Usefulness of Unipolar Epicardial Ventricular Electrogram for Triggering of Intraaortic Balloon Counterpulsation During Off-Pump Coronary Artery Bypass Surgery in Patients with Hemodynamic Instability Complicating Acute Coronary Syndromes

Abstract

During off-pump coronary artery bypass graft (CABG) surgery for patients requiring preoperative intraaortic balloon counterpulsation (IABP) support, errors in timing of IABP during mobilization of the heart can induce unstable hemodynamic conditions. We applied a simple technique for triggering IABP accurately during off-pump CABG: one end of an epicardial pacemaker wire is placed on the surface of the left ventricle, and the other end is linked to one of the precordial V leads. This technique provided an exact detection of R-wave potentials in any position of the heart during the off-pump CABG procedure. Ten patients with hemodynamic instability complicating acute coronary syndromes underwent off-pump CABG with this technique, including grafting to the circumflex system, safely and successfully.

In Brief

IMPLICATIONS: During off-pump coronary-artery-bypass surgery (OPCAB), errors in timing of intraaortic balloon counterpulsation (IABP) caused by mobilization of the heart could induce hemodynamic instability. We have used a temporary epicardial electrode to trigger the IABP accurately during OPCAB. This technique is useful for performing OPCAB safely when hemodynamic stability depends on effective IABP support.

Coronary artery bypass graft (CABG) surgery on the beating heart without cardiopulmonary bypass (CPB), so-called off-pump CABG (OPCAB), has come into common use worldwide (¹). OPCAB could theoretically confer the benefits of surgical CABG without incurring the risks associated with the use of CPB. However, patients requiring conversion to CPB during the OPCAB procedure have a significantly increased risk of morbidity and mortality than patients whose initial procedure is on-pump CABG (^2,3). Predictors of conversion to CPB include surgeon experience with the technique and hemodynamic instability, such as cardiogenic shock and congestive heart failure (³).

Intraaortic balloon counterpulsation (IABP) provides hemodynamic assistance to patients with hemodynamic instability complicating acute coronary syndromes (^4,5). IABP is conventionally synchronized using either the body surface electrocardiogram (ECG) or the arterial blood pressure waveform. During OPCAB, displacement of the heart from its anatomical position leads to a drastic change of the ECG. Therefore, triggering IABP by ECG causes errors in timing of IABP during mobilization and stabilization of the heart. From our experience, we can distinguish three types of inaccurate triggering of IABP. First, the potentials of the R-wave decrease (Fig. 1, A and B), and IABP triggering fails completely. Second, the potentials and configurations of the P-, R-, and T-waves are continuously changing (Fig. 1C), and IABP can be incorrectly triggered by P- or T-waves rather than R-wave. Third, the potentials of the P- or T- waves may be equal to that of the R-wave (Fig. 1D), and IABP can be continuously triggered by not only the R-wave, but also by P- or T-waves. In addition, in patients with hemodynamic instability, mobilization and stabilization of the heart easily decrease arterial blood pressure. Therefore, triggering of IABP by detecting an altered arterial blood pressure waveform could also cause errors of timing, increasing risk for conversion to CPB. We have applied a simple technique for triggering IABP accurately during the OPCAB procedure. We report our early experience with this technique during OPCAB in patients with hemodynamic instability complicating acute coronary syndromes.

Figure 1.: Electrocardiography (ECG) during off-pump coronary artery bypass surgery (OPCAB). (A) Preoperative ECG. (B) The potentials of the R-wave are too small, and intraaortic balloon counterpulsation (IABP) is not triggered. (C) The potentials and configurations of the P- and R-waves are continuously changing, and IABP is triggered by the P-waves (arrows) rather than the R-wave. (D) The potentials of P-waves are equal to that of the R-wave, and IABP is trigged by both the P- and R-waves.

Methods

Between September 2002 and December 2003, 126 consecutive patients were referred for OPCAB. Among them, 33 (26.1%) patients presented to the operating room with IABP having been placed before surgery by the cardiologist. Of 33 patients with IABP, 10 patients had hemodynamic instability complicating acute coronary syndromes. We define hemodynamic instability as low systolic arterial blood pressure (<90 mm Hg) or low cardiac output (<1.8 L · min⁻¹ · m²) with use of inotropic drugs before IABP insertion. For these 10 patients, we used a technique in triggering IABP accurately during OPCAB. One end of a temporary epicardial pacemaker wire was applied to the anterior surface of the left ventricle (Fig. 2A) and the other end linked to one precordial V-lead (Fig. 2B). We obtained the approval from our institutional ethics committee for using this method during OPCAB, and all patients gave written informed consent.

Figure 2.: (A) During the mobilization and stabilization procedure, one end of the temporary epicardial pacemaker wire is placed on the surface of left ventricle. (B) The other end is connected to one of the precordial V electrodes. (C) This technique usually provides an accurate and continuous detection of the R-wave (arrows) throughout the off-pump coronary-artery-bypass (OPCAB) surgery procedure. (D) The direct triggering technique may result in biphasic configurations.

Results

The clinical and hemodynamic characteristics of 10 patients are described in the Table. Seven patients were men, and three were women. The average age was 71.3 yr (58–84 yr). Seven patients had a history of diabetes mellitus, and two patients had a history of previous CABG. Cardiac catheterization revealed that all patients had triple vessel disease, four patients also had critical left main trunk stenosis (>90% stenosis), and mean left ventricular ejection fraction was 29.7% (18%–43%). Five patients were diagnosed with acute myocardial infarction and the other five with unstable angina pectoris. In eight patients (Patients 2–9) with low systolic blood pressure, IABP was inserted emergently in the catheterization room, and the subsequent measured average cardiac output was 2.16 L · min⁻¹ · m² (1.9–2.6 L · min⁻¹ · m²). In two patients (Patient 1 and 10) with low cardiac output (1.3 and 1.6 L · min⁻¹ · m², respectively), an IABP was inserted electively. All patients underwent OPCAB within 24 h after admission to our center. The mean time on the IABP before OPCAB was 3.4 h (1–9 h). In all 10 patients, temporary epicardial electrode provided an exact and continuous detection of R-wave potentials in any position of the heart throughout the entire OPCAB procedure (Fig. 2C), and complete revascularization, including grafting to the circumflex system, was performed successfully with hemodynamic stability. No patient required conversion to CPB. All patients were tracheally extubated and weaned from IABP within 1 wk after the operation. They were discharged from the hospital within 3 wk.

Table 1: Clinical Data of Patients Undergoing Off-Pump CABG with Direct Triggering Technique

Discussion

In our institute, prior to using this method, IABPs were synchronized with the arterial blood pressure waveform during the OPCAB procedure. In two previous patients with cardiogenic shock complicating acute myocardial infarction however, mobilization and stabilization of the heart induced a decrease in arterial blood pressure, resulting in errors of triggering the IABP. These errors in timing of IABP induced critical hemodynamic conditions, mandating conversion to CPB. These experiences motivated us to use unipolar epicardial ventricular electrogram for triggering of IABP accurately during OPCAB. We found that this method provides excellent timing of IABP during all stages of the OPCAB procedure.

Initially, we used this method in patients with hemodynamic instability complicating acute coronary syndrome requiring IABP support. In these patients, because intraoperative stable hemodynamic conditions largely depend on effective IABP support, errors of triggering of IABP can cause critical hemodynamic conditions. Without this method, these patients can be at high risk for converting to CPB, and we believe that the epicardial lead for timing IABP can reduce this risk for these patients. Conversely, some surgeons prefer “beating heart” CABG with a use of CPB. We cannot comment on our triggering technique in the latter scenario.

In the patients with hemodynamic instability, even without acute coronary syndromes, anesthetic management is often demanding: requirements for IV fluid volume and inotropic drugs are increased to maintain stable hemodynamic conditions. We found that an accurate triggering of IABP with our method could reduce the requirement for IV fluid volume and inotropic drugs. Prophylactic use of IABP with effective methods for triggering may be useful for intraoperative anesthetic management of these patients.

One problem with this technique is selecting the best location for the temporary epicardial pacemaker wire. Placement on the anterior surface or around the apex of the left ventricle usually provides a good configuration of R-wave (Fig. 2C). However, the configuration of the epicardial ventricular electrogram can also change during OPCAB, rarely giving biphasic signals and resulting in double triggering of IABP (Fig. 2D). In this case, the anterior surface of the right ventricle might be an alternative location.

In summary, we found the unipolar epicardial ventricular electrogram for triggering of IABP to be a useful technique for intraoperative anesthetic management of patients with hemodynamic instability complicating acute coronary syndromes undergoing OPCAB.