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Ann Thorac Surg 1998;66:1097-1100
© 1998 The Society of Thoracic Surgeons

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Supplement

Transit-time flow measurement for detection of early graft failure during myocardial revascularization

^a Department of Thoracic and Cardiovascular Surgery and Division of Cardiology, University Hospital, Berne, Switzerland

Address reprint requests to Dr Walpoth, Thoracic and Cardiovascular Surgery, University Hospital, Insel, CH-3010 Bern, Switzerland
e-mail: (beat.walpoth{at}insel.ch)

Presented as a Poster at "Facts and Myths of Minimally Invasive Cardiac Surgery: Current Trends in Thoracic Surgery IV", New Orleans, LA, Jan 24, 1998.

Abstract

Background. A low-flow situation in arterial and venous grafts has been associated with high rates of perioperative infarction and mortality. This study was designed to look at intraoperative graft flow and resistance in patients with coronary artery disease.

Methods. Coronary artery bypass graft flow was measured in 46 patients. Transit-time flow was used for coronary flow measurements at rest as well as after maximal vasodilation with adenosine infusion.

Results. Forty-three of the 46 patients showed normal internal mammary artery graft flow (>20 mL/min); 3 patients had no or minimal graft flow. Redoing the graft anastomosis in these 3 patients resulted in normalization of graft flow. The mean flow increased significantly after correction from 0.5 ± 0.7 mL/min to 15.7 ± 9.6 mL/min (p < 0.02). Conversely, vascular resistance decreased significantly from 138 ± 10 to 4.8 ± 1.8 Ohmv (p < 0.0001), as did the pulsatility index (from 146.9 ± 95.7 to 3.4 ± 1.8; p < 0.001). After correction, coronary flow reserve was 2.5 ± 1.1.

Conclusions. Measurements of intraoperative flow and resistance as well as derived variables allow assessment of early graft function and thus help prevent graft failure and reduce perioperative infarction. Transit-time volume flow might be a simple tool for quality control in coronary bypass procedures.

Coronary flow measurement allows functional evaluation of coronary artery bypass grafts and may be predictive of the patient’s immediate and late outcome after myocardial revascularization [1–4]. With the advent of minimally invasive coronary artery bypass grafting, including multivessel revascularization on the beating heart, quality control of the anastomoses becomes particularly important [5–8]. Easily accessible flow-measuring devices are prerequisites for a reliable assessment of coronary grafts, especially to detect technical failure at the anastomotic site early and to identify low-flow situations resulting from vasospasm or poor runoff [9–13]. Recently, the transit-time ultrasound principle has been introduced into cardiac surgery to measure blood volume flow. Several studies [14–16] have demonstrated the particular value of this method to determine coronary graft flow after revascularization.

In the present study, intraoperative transit-time flow recordings were used to compare the flow characteristics of 3 patients with failure of the coronary anastomosis with those of 43 control patients at baseline and after maximal vasodilation with adenosine.

Patients and methods

Forty-six patients underwent coronary artery bypass grafting using the left internal mammary artery (IMA). In our institution, intraoperative transit-time flow measurement is routinely performed in patients who are undergoing coronary artery bypass procedures and are at increased perioperative risk because of hemodynamic instability, severely reduced left ventricular function, or diffuse coronary artery disease. Our operative technique includes moderate hypothermic cardiopulmonary bypass (32°C) and antegrade cold blood cardioplegia. After the completion of all distal and proximal anastomoses and the weaning of the patient from cardiopulmonary bypass, coronary flow is measured in the arterial and venous grafts at baseline and after maximal vasodilation with adenosine, 23 µg · kg^-1 · min^-1 injected into the left ventricle. This adenosine dosage was found to induce maximal vasodilation in a previous trial [17].

Flow was measured by the transit-time method with the CardioMed Trace System (CM 4008; Medi-Stim AS, Oslo, Norway) and probes of 3 and 4 mm to fit the actual vessel size [15]. Invasive arterial pressure monitoring was done through a radial artery catheter. Simultaneously, the electrocardiogram (leads V₅ and II) was recorded for the timing of systolic and diastolic flow patterns. The arterial pressure and electrocardiogram were interfaced from a Hellige monitor to the flowmeter to evaluate the vascular resistance and the diastolic filling pattern.

The transit-time method is based on the fact that the time required for ultrasound to pass through blood is slightly longer upstream than downstream. As the ultrasound beam is wider than the diameter of the vessel lumen, the ultrasound wave will cover every flow vector in the vessel, thus making the transit-time difference proportional to the true volume of blood flow in milliliters per minute. The following variables were calculated: mean systolic and diastolic flow in milliliters per minute; pulsatility index ; early diastolic backflow expressed as percent insufficiency (volume of backward flow/volume of forward flow); fast Fourier transformation of the flow curve; and systolic to diastolic flow and time ratios. In addition, heart rate and radial artery mean, systolic, and diastolic pressure were recorded. Vascular resistance was calculated using the equation Coronary flow reserve was calculated from the maximum flow assessed during adenosine infusion divided by flow at baseline.

Results

In 43 patients, flow through the IMA graft was normal (>20 mL/min). In 3 patients, a low-flow situation (mean flow, < 0.5 ± 0.7 mL/min) was found in the left IMA to the left anterior descending coronary artery. Calculated vascular resistance was significantly elevated (138 ± 10 Ohmv or mm Hg · mL^-1 · min^-1), pulsatility index was pathologically high (147 ± 96), diastolic backflow showed an insufficiency of more than 50%, and diastolic filling was absent (Table 1). The reason for the low-flow situation was distal IMA dissection in 1 patient, an obstructing intimal IMA flap in 1, and an intramural hematoma with compression of the IMA anastomosis in 1. In only 1 patient transient electrocardiographic changes and poor contractility of the anterior wall of the left ventricle were observed. In all 3 patients, the distal anastomosis was reconstructed with the patient on cardiopulmonary bypass.

View larger version (91K): [in this window] [in a new window]   Fig 1. Transit-time flow measurement of left internal mammary artery (LIMA) to left anterior descending coronary artery. Shown are coronary bypass flow (Q1) (top signal), aortic pressure (P1) (second signal), standard-lead electrocardiogram (ECG) (third signal), vascular resistance (fourth signal), and fast Fourier transformation of flow FFT(Q1)) (bottom signal) at baseline in the presence of graft occlusion (A), after redoing of the anastomosis (B), and after maximal vasodilation with adenosine (25 µg · kg-1 · min-1) (C). Note the flow increase from 1.0 to 25 mL/min after distal anastomosis was redone and further increase to 46 mL/min after infusion of adenosine. At the same time, there is a marked diastolic flow pattern to the IMA flow curve. After the corrective operation and especially after adenosine infusion, fast Fourier transformation shows a harmonic repetitive signal with a limited amount of frequency content.

Low coronary artery bypass flow is associated with early graft failure and high risk of perioperative myocardial infarction [18, 19]. Thus, early recognition of low graft flow will alert the surgeon and avoid early graft occlusion. The purpose of the study was to evaluate the impact of transit-time flow and resistance measurements on graft function and patency in 46 patients with coronary artery disease undergoing coronary artery bypass grafting. In 43 patients, good functional reserve was observed with a mean flow of 33 ± 27 mL/min at rest, which increased to 45 ± 18 mL/min after adenosine infusion.

In 3 patients, a low-flow situation was detected by the flow probe, and a redo procedure was immediately performed. This intervention led to improvement in IMA graft flow, which increased significantly from 0.5 ± 0.7 mL/min to 15.7 ± 9.6 mL/min. Coronary flow reserve after adenosine infusion amounted to 2.5 ± 1.1, which is normal. After the anastomosis was redone, there was a significant decrease in vascular resistance from 138 ± 10 to 4.8 ± 1.8 (see Table 1). After adenosine, the systolic to diastolic flow ratio decreased from 61% to 26% (p < 0.05). These data indicate that reoperation was successful (see Table 2).

Comparison with other techniques
Other techniques that may help detect early graft failure include: electromagnetic or Doppler flow measurements or direct coronary angiography [5, 14, 20]. Electromagnetic flow measurement is often difficult and is sensitive to technical errors such as drift and zero adjustment. Doppler flow measurement is affected by insonation angle and indicates only velocity. Intraoperative angiography is often unsatisfactory because of poor vessel filling, low resolution, and problems related to the optimal projection. Transit-time volume flow measurement may help overcome these problems. The method is simple and reliable, provides accurate flow measurement, and is highly reproducible [14–16]. A major advantage of this technique is its use for measuring coronary bypass flow during minimally invasive coronary bypass procedures, as it may detect early graft failures in a timely fashion [7, 8].

Clinical implications
Graft failure remains one of the most important aspects of coronary artery bypass grafting. Early intraoperative recognition is essential for immediate correction of any problem. On the basis of our experience with transit-time flow measurement, we have reached the following conclusions:

1. Transit-time volume flow measurement is simple, reliable, and easy to perform.
   
2. Low flow in a coronary bypass graft requires reexploration of the anastomosis unless severe spasm of the conduit or poor runoff is strongly suspected.
   
3. Redoing the distal anastomosis leads to significant improvement in flow and decreases vascular resistance in the presence of anastomotic failure.
   
4. Determination of coronary flow reserve with adenosine may help differentiate vasospasm from anastomotic graft failure.
   
5. Early recognition of graft failure is cost-effective and possibly prevents hemodynamic instability in the intensive care unit during the early postoperative period.
   

Measuring intraoperative flow may help detect bypass failure early and thus improve postoperative outcome in patients undergoing coronary artery bypass grafting. This might be of great importance in patients having minimally invasive surgical procedures.

Acknowledgments

This work was supported in part by a grant from The Swiss Heart Foundation.

References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Beat H. Walpoth Beat Kipfer Ulrich Althaus Thierry P. Carrel Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Walpoth, B. H. Articles by Carrel, T. P. Search for Related Content PubMed PubMed Citation Articles by Walpoth, B. H. Articles by Carrel, T. P.