14].

However, manipulation of the heart required to do OPCAB can bring about haemodynamic and electrical changes [15–18]. In a small proportion of OPCAB patients, it may be necessary to institute cardiopulmonary bypass (CPB) as an emergency because of acute haemodynamic or electrical instability, cardiac arrest or haemorrhage. We refer to this as ‘acute conversion’.

The risk of acute conversion, and outcome compared to unconverted patients, has been described in two large series of OPCAB patients [17,18]. Observed risks of acute conversion were 1.7% (28/1644) and 3.0% (50/1678). Converted OPCAB patients were reported to have a 12-fold and an 8-fold higher mortality in hospital than CABG-CPB patients [17] and unconverted OPCAB patients [18] (absolute risks of 18% and 12%, respectively). Compared with unconverted OPCAB patients, converted patients also had a six-fold increased risk of stroke and similar increased risks of other serious post-operative complications [18]. A higher risk of mortality has also been observed in a large cardiac registry [19] but the available data did not allow acute conversions to be distinguished from other, non-acute conversions, e.g. because of the coronary anatomy.

These findings contrast with our own experience. We describe the risk of conversion in OPCAB patients and compare their outcomes with outcomes for similar unconverted OPCAB and CABG-CPB patients. Both comparisons were of interest because the reported benefits of OPCAB compared to CABG-CPB may offset increased morbidity arising from conversion.

	2. Materials and methods

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

 
2.1 Patients
Consecutive patients having CABG only operations between 1st April 1996 and 28th September 2004 were considered. This period included all patients who had had OPCAB surgery in our institution, the first such operation having been carried out on 8th May 1996.

2.2 Definition of acute conversion
‘Intention to perform OPCAB’ and ‘conversion’ can be difficult to define, since decisions to switch from planned OPCAB to CABG-CPB can be made at any time and for various reasons, e.g. change in operative staff, change in the clinical state of the patient, coronary anatomy, acute haemodynamic or electrical instability. Reasons other than acute problems during the operation may arise from differences in patients’ characteristics but only emergency conversion should cause morbidity directly for operative reasons [17]. Hence, we distinguish acute conversions from other conversions (including conversions after opening the chest but for non-acute reasons, e.g. because of concern about the coronary anatomy; most such instances occurred early when our technique was evolving and we were gaining experience of how best to access the distal marginal branch of the circumflex artery).

2.3 Data collection
We extracted data from our institutional database for all patients who had isolated CABG operations during the study period. Risk factors [20] and adverse outcomes in hospital [21] were defined as previously. Myocardial infarction was identified clinically (from continuous ECG monitoring in intensive care, and ECG carried out on the basis of clinical suspicion thereafter) and deep sternal wound infection was defined as need for rewiring because of sternal infection. OPCAB patients were classified as converted or not by the following methods: between 1st April 1996 and 31st March 2003, acute conversions were identified retrospectively by hand searching all operation notes; from 1st April 2003, additional fields were added to the database to record conversions. Operation notes for all OPCAB patients recorded as having been converted were examined and classified as acute conversions or not by one of us (G.D.A.).

2.4 Data analysis
Patients with single vessel disease, or who required emergency or salvage operations were excluded because no patient who had an acute conversion had these characteristics. A small number of patients in a trial of beating heart surgery combined with CPB were also excluded. Separate logistic regression models were fitted to data for (a) all OPCAB patients and (b) all patients who had had CPB (i.e. including acute conversions from OPCAB) to estimate propensity scores for acute conversion. The small number of acute conversions meant propensity scores could only be estimated from a small number of variables, which sometimes had to be ‘grouped’. Demographic and pre-operative risk factors included the following: age group, sex, period of operation, extent of coronary disease, Canadian Cardiovascular Society class, New York Heart Association class, ejection fraction, Parsonnet score, diabetes, operative priority.

Up to 11 unconverted OPCAB and CABG-CPB patients were matched by propensity score with each acutely converted OPCAB patient, using a GREEDY algorithm. One converted patient could not be matched with any unconverted OPCAB patients, and two others could only be matched with one and six unconverted OPCAB patients, respectively, giving a total of 271 in the unconverted OPCAB group. Eleven CABG-CPB matches were identified for each acute conversion.

In-hospital outcomes for acutely converted patients were compared with propensity-matched unconverted OPCAB and CABG-CPB patients by logistic regression, calculating robust standard errors to take account of clustering by matched set. Mid-term survival was also compared using Cox proportional hazards regression. All analyses were carried out using STATA v8.2 (Stata Corporation, Texas).

	3. Results

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

 
A total of 6675 patients underwent OPCAB or CABG-CPB operations between 1st April 1996 and 28th September 2004, including consultant and resident-led operations. The percentage of isolated CABG operations carried out using OPCAB increased up to 2002/2003. Since 2002/2003, the percentage has been stable at about 60% of all isolated CABG operations. OPCAB was implemented first by one consultant (G.D.A.), and was then taken up gradually by other consultants and residents. During recent years, surgeon preference has been the main determinant of whether a patient has OPCAB or CABG-CPB. Currently, four of eight consultant cardiac surgeons prefer OPCAB to CABG-CPB and one uses both techniques in a similar percentage. The study period included the ‘learning curves’ for all surgeons.

The overall risk of acute conversion among all OPCAB operations during the study period was 1.1% (27/2492; 95% confidence interval (CI) 0.7–1.6%). The risk varied over time: 5.1% (5/99; 95% CI 1.7–11.4%) between 1st April 1996 and 31st March 1998; 2.2% (3/137; 95% CI 0.5–6.3%) between April 1998 and March 1999; 0.8% (19/2256; 95% CI 0.5–1.3%) thereafter, ranging from 0.3% to 1.5%.

The operating details of conversion, the timing and reason for conversion were as follows. Six patients required conversion during or immediately after grafting of the left anterior descending artery. Three of them experienced electrical and haemodynamic instability; these conversions occurred during the early stage of our OPCAB experience when no intracoronary shunts were used. One patient experienced air embolisation into the coronaries, one required an extensive endoarterectomy and one suffered a cardiac arrest immediately on completion of the anastomosis, which was subsequently regarded by the surgeon as technically inadequate.

Eleven patients required conversion during grafting of the circumflex artery or its branches. One experienced a sudden cardiac arrest when grafting an intramyocardial vessel, in spite of the use of a shunt. The remaining 10 had ischaemia hypotension during grafting.

Six patients required conversion during grafting of the right coronary artery or its branches, four during grafting of the posterior descending artery because of ischaemia hypotension, one during grafting of the main right (no shunt was available at that time), and one in the middle of a complex endoarterectomy into the main right coronary artery.

The four remaining patients required establishment of cardiopulmonary bypass prior to grafting. Two experienced electro-mechanical instability unresponsive to treatment (one fast atrial fibrillation, one ventricular tachycardia) and the other two experienced haemodynamic instability on manipulation of the heart and exposure of the target coronary vessels.

Table 1 shows the characteristics of acutely converted OPCAB patients, and unconverted OPCAB and CABG-CPB patients with matching propensity scores. The groups were well matched both on prognostic factors used to derive the propensity scores and other prognostic factors. The mean numbers of grafts in the three groups were 2.8, 2.7 and 3.1, respectively.

Survival analyses showed that patients who were acutely converted from OPCAB to CABG-CPB had a significantly increased risk of death during follow-up compared to unconverted OPCAB patients (hazard ratio 3.21, 95% CI 1.20–8.59, p = 0.02) and CABG-CPB patients (hazard ratio 3.23, 95% CI 1.41–7.39, p = 0.006). The proportional hazards assumption was satisfied in both analyses. However, the tests for proportionality had low power because of the small number of deaths (7/27, 25/271 and 28/297 in converted OPCAB, unconverted OPCAB and CABG-CPB groups, respectively). Kaplan–Meier cumulative mortality graphs are shown in Fig. 1 .

View larger version (24K): [in this window] [in a new window]   Fig. 1. Kaplan–Meier cumulative mortality graphs for propensity-matched groups of converted OPCAB patients, unconverted OPCAB patients and CABG-CPB patients.

	4. Discussion

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

• The risk of conversion is low after the initial learning curve period.

• Acutely converted patients tended to have higher risks of mortality and morbidity in hospital but because acute conversions were few and complications infrequent, the risks of specific complications were rarely statistically significant.

• The tendency for a higher risk of operative mortality and morbidity was similar whether acute conversions were compared with propensity-matched unconverted OPCAB or CABG-CPB patients.

• Patients who have acute conversions appear to have an increased risk of death or serious cardiac-related event for 2–3 years after surgery.

This is the largest series of OPCAB operations for which the risk of acute conversion has been described. Our findings differ somewhat compared to previous reports describing outcome after acute conversion. First, the overall risk of conversion is relatively low, e.g. 1.1% compared to 1.7% (28 urgent/emergent conversions of 1644 OPCAB operations [17]) and 3.0% (50/1678) [18]. Second, in our institution the relative risks of serious adverse short-term complications (stroke, MI, deep sternal wound infection, reoperation for bleeding, sepsis, GI bleed or perforation, dialysis for renal failure, prolonged ventilation or reintubation, in-hospital mortality) attributable to acute conversion appear to be lower than previously reported.

4.1 Risk of conversion
The risk of acute conversion reported here is representative of our entire experience with OPCAB. Absolute risks of acute conversion are difficult to compare; other surgeons have reported risks of 0.4% [1], 2.8% [22] and 5.4% [23]. Our low risk of conversion may arise in part from varying durations of learning curve. Edgerton and colleagues [17] found that prior OPCAB experience was a highly significant predictor of the risk of conversion. Patel and colleagues [18] reported a risk of 0.7% for their most recent data, although this figure was based on only two acute conversions out of about 285 OPCAB operations. Comparing the decline in our rate of conversion with increasing experience with that of Patel and colleagues, equivalent conversion rates were reached after about 250 and 1400 operations, respectively.

We believe that the method of introduction of OPCAB is an important factor in reducing the risk of conversion. Patel and colleagues [18] carried out 330 and 500 OPCAB operations in their first and second years after introducing the technique, compared to 20 and 79 in our institution. One surgeon in our institution acquired expertise with OPCAB before other surgeons adopted the technique, whereas Patel and colleagues appear to have taken up OPCAB at a similar time. The method of introduction of a new technique is linked to the collective institutional expertise. We have previously described the need to distinguish between (a) acquiring knowledge of how best to implement OPCAB (i.e. evolution of the technique) and (b) gaining expertise to apply the knowledge [20]. We attribute our low conversion rate, despite uptake of OPCAB by other consultants and resident surgeons, primarily to this controlled method of introduction.

Other technical differences in OPCAB technique may also influence the conversion rate. Patel and colleagues [18] suggested that the reduction in their rate of conversions was achieved partly through the introduction of an apical suction cardiac positioning device. We do not use it but we agree that achieving satisfactory exposure and stabilisation is critical. We have been using an intracoronary shunt when constructing most anastomoses for 8 years [24] and believe this is important to prevent ischaemia but do not know how much other surgeons use this device. A surgeon's threshold for conversion may also be important, with outcomes being better if CPB is instituted relatively early rather than waiting to the last minute, although a lower threshold for conversion would be expected to be associated with a higher risk of conversion.

4.2 Relative risk of serious short-term complications
Compared to unconverted OPCAB patients, the relative risks of serious adverse short-term complications ranged from 0 (when no events were observed in patients who had acute conversions, i.e. stroke, MI, sepsis, GI bleed or perforation) to about 4.5, compared to 1.8–10.0 for Patel and colleagues (unadjusted ORs calculated from published data [18]). Our relative risks for converted OPCAB versus CABG-CPB patients cannot easily be compared with the findings of Edgerton and colleagues because different complications were reported. Nevertheless, their relative risks for vascular morbidity, coma > 24 h, peri-operative MI, multisystem organ failure or cardiac arrest seem high, ranging from 6.5 to 22.9 (unadjusted ORs calculated from published data [17]).

Although in-hospital mortality tended to be higher for our acutely converted patients (consistent with the previous reports but to a lesser degree), this finding was based on only two deaths in acutely converted patients. These patients died following cardiac arrest, one because of a technical difficulty in performing the anastomosis and the other because of an intramyocardial severely diseased circumflex. These deaths may have occurred because the surgeon waited too long before establishing CPB.

There are several possible explanations for the lower relative risks we observed:

A. Some of the acute conversions that we included might not have been truly acute, reducing the overall increased risk of morbidity.

B. The risk of adverse outcomes in unconverted OPCAB patients may have been higher in our patients than those of Patel and colleagues [18], reducing the relative risk between converted and unconverted.

C. There may have been subtle differences in the criteria for reporting complications, which could have caused us to identify complications less often in acutely converted patients.

D. Some previous comparisons were not adjusted for patients’ preoperative risk of morbidity [18]; if, before surgery, converted patients have a higher risk of morbidity, unadjusted comparisons would tend to overestimate the relative risks.

Explanation A is unlikely for two reasons. First, we carefully reviewed the operation notes and only classified as acute conversions patients who met the same criteria as described by Patel and colleagues [18]. Second, if some non-acute conversions were misclassified as acute, the true risk of acute conversion in our institution would be even lower than reported.

The summary data shown in Table 1 appear to rule out explanation B. A related possibility is that surgeons’ thresholds for conversion are lower in our institution. This is unlikely, given the low risk of conversion. Surgeons established bypass as soon as warning signs were apparent for all conversions except when the two deaths occurred (see above).

Explanation C is possible since institutions do not use identical protocols. However, the similarity of the risks of complications in unconverted OPCAB patients make it unlikely. Explanation D is also possible, since acutely converted patients of Patel and colleagues [18] had worse preoperative risk factors for post-operative morbidity than unconverted patients.

In view of C and D, the relative risks of acute conversion in our institution may not truly be lower. However, the estimates we report here are more valid, since (i) propensity-matching controls as far as possible for confounding and (ii) identical criteria were used for identifying complications in patients who were and were not acutely converted. Uncertainty in estimating the relative risks is a separate issue, which is inevitable when acute conversion occurs so rarely. Our series of OPCAB patients is larger than previous ones but the low risk of conversion limits the precision of the estimates.

Findings were similar when acutely converted patients were compared with unconverted OPCAB and CABG-CPB patients. Therefore, if one considers the initial choice between OPCAB and CABG-CPB, reductions in the risk of operative morbidity for OPCAB [1–8] do not appear to be offset by increased morbidity from acute conversion. However, the small number of acute conversions limited our ability to quantify any such effect and, overall, ORs for the comparison with CABG-CPB tended to be smaller than for unconverted OPCAB. Is the risk of morbidity from acute conversion an important factor to consider when choosing between OPCAB and CABG-CPB? The answer depends critically on three factors: the absolute risk of conversion, the relative risks of complications and their seriousness. The risk of conversion and the relative risks of complications are likely to vary by institution and, increasingly, these are available from institutional databases. The seriousness of complications should be judged primarily by patients, not surgeons and cardiologists [25], and be ‘weighed’ against the value that patients attach to the likely benefits of OPCAB [8].

4.3 Outcome after discharge from hospital
It is unclear whether the consequences of conversion persist. Both comparisons found that patients who had acute conversions appear to have an increased risk of death during follow-up but the graphs suggest that the risks are confined to the early post-operative period (the proportional hazards test has low power, especially when there are few deaths). Any longer term risk could arise from enduring effects of conversion, for example, poorer quality anastomoses. Alternatively, converted patients may share some characteristic that predisposes them to require acute conversion and, subsequently, to die earlier than unconverted patients, e.g. an underdeveloped or absent collateral circulation.

	5. Conclusions

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

 
Experienced OPCAB surgeons have a risk of acute conversion of about one percent. This low level of risk can be achieved over a small number of operations if the technique is introduced slowly with experienced OPCAB surgeons passing on their ‘know-how’ to less experienced surgeons. Acutely converted patients have moderately increased risks of death or morbidity in hospital. These risks are difficult to quantify precisely because the patients are small in number.

	Acknowledgments

 
This study was carried out with the support of funding from the British Heart Foundation and the Garfield Weston Trust.

	References

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion 5. Conclusions References

 

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