Eur J Cardiothorac Surg 2003;23:175-178
© 2003 Elsevier Science NL
Effects of atrial fibrillation on coronary artery bypass graft flow
Hankei Shina*,
Kenichi Hashizumea,
Yoshimi Iinoa,
Kiyoshi Koizumia,
Toru Matayoshib,
Ryohei Yozua
a Division of Cardiovascular Surgery, Department of Surgery, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan
b Medical Engineering Center, Keio University School of Medicine, Tokyo, Japan
Received 5 May 2002;
received in revised form 6 October 2002;
accepted 21 October 2002.
* Corresponding author. Tel.: +81-3-5363-3804; fax: +81-3-5379-3034
e-mail: h-shin{at}sc.itc.keio.ac.jp
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Abstract
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Objectives: No detailed studies exist of coronary artery bypass graft flow during atrial fibrillation. We examined the effects on bypass graft flow of atrial fibrillation following coronary artery bypass grafting. Methods: Immediately after surgical revisualization, atrial fibrillation was induced in 18 patients by high frequency atrial pacing. Hemodynamic variables were measured in sinus rhythm and atrial fibrillation. The graft flow in pedicled left internal thoracic artery grafts and in saphenous vein grafts was also measured using transit-time flowmetry. Results: Left internal thoracic artery graft flow had a greater diastolic component than saphenous vein graft flow, as shown by the percent diastolic time-flow integral (86±10% in the left thoracic artery and 62±12% in the saphenous vein, P<0.0001). The induced atrial fibrillation caused significant deterioration in hemodynamics: heart rate and central venous pressure increased, and mean arterial pressure and cardiac index decreased (all P<0.0025). In left internal thoracic artery grafts (n=18) and also in saphenous vein grafts (n=20), graft flow decreased significantly with atrial fibrillation (44.3±26.2 to 26.2±20.7 ml/min in the left internal thoracic artery, P=0.0003; 39.7±15.6 to 33.3±14.3 ml/min in the saphenous vein, P=0.001). The reduction in graft flow due to atrial fibrillation was much larger in left internal thoracic artery grafts than in saphenous vein grafts (P=0.0008). Conclusions: Direct measurement of coronary artery bypass graft flow shows that atrial fibrillation after surgery significantly reduces graft flow. The effect is much larger in left internal thoracic artery grafts with their strong diastolic component than in saphenous vein grafts.
Key Words: Coronary artery bypass grafting • Coronary artery bypass graft flow • Atrial fibrillation • Transit-time flowmetry
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1. Introduction
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Atrial fibrillation (AF) is the most common arrhythmia following coronary artery bypass grafting (CABG), with an incidence of 10–40% [1–5]. Even AF that is perceived as benign is likely to lead to serious complications such as hemodynamic instability and thromboembolic events, and is associated with prolonged hospitalization [4–7]. Management of AF following CABG is directed initially at controlling the ventricular rate; pharmacological and electrical defibrillation are options when the arrhythmia is prolonged or when hemodynamic instability becomes serious [8,9]. This therapeutic strategy does not take into account the direct effects of AF on the bypass graft. Here, the direct effects on bypass graft flow of AF following CABG are studied by inducing AF immediately after surgery.
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2. Patients and methods
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2.1. Patients
This article studies 18 patients who underwent elective, isolated CABG, and in whom AF was induced. Their mean age was 65±8 years and 12 of the patients were male. Seventeen patients underwent conventional CABG with cardiopulmonary bypass; the remaining patient underwent off-pump CABG. In all patients a pedicled left internal thoracic artery (LITA) was anastomosed to the left anterior descending coronary artery, and the saphenous vein (SV) and/or other arteries (radial artery, right gastroepiploic artery, and/or right internal thoracic artery) were used for additional bypass conduits in all patients but one. All grafted vessels had 75% stenosis or more. All patients underwent complete revascularization and were weaned successfully from cardiopulmonary bypass. In the perioperative period, nitroglycerin or nitrosorbide (0.5–1.0 µg kg-1 min-1) was used in all patients, and a low dose of inotropic support (3 µg kg-1 min-1 of dobutamine or dopamine) was used in most patients. An additional low dose of diltiazem (0.5–1.0 µg kg-1 min-1) or milrinone (0.25–0.5 µg kg-1 min-1) was administered to patients who had radial artery grafts. Table 1 summarizes the characteristics of the patients. The procedures of inducing AF and measuring graft flow were approved by the Ethics Committee of our institute, and informed consent for the procedure was obtained in advance from all patients.
2.2. Study protocol
In each patient the study began after a 30 min stabilization period once revascularization was completed and before the sternum was closed. At this time, all patients were in sinus rhythm (SR) with stable hemodynamic status. The following hemodynamic variables were recorded in SR: heart rate (HR), mean arterial pressure (MAP), central venous pressure (CVP), mean pulmonary arterial pressure (MPAP), and cardiac index (CI). To obtain the CI on the spot, a thermodilution method was used with bolus injection of 10 ml cold saline into a pulmonary artery catheter (Swan-Ganz CCOmbl CCO/SvO2/VIP, Edwards Lifesciences LLC, Irvine, CA, USA). Each graft flow was measured at the same time using a transit-time flowmeter (CardioMed CM2005, MediStem AS, Oslo, Norway). The percent diastolic time-flow integral (%DTFI) of graft flow was determined; this gives the ratio of the diastolic antegrade graft flow volume to the total antegrade graft flow volume, and provides insight into the relation between graft flow characteristics and effects of AF on bypass graft flow (Fig. 1)
. The measurement required image processing and analysis software (Socion Image Beta 3b, Socion Corporation, Frederick, MD, USA).
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Fig. 1. LITA graft (above) and SV graft (below) flow curves in sinus rhythm. LITA graft flow is much more diastolic-predominant than SV graft flow, as shown by %DTFI. LITA, left internal thoracic artery; SV, saphenous vein; %DTFI, percent diastolic time-flow integral.
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After measurements of the hemodynamics and graft flow in SR, AF was induced by high frequency bipolar atrial pacing, using a temporary pacemaker (Model 5348, Medtronic Inc., Minneapolis, MN). One temporary pacing lead was placed in the Backmann's bundle, and the other in the interatrial sulcus. Thirty seconds of atrial pacing was then given at a frequency of 800/min and 10 V output. This pacing induced sustained AF in 11 patients. Even repetition of this stimulation did not induce AF in the other seven patients, and high frequency atrial pacing was continued to maintain AF during the measurements of hemodynamics and graft flow. After a 5 min period to allow the AF to stabilize, the measurements were performed. Electrical defibrillation was then performed to restore SR; seven patients recovered SR as soon as the high frequency pacing ceased. After a further 5 min stabilization period, the variables were measured again in SR. The quoted SR value is the mean of the pre- and post-AF values.
2.3. Data analysis
All values are presented as the mean±SD. The Wilcoxon signed rank test was used to compare SR and AF, and the Mann–Whitney U-test was used to compare the %DTFI and the degree of graft blood flow reduction. Values of P less than 0.05 were considered statistically significant.
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3. Results
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AF that was induced immediately after CABG caused a clear deterioration in the hemodynamics: HR and CVP increased, and MAP and CI decreased. Significant differences were observed in these four variables between SR and AF (P=0.0019, 0.0004, 0.0011, and 0.0025, respectively). MPAP did not change significantly. The LITA graft flow was 44.3±26.2 ml/min in SR, and 26.2±20.7 ml/min in AF (P=0.0003). The SV graft flow was 39.7±15.6 ml/min in SR, and 33.3±14.3 ml/min in AF (P=0.001). The graft flow decreased in both LITA and SV grafts with AF; there was a greater reduction in the LITA graft than in the SV graft (39.8±18.1% vs. 17.3±15.6%, P=0.0008). Table 2 summarizes these differences. In the other grafts (radial, right gastroepiploic, and free right internal thoracic artery) the graft flow reduced during AF, but statistical analyses were not performed because of the small number of subjects.
The graft flow curve indicates that the LITA graft flow is more diastolic-predominant than the SV graft flow, based on %DTFI (86±10% and 62±12% in LITA grafts and SV grafts, respectively, P<0.0001).
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4. Discussion
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AF is one of the most common postoperative problems following CABG. This arrhythmia is generally perceived as benign, but is likely to result in serious morbidity and to lengthen the duration of hospitalization [4–7]. Extensive efforts have been made to achieve prophylaxis of postoperative AF, both pharmacologically and electrophysiologically. There has been some success, but complete prophylaxis remains elusive and the incidence is still substantial [10–15].
The therapeutic strategy for the management of AF is conservative. Control of the ventricular rate by drugs such as digitalis and Ca-blocker is preferred over defibrillation provided that hemodynamic conditions including systemic blood pressure and CI are within an acceptable range. When AF continues with no sign of restoration of SR, pharmacological defibrillation is initiated using antiarrhythmic drugs. Defibrillation using direct current is used if systemic hemodynamic conditions deteriorate or if pharmacological defibrillation is unsuccessful [8,9]. This strategy is based on systemic hemodynamic status, but does not take the bypass graft flow into account. In present therapeutic strategies against postoperative AF following CABG, this is a major deficiency. Postoperative AF usually occurs on or after the second postoperative day [5,8]. At this stage it is difficult to determine the graft flow directly, but recent TTFM advances now allow graft flow measurement intraoperatively. We therefore induced AF immediately after CABG in order to study the effects of AF on coronary artery bypass graft flow.
The induced AF caused clear hemodynamic changes: the HR increased, CVP rose, MAP dropped, and CI declined significantly. These changes are similar to those in postoperative AF encountered in clinical practice. Graft flow decreased with AF in both LITA and SV grafts. The reduction in SV graft flow (17.3%) was almost identical to the MAP or CI reduction, whereas the reduction in LITA graft flow (39.8%) was much greater than the reduction in MAP or CI. This difference is probably a consequence of the graft flow characteristics, since the total LITA graft flow depends much more on the diastolic flow than the SV graft flow does, as indicated by the %DTFI (Fig. 1). In AF, loss of atrial contraction causes the hemodynamics to deteriorate. Ventricular filling caused by atrial contraction at the end of the diastole abruptly elevates ventricular end-diastolic pressure and volume [16]. This allows the mean ventricular pressure to be lower through most of the diastole than without effective atrial contraction. As a result, the ventricular end-diastolic volume is lower, and the mean ventricular pressure throughout the diastole is higher, in AF without atrial contraction than in SR with atrial contraction. A reduction in ventricular end-diastolic volume leads to a reduction in stroke volume and CI. Reduction in the stroke volume and CI is associated with arterial hypotension. Arterial hypotension due to a decrease in CI causes a reduction in graft flow. Furthermore, an increase in left ventricular pressure during the diastole reduces graft flow, because the graft perfusion pressure during the diastole arises from the pressure difference between the arterial pressure and the left ventricular pressure. The graft flow therefore decreases in AF, and the effect of AF is probably larger on LITA graft flow, with its diastolic predominance, than on SV graft flow. Direct measurement of the difference between the arterial pressure and the left ventricular pressure would test this claim. As well as AF, tachycardia might itself impair graft flow, especially when the total graft flow depends on the diastolic graft flow. Ferro and associates indicated that the duration of the diastole was a factor limiting coronary blood flow, since coronary blood flow to the left ventricle occurs mainly during the diastole [17]. This argument probably applies to bypass graft flow, and LITA graft flow with its strong diastolic component decreases more readily in tachycardia. In tachycardic AF, loss of atrial contraction and a rapid ventricular rate both appear to affect graft flow, particularly LITA graft flow.
The present study shows that postoperative AF has a detrimental effect on bypass graft flow. Maintenance of good graft flow is vital in avoiding myocardial ischemia and early graft occlusion. Once AF has occurred after CABG, therefore, control of the preload or the afterload, to aid in correcting the effect of AF, would be helpful in clinical practice in addition to prophylaxis of AF. Aggressive strategies may also be desirable to restore SR in the early phase.
The present study possibly contains an artifact. In clinical practice, AF usually occurs on or after the second postoperative day, and occurs only rarely immediately after surgery [5,8]. The heart and bypass grafts are particularly vulnerable immediately after CABG. The effects of AF induced immediately after surgery, as here, might therefore be greater than those usually observed in clinical practice. Gurné and associates reported that the LITA graft flow, which was primarily diastolic soon after surgery, became primarily systolic in the late period [18]; however, Ichikawa and associates evaluated LITA graft flow after surgery over 10 years and found that LITA graft flow maintained its diastolic predominance even 10 years after surgery [19]. Evidently, further investigations are needed to clarify the effects of AF on bypass graft flow in both the early and late postoperative periods.
In summary, AF was induced immediately after CABG, and its effects on coronary artery bypass graft flow were examined. Direct measurement of the graft flow verifies that AF significantly reduces the graft flow. The effect is much greater in LITA grafts with strong high diastolic dependence than in SV grafts. To maintain good bypass graft flow, in addition to prophylaxis of AF, once AF after CABG has occurred, early defibrillation may be desirable.
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Abstract
1. Introduction
