Surgeon’s prior experience with OPCAB
Previous CABG
Congestive heart failure
Previous myocardial infarction
Urgent states
Low ejection fraction
Preoperative hemodynamic instability
Extremely dilated heart
Higher New York Heart Association class
Mitral regurgitation
Chronic obstructive pulmonary disease
Cardiomegaly
Smaller body surface area
8.2.1.1 Intraoperative Factors
On-pump conversion during OPCAB occurs unexpectedly. Urgent conversion to CPB may be needed during positioning of the heart or grafting or in patients with acute ischemia when there is hemodynamic collapse or ventricular arrhythmias. Conversion needs to be performed most frequently during an attempt to expose the posterior-lateral wall of the heart. Heart displacement to expose the posterior-lateral wall causes mechanical alterations of the normal cardiac geometry such as right ventricular compression, right ventricular outflow tract obstruction, mitral annular deformation leading to functional mitral regurgitation, and impaired left ventricular filling [5]. These changes take place whenever there is displacement of the heart, even when there is normal cardiac function preoperatively.
The other cause of hemodynamic instability is ischemia . Coexistent ischemia compounds the effects of mechanical dysfunction resulting in the need for urgent institution of CPB. During anastomosis, the proximal coronary artery is snared with a silicon-coated suture in our institute for adequate visualization. When the target coronary artery is large, ischemia may be a major cause for conversion, especially in cases with poor collateral vessels. Ischemia in cardiac conduction system supplied by the right coronary artery sometimes causes severe bradycardia. The surgical techniques for prevention of hemodynamic instability are described in the next paragraph.
To achieve a clear field to perform the anastomosis, a low-pressure carbon dioxide gas blower is used widely. It is very helpful, but results in a drop in myocardial temperature that may cause arrhythmias. Excess use of low-pressure carbon dioxide gas should be avoided. In emergent cases, especially acute myocardial infarction, the hemodynamics are sometimes unstable before CABG. In such cases, on-pump CABG should be selected rather than OPCAB, because ventricular arrhythmias and hypotension occur easily. The other circumstances leading to pump conversion intraoperatively are described in Table 8.2 [5].
Table 8.2
Intraoperative circumstances leading to conversion
Inappropriate heart displacement |
Ongoing ischemia |
Hypothermia |
Deep intramyocardial vessels |
Calcified or diffusely diseased target arteries |
Diminutive vessels |
Need for extended endarterectomy of the target vessel |
Cardiomegaly |
Arrhythmias |
8.3 How Can We Prevent Urgent Pump Conversion?
8.3.1 Operative Monitoring
8.3.1.1 Parameters
For intraoperative monitoring, a combination of leads II and V5 is continuously displayed and used for ST-segment trend analysis. Radial artery pressure and central venous pressure are also monitored. In patients with poor left ventricular function (LVEF <35 %), pulmonary artery pressure should be monitored via a Swan-Ganz catheter, and the femoral artery cannulation site should be secured for emergency insertion of an intra-aortic balloon pumping (IABP). Patient temperature is constantly monitored via a properly placed rectal or bladder temperature probe and maintained at >35 °C. Normothermia also helps to achieve hemostasis and early postoperative extubation. Parameters are recorded at baseline, with the heart in the normal position. A CPB setup is kept ready, but not primed, and a perfusionist is readily available.
8.3.1.2 Transesophageal Echocardiography (TEE)
Continuous intraoperative TEE monitoring is very useful. A baseline TEE examination should be done before sternotomy to assess the following: (1) global left ventricular function, (2) regional ventricular wall motion, (3) mitral regurgitation, (4) right ventricular function, (5) right ventricular outflow tract and pulmonary artery, (6) tricuspid regurgitation, (7) aortic valve, and (8) thoracic aorta [7, 9].
TEE during heart displacement and grafting is used to monitor the following parameters: (1) deterioration of global left ventricular function, (2) regional wall motion abnormalities and left ventricular filling, (3) deterioration of mitral regurgitation, (4) right ventricular outflow tract and pulmonary artery obstruction, (5) right atrial filling and tricuspid regurgitation, and (6) aortic regurgitation [7, 9]. Multiple views are monitored during the anastomosis because of difficulties in imaging due to movement of the heart, pericardial traction, and vertical positioning of the heart.
8.3.2 Surgical Techniques
8.3.2.1 Sequence of Anastomosis
The grafting strategy plays an important role in preventing ischemia that potentiates hemodynamic compromise. The anteroseptal vessels are revascularized and perfused first, especially in patients with poor left ventricular function or acute ischemia preoperatively. Heart positioning for exposure of the left anterior descending coronary artery is easy and seldom causes hemodynamic compromise. After revascularization of the anteroseptal vessels, the heart is displaced to graft the remaining target vessels [4]. With a free graft, the proximal anastomosis should be completed before tackling another stenosed vessel if hemodynamics are unstable. In addition, the collateralized vessels should be grafted before collateralizing vessels.
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