Little is known about the changes in the coronary flow velocity reserve (CFVR) of the left anterior descending artery (LAD) before and after coronary artery bypass grafting (CABG). The present study aimed to evaluate the feasibility of measuring the CFVR of the LAD using transthoracic Doppler echocardiography before and after CABG. We prospectively measured the CFVR before and after CABG in 56 patients. The flow velocity in the LAD was measured using transthoracic Doppler echocardiography both at rest and during intravenous infusion of adenosine. The CFVR was calculated as the ratio of hyperemic to the basal peak and mean diastolic flow velocities. Coronary angiography was also performed to assess graft patency after CABG in all patients. Furthermore, we compared the differences between the pre- and postoperative CFVR in patients with and without a diffusely diseased LAD (lesion length >2 cm). All grafts were angiographically patent. The postoperative peak and mean CFVR were significantly increased compared to the preoperative peak and mean CFVR (both peak and mean 2.7 ± 0.9 vs 1.5 ± 0.6, respectively; p <0.0001). The preoperative peak CFVR was significantly lower in patients with a diffusely diseased LAD than in those without a diffusely diseased LAD (1.3 ± 0.5 vs 1.6 ± 0.5, respectively; p = 0.04). The postoperative peak CFVR of the 2 groups was almost identical (2.5 ± 0.6 vs 2.9 ± 1.0; p = 0.07). In conclusion, assessment of the CFVR of the LAD using transthoracic Doppler echocardiography was useful after CABG for confirming graft patency.
The coronary flow velocity reserve (CFVR) with pharmacologic vasodilation is a more useful method to assess the functional significance of native coronary artery stenosis and graft stenosis than flow velocity assessment alone. The diagnostic accuracy of CFVR has been shown to be high (sensitivity of 90% and specificity of 93%) in predicting significant stenosis of the left anterior descending artery (LAD) with a cutoff value of <2.0. However, to the best of our knowledge, little information is available regarding the changes in the CFVR in the LAD before and after coronary artery bypass grafting (CABG). The present study evaluated the changes in the CFVR of the LAD before and after CABG to assess graft function. Furthermore, we compared the differences between the pre- and postoperative CFVR in patients with and without a diffusely diseased LAD.
Methods
We studied 56 selected patients who had undergone isolated CABG at the Sakakibara Heart Institute. The exclusion criteria were severe chronic pulmonary obstructive disease and atrial fibrillation. The patients who had undergone concomitant procedures were also excluded from the present study. Our institutional review committee approved the study protocol. Each patient provided informed consent for postoperative angiography and pre- and postoperative echocardiography with a stress test.
Our strategy for isolated CABG was directed toward obtaining complete myocardial revascularization with an off-pump technique whenever feasible. A median sternotomy was performed in all patients. Left and right internal thoracic artery (ITA) grafts were harvested in a skeletonized fashion. The techniques of off-pump CABG have been previously described. We bypassed all significantly diseased coronary vessels >1 mm in diameter. Anastomosis was performed with an 8-0 polypropylene running suture using the parachute technique. If necessary, concomitant long segmental reconstruction of the LAD with or without endarterectomy was performed in patients with a diffusely diseased LAD. The length of a long segment was defined as >2.0 cm.
We routinely prescribed low-dose aspirin to all patients after CABG, continued indefinitely. Warfarin (maintained with a target international normalized ratio of 2.0) was used for patients with a saphenous vein graft, in addition to low-dose aspirin. The administration of warfarin was stopped after 3 months. In patients with a radial artery graft, nicorandil was administered systemically for the prophylaxis of perioperative vasospasm. An intravenous infusion of nicorandil (0.5 μg/kg/min) was started after the induction of general anesthesia and continued until the second postoperative day. Oral nicorandil 15 mg/day was then prescribed for ≥1 year.
Preoperative angiography was performed within 2 months before CABG in all patients. Postoperative angiography was performed to assess graft patency at 11.2 ± 5.1 days after surgery. The angiographic studies were reviewed and evaluated by 2 cardiologists.
Echocardiography was performed using a SONOS 7500 digital ultrasound system (Philips Medical Systems, Andover, Massachusetts) using a high-frequency transducer (5 to 7 MHz). For color Doppler flow mapping, the velocity range was set at 9.6 to 16.0 cm/s. Adequate filtering was used to minimize low-frequency wall motion artifacts. Echocardiographic images were obtained from the acoustic window around the midclavicular line in the fourth and fifth intercostal spaces in the left lateral decubitus position. After the lower position of the interventricular sulcus was located in the long-axis cross section, the ultrasound beam was rotated laterally, visualizing the distal portion of the LAD under color flow mapping guidance. The color flow was visualized using a high-frequency color Doppler technique (3.5 MHz). The blood flow velocity was measured using pulsed wave Doppler echocardiography (frequency 3.5 MHz), using a sample volume (1.5 to 2.0 mm) placed on the color signal in the distal LAD. We attempted to align the ultrasound beam direction with the distal LAD flow as parallel as possible. All off-line analyses were performed with Xcelera (Philips Medical Systems). The baseline spectral Doppler signal in the LAD was recorded first. Adenosine was administered by intravenous infusion (0.14 mg/kg/min) for 2 minutes to record the spectral Doppler signals during hyperemia. This enabled the attainment of a peak flow response induced by dilation of he coronary microvessels. The electrocardiogram and heart rate were monitored continuously during the patients’ examination. The blood pressure was recorded at baseline and every 1 minute after intravenous adenosine was started. Measurements of the blood flow velocity were performed off-line by tracing the contour of the spectral Doppler signal. An average of the measurements was obtained in 3 cardiac cycles. The CFVR was calculated as the ratio of the hyperemic to basal peak (peak CFVR) and mean (mean CFVR) diastolic flow velocities. From previously reported studies, a cutoff value of <2.0 for the CFVR was adopted for the diagnosis of stenosis of each vessel. A preoperative examination was performed 2.0 ± 2.3 days before surgery and a postoperative examination 10.5 ± 4.2 days postoperatively.
All statistical analyses were performed using the StatView, version 5.0, software package (SAS Institute, Cary, North Carolina). Continuous variables are reported as the mean ± SD. Continuous variables were compared using the Student t test, and discrete variables were compared using the chi-square test or Fischer’s exact test. Differences were considered significant at p <0.05.
Results
A total of 56 patients (46 men and 10 women) met the inclusion criteria for the present study. The preoperative patient characteristics are listed in Table 1 .
| Variable | All Patients (n = 56) | Long Group (n = 25) | Short Group (n = 31) | p Value ⁎ |
|---|---|---|---|---|
| Age (years) | 67 ± 9 | 69 ± 6 | 65 ± 10 | 0.14 |
| Women | 10 (18%) | 4 (16%) | 6 (19%) | >0.99 |
| Hypertension † | 37 (66%) | 17 (68%) | 20 (65%) | >0.99 |
| Diabetes mellitus | 30 (54%) | 15 (60%) | 15 (48%) | 0.55 |
| Hyperlipidemia ‡ | 30 (54%) | 14 (56%) | 16 (52%) | 0.95 |
| Smoker | 36 (64%) | 16 (64%) | 20 (65%) | >0.99 |
| Previous stroke | 9 (16%) | 2 (8%) | 7 (23%) | 0.17 |
| Peripheral vascular disease | 6 (11%) | 3 (12%) | 3 (10%) | >0.99 |
| Creatinine (mg/dl) | 1.0 ± 0.3 | 1.0 ± 0.3 | 0.9 ± 0.4 | 0.39 |
| Body surface area (m 2 ) | 1.7 ± 0.1 | 1.7 ± 0.1 | 1.7 ± 0.2 | 0.39 |
| Previous myocardial infarction | 32 (57%) | 14 (56%) | 18 (58%) | >0.99 |
| Canadian Cardiovascular Society score | 1.7 ± 0.6 | 1.6 ± 0.6 | 1.7 ± 0.7 | 0.55 |
| Ejection fraction (%) | 56 ± 11 | 54 ± 11 | 58 ± 11 | 0.18 |
| Number of narrowed arteries | 2.7 ± 0.5 | 2.8 ± 0.4 | 2.6 ± 0.6 | 0.20 |
| Left main narrowing | 10 (18%) | 5 (20%) | 5 (16%) | 0.74 |
⁎ Comparison between long and short groups.
† Defined by history of elevated blood pressure in the medical chart and/or ongoing antihypertensive therapy.
‡ Defined by fasting blood cholesterol level >240 mg/dl or use of lipid-lowering medication.
CABG was successfully performed in all patients using the off-pump technique. The mean number of anastomoses per patient was 3.7 ± 1.2. Long segmental reconstruction of the LAD was performed in 26 patients (46.4%). The mean length of long segmental reconstruction was 5.0 ± 1.4 cm. Revascularization of the LAD was performed with a left ITA graft in 91.1% of the patients and with a right ITA graft in 8.9%.
No operative mortalities occurred. Furthermore, postoperative major complications (e.g., bleeding requiring re-exploration, low output, perioperative myocardial infarction, respiratory failure, stroke, new hemodialysis, and mediastinitis) were not observed in any patient. Of the 56 patients, 13 (23.2%) had paroxysmal atrial fibrillation that was treated with antiarrhythmic medications.
Preoperative coronary angiography was performed within 2 months before surgery in all patients. Significant 3-vessel disease was found in 73.2% of the patients; 26.8% had 2-vessel disease with significant LAD disease that was not an indication for percutaneous coronary intervention. Postoperative angiography demonstrated no stenosis in the graft or anastomosis site in all patients. Furthermore, no new stenosis lesions were found in any of the coronary arteries.
The pre- and postoperative coronary Doppler velocities (peak and mean), heart rate, and blood pressure at rest and during hyperemia are listed in Table 2 . The parameters of the velocity profiles in the LAD were obtained for all patients before and after CABG. The postoperative peak CFVR was significantly greater than preoperative peak CFVR (preoperative 1.5 ± 0.6 vs postoperative 2.7 ± 0.9; p <0.0001; Fig 1 ). The postoperative mean CFVR was significantly greater than the preoperative mean CFVR (preoperative 1.5 ± 0.6 vs postoperative 2.7 ± 0.9; p <0.0001; Fig 1 ). A preoperative mean CFVR >2.0 was observed in 9 patients. All 9 patients had a significant increase in the postoperative CFVR compared to preoperatively (3.3 ± 1.1 vs 2.4 ± 0.3, respectively; p = 0.04). A postoperative mean CFVR of <2.0 was observed in 8 patients. In all 8 patients, the postoperative CFVR was significantly increased compared to preoperatively (1.7 ± 0.3 vs 1.2 ± 0.6, respectively; p = 0.01).
| Variable | Preoperative | Postoperative | p Value |
|---|---|---|---|
| Peak flow velocity (cm/s) | |||
| At rest | 23.3 ± 11.4 | 22.0 ± 7.2 | 0.40 |
| Hyperemia | 34.9 ± 20.2 | 58.2 ± 19.1 | <0.0001 |
| Peak coronary flow velocity reserve | 1.5 ± 0.6 | 2.7 ± 0.9 | <0.0001 |
| Mean flow velocity (cm/s) | |||
| At rest | 17.9 ± 8.2 | 17.7 ± 6.7 | 0.89 |
| Hyperemia | 27.8 ± 16.5 | 43.7 ± 15.4 | <0.0001 |
| Mean coronary flow velocity reserve | 1.5 ± 0.6 | 2.7 ± 0.9 | <0.0001 |
| Heart rate (beats/min) | |||
| At rest | 64 ± 12 | 77 ± 14 | <0.0001 |
| Hyperemia | 69 ± 11 | 80 ± 13 | <0.0001 |
| Systolic blood pressure (mm Hg) | |||
| At rest | 132 ± 20 | 122 ± 17 | 0.0003 |
| Hyperemia | 118 ± 17 | 107 ± 15 | <0.0001 |
| Diastolic blood pressure (mm Hg) | |||
| At rest | 71 ± 10 | 69 ± 10 | 0.30 |
| Hyperemia | 65 ± 11 | 61 ± 11 | 0.05 |
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