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Eur J Cardiothorac Surg 2004;25:26-34
© 2004 Elsevier Science NL

Analysis of thoracic aortic blood flow during off-pump coronary artery bypass surgery

Christophe Decoenea, Thomas Modineb, Sharif Al-Ruzzehc*, Thanos Athanasiouc, Dina Fawzia, Richard Azzaouib, Annie Pola, Georges Fayadb

a Service d'anesthésie-réanimation cardiologique, Hopital cardiologique, CHRU de Lille, France
b Service de chirurgie cardiovasculaire, Pr. Henri Warembourg, Hopital Cardiologique, CHRU de Lille, France
c The National Heart and Lung Institute, Imperial College of Science, Technology and Medicine, London, UK

Received 21 May 2003; received in revised form 24 September 2003; accepted 28 September 2003.

* Corresponding author. MRS Unit, Harefield Hospital, Middlesex UB9 6JH, UK. Tel.: +44-79-68-025-332; fax: +44-1895-828-684
e-mail: sharifalruzzeh{at}hotmail.com


   Abstract
Top
 Abstract
1. Introduction
 2. Patients and methods
 3. Results
 4. Discussion
 References
 
Objectives: The non-invasive monitoring of thoracic aortic blood flow (TABF) during off-pump coronary artery bypass (OPCAB) surgery is becoming more commonly used and proved to be invaluable in the early detection of haemodynamic compromise due to heart displacement. The aim of this study was to analyze the changes in the TABF during OPCAB using transoesophageal Doppler and compare them with the changes observed by other monitoring methods as cardiac output, invasive pulmonary and radial pressures and mixed venous oxygen saturation. Methods: The measurements obtained from classic haemodynamic monitoring methods including the radial artery line and the pulmonary artery catheter with continuous monitoring of the cardiac output and mixed venous blood oxygen saturation were compared to the measurements of TABF obtained from a transoesophageal Doppler probe in 15 consecutive patients who underwent OPCAB surgery. Results: The TABF decreased significantly during the construction of coronary anastomoses from 3.42±0.94 l/min (baseline) to 2.2±0.8 l/min during the first coronary anastomosis and then to 2.14±1.12 l/min during the second coronary anastomosis (F=4.29, P=0.008). TABF returned to the baseline values (2.85±1.19 l/min) at chest closure. The cardiac output measurement showed no significant decrease compared to baseline. Conclusions: Low TABF occurred without significant changes in the measurements obtained from classic haemodynamic monitoring methods during OPCAB surgery. This finding could be of vital importance in helping improve the monitoring and consequently the management of patients undergoing OPCAB surgery.

Key Words: Off-pump surgery • Transoesophageal Doppler • Thoracic aortic blood flow


   1. Introduction
Top
 Abstract
 1. Introduction
2. Patients and methods
 3. Results
 4. Discussion
 References
 
Off-pump coronary artery bypass (OPCAB) surgery has been recently shown to be superior to conventional coronary artery bypass grafting (CABG) using cardiopulmonary bypass (CPB) in high-risk patients with renal disease or severe aortic atherosclerosis [1]. The two main challenges that face the operating team during OPCAB are firstly to detect and prevent any myocardial ischaemia and secondly ensure adequate blood delivery to the brain, kidney, liver and gut, and both challenges are closely related [2]. This underlines the need for reliable and sensitive monitoring tools [3]. Little is known about aortic blood flow distribution during OPCAB, although the period of haemodynamic instability could last for a relatively long time, particularly in cases requiring three or four coronary anastomoses. Global haemodynamic disturbance during OPCAB has been previously studied by the use of the pulmonary artery catheter and the radial arterial line [4], although the accuracy of the pulmonary artery catheter measurements in such cases, where the heart is twisted, may not be perfectly reliable.

Aortic blood flow measurement by an oesophageal pulse Doppler velocimeter is often used as a non-invasive method for cardiac output monitoring [5], but only successfully used in measuring thoracic descending aortic blood flow [6].

The aim of this study was to analyze the changes in the thoracic aortic blood flow (TABF) during OPCAB using a transoesophageal Doppler device (Hemosonic®) and compare them with the changes observed by other monitoring methods as cardiac output, invasive pulmonary and radial pressures and mixed venous oxygen saturation.


   2. Patients and methods
Top
 Abstract
 1. Introduction
 2. Patients and methods
3. Results
 4. Discussion
 References
 
The study was approved by the institution review board. Fifteen patients undergoing elective OPCAB surgery for more than two grafts were studied. Patients were excluded if they had gastric or oesophageal disease. The preoperative characteristics of the study patients are presented in Table 1.


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  		Table 1. Preoperative characteristics

 
2.1. Anaesthetic and monitoring techniques
Premedication consisted of 0.1 mg/kg of oral midazolam 1 h before induction of the anaesthesia. After catheterization of a peripheral vein and the radial artery under local anaesthesia (EMLA® patch) general anaesthesia was induced with propofol via a target control infusion (1.5–2 ng/ml), sufentanil (0.5 µg/kg) for analgesia and pancuronium (0.1 mg/kg) for muscle relaxation. Following induction, a pulmonary catheter (CCOmbo®, Edwards Life Science LLc, Irvine, CA) and a central venous catheter (in the right jugular vein), a urinary catheter and a rectal temperature probe were inserted. Anaesthesia was maintained with sufentanil, a low dose of isoflurane (MAC <1%) and propofol infusion as required. In all patients, normothermia was maintained by warm fluid infusion and forced warm air blankets on the head and the non-harvested radial artery arm.

Peri-operative monitoring included continuous electrocardiogram with ST segment analysis, continuous end-tidal carbon dioxide measurement, arterial, pulmonary and atrial blood pressure monitoring, intermittent wedge pressure measurements, capillary pulse oxymetry and rectal temperature (SC 9000XL®, Siemens Medical systems Inc., Dawers, MA). Continuous monitoring of venous mixed blood oxygen saturation and cardiac output were performed by the pulmonary catheter. No intermittent thermodilution cardiac output was performed.

A nasal transoesophageal Doppler probe was inserted preoperatively and was connected to a specific monitor allowing an instantaneous M-mode echographic measurement of the aortic diameter (Hemosonic® Arrow Int. Inc., Reading, PA). Doppler measurements were only performed when the best aortic diameter and Doppler signal were obtained.

2.2. Surgical technique
Left internal thoracic artery (LITA), left radial artery (LRA) and occasionally saphenous vein (SV) grafts were used for all patients. The sequence of coronary anastomoses started with the LITA graft to the left anterior descending artery (LAD), followed by revascularization of the marginal arteries and lastly, if planned, the posterior descending artery (PDA). The operation was performed through median sternotomy using Octopus III® (Medtronic Inc., Minneapolis, MN). Heart displacement into various grafting positions was aided by the use of deep pericardial retraction sutures facilitating exposure of lateral and posterior walls. No suction devices were used to assist immobilization. The Trendelenburg position was used and the table was tilted toward the patient's right hand side during marginal and PDA anastomoses.

2.3. Haemodynamic management
Haemodynamic management aimed to maintain mean arterial blood pressure (MAP) above 70 mmHg first by intravenous fluids and then by dobutamine at a low dose rate (<5 µg/kg per min).

The recorded parameters included: MAP, right atrial blood pressure (RAP), pulmonary capillary wedge pressure (PCWP), cardiac output by the continuous thermodilution method (COTD), and mixed venous blood oxygen saturation (SVO2).

The Hemosonic® Doppler probe recorded the TABF and, based on this, the cardiac output (COABF) was calculated. Thoracic aorta diameter (TAD) was measured by the M-mode echographic method.

The haemodynamic measurements were performed at three time points: baseline preoperative measurement just before the chest skin incision (T0), prior to the first coronary anastomosis and before any heart manipulations (T1), during the first coronary anastomosis (T2), during the second coronary anastomosis (T3) and at chest closure (T4). Measurements were performed exactly after finishing half the coronary anastomoses at T2 and T3. No measurements were performed during any PDA anastomoses being always third in sequence.

2.4. Statistical analysis
All measurements are given as mean±standard deviation. One-way analysis of variance (ANOVA) was used to identify significant haemodynamic changes between different measurement points (T0–T4). Before performing the analysis we evaluated normality and homogeneity between groups by using the Kolmogorov–Smirnov test and the Levene test, respectively. The Bonferroni test was used for post-hoc comparisons. Correlations between variables of interest were performed using the Pearson correlation test.

A P value less than 0.05 was considered statistically significant. Statistical analysis was performed using the SPSS 11.0 for Windows software package and for power analysis the Sample Power 2.0 for Windows (SPSS Inc., Chicago, IL).

The COABF was compared to the COTD by a Bland–Altman plot for each time point of the study [7].


   3. Results
Top
 Abstract
 1. Introduction
 2. Patients and methods
 3. Results
4. Discussion
 References
 
Two patients required dobutamine infusion (<5 µg/kg per min) during coronary anastomoses, which was maintained until the patient was extubated. Preoperative characteristics of the patients included in the study are presented in Table 1. The postoperative time ventilation time was 6.4±2.1 h and all patients were discharged from the intensive care unit (ICU) within 36 h postoperatively. No postoperative complications were observed in the study patients and the mean postoperative serum troponine level was 2.1±1.5 ng/ml. The average time period of ‘myocardial mobilization and ischaemia’ from the LAD exposure to the completeness of the last coronary suture was 76+9 min. The study patients were given an average amount of intravenous (IV) fluids of 1250±250 ml intraoperatively.

Descriptive statistics for the haemodynamic measurements, results of ANOVA to identify differences between groups and post-hoc comparisons to identify which groups were statistically significantly different from the others are presented in Tables 2–4, respectively.


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  		Table 2. Haemodynamic measurements during OPCAB surgery

 

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  		Table 3. ANOVA results

 

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  		Table 4. Bonferroni test results

 
The heart rate, pulmonary filling pressures and cardiac output, measured by thermodilution, did not change significantly during the procedure.

A significant decrease in MAP was observed between groups (T0–T4) (F=3.7, P=0.009). This change was related to a significant reduction of MAP during the LAD anastomosis (T2) compared to T1 (64±13 vs. 86±18 mmHg, with 95% CI of 4.14–40.45, P<0.01), which improved during marginal coronary anastomosis (72±20 mmHg).

Right atrial pressures differed between groups (T0–T4) (F=4.9, P=0.002) especially at T4 in comparison to baseline (14±7 vs. 7±4 mmHg, with 95% CI of 0.48–12.9, P=0.02) and in comparison to T2 (14±7 vs. 7±3 mmHg, with 95% CI of 0.94–13.4, P=0.01). Also a significant decrease of RAP was observed at T2 in comparison to T3 (7±3 vs. 12±5 mmHg, with 95% CI of -10.6 to -0.08, P=0.04) and in comparison to T4 (7±3 vs. 14±7 mmHg, with 95% CI of -13 to -0.94, P=0.01).

SVO2 showed a significant change between groups (T0–T4) (F=3.8, P=0.007) especially at T2 in comparison to T3 (76±8% vs. 66±10%, with 95% CI of 1.28–18.8, P=0.01) and at T3 in comparison to T1 (66±10% vs. 76±4% with 95% CI of -18 to -1.4, P=0.01) and T2 (66±10% vs. 76±8% with 95% CI of -18 to -1.2, P=0.01).

TABF was significantly different between groups (T0–T4) (F=4.9, P=0.008); a significant reduction was observed during the first anastomosis and continued to decrease during further heart manipulation from 3.72±0.72 l/min at T1 to 2.2±0.8 l/min at T2 (95% CI of -2.9 to -0.05, P=0.03) and to 2.14±1.12 l/min at T3 (95% CI of -3 to -0.1, P=0.03). The aortic blood flow returned to the T0 values at chest closure.

Regarding the changes in TAD, we found that although a marginally significant change was observed between groups (T0–T4) (F=2.6, P=0.04) in the post-hoc comparison none of the groups was found to be responsible for a statistically significant difference.

Simple correlation analysis showed significant relations between MAP and TABF (correlation coefficient=0.48, P=0.005), between MAP and TAD (correlation coefficient=0.35, P=0.005) and between SVO2 and TAD (correlation coefficient=0.29, P=0.01).

Analysis of the agreement between COABF and COTD is presented in Fig. 1 . Before heart displacement, bias was 0.19 l/min and precision was 1.08 l/min. During heart displacement a discrepancy appeared between the two methods. At T2 bias was 1.4 l/min and precision was 2.6 l/min. At T3 bias was 1.5 l/min and precision was 2.3 l/min. At chest closure bias and precision decreased to 0.4 and 1.2 l/min, respectively.



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  		Fig. 1. Bland and Altman [7] test for agreement between cardiac output measured by thermodilution and transoesophageal Doppler. Agreement between COTD and COABF. T0, preoperative baseline before performing LAD anastomosis (A); T2, preoperative baseline during LAD anastomosis (B); T3, preoperative baseline during marginal artery anastomosis (C); T4, preoperative baseline at chest closure (D). During heart manipulation (T2 and T3), there is a lack of agreement between the two methods as shown by the high bias values (1.4 and 2.6 l/min, respectively) and loss of precision (1.5 and 2.3 l/min, respectively).

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Patients and methods
 3. Results
 4. Discussion
References
 
We observed a significant decrease in TABF in the study patients undergoing OPCAB surgery for multi-vessel disease, although the classic haemodynamic monitoring methods showed little changes, except for the SVO2, which followed with some delay the TABF decrease. The agreements between COABF and COTD at baseline and at chest closure were acceptable and confirm previous studies, however, this agreement was lost during heart displacement.

Haemodynamic changes during heart displacement have been studied in both animals and humans [2–4,8–10], and were found to be significant during the marginal and right coronary artery anastomoses rather than during the LAD anastomosis; however, they were thought always to be within the physiologic limits. Our study confirms the findings of those previous studies in that respect.

Transoesophageal Doppler directly measures TABF and calculates the CO as the descending TABF is part of the aortic blood flow through the aortic valve minus the cerebral, coronary and upper arms blood flow [5,6]. The ascending/descending ratio commonly represents this division and is estimated to be in the region of 70% at rest. During OPCAB, many factors can change this ratio. The repeated transient unstable haemodynamic periods that occur during heart positioning can induce a redistribution of the flow to the brain and the coronary circulation changing the ascending/descending flow ratio. Patient warming is mainly performed on the upper part of the body and can induce a vasodilatation increasing the blood flow to this part of the body. Furthermore, placing the patient in the Trendelenburg position also affects the ascending/descending flow ratio in a way that is very difficult to predict [11]. It probably increases blood flow to the upper part of the body, while it decreases the cephalic venous return. In normal patients no appreciable change in cerebral blood flow has been observed until MAP falls to below 50 mmHg [11].

Although none of the study patients had any postoperative complications, the decrease of TABF underlines the risk of low blood delivery to the liver, the kidney and the gut during OPCAB. This decrease can be deleterious in patients with previous critical low blood flow delivery to the kidneys or the gut before surgery. Ascione and associates have found that OPCAB decreases the incidence of further renal deterioration in patients with preoperative renal insufficiency [12]. The benefits of avoiding CPB, aortic cannulation, systemic inflammatory reaction and cerebral emboli may be outweighed by the risk of renal and gut hypoperfusion especially in patients with renal insufficiency [13–15].

The favourable OPCAB results mainly depend on successful intraoperative haemodynamic management, which should be based on reliable and sensitive monitoring methods. Continuous monitoring of cardiac output and filling pressures was not sufficient in our study to detect the decrease in TABF. Continuous cardiac output monitoring is often used in cardiac surgery but has an in vitro response time between 5 and 15 min to detect a rapid change in cardiac output [16]. This delay is too long to be useful in OPCAB and repeated intermittent measures are time consuming especially during the critical period of heart displacement. We deliberately chose to perform this study with continuous cardiac output monitoring to reflect the usual monitoring conditions during OPCAB surgery, although the conventional method with a triplicate injection of ice bolus could have been more appropriate.

The observed discrepancy between the two methods is probably partially related to blood redistribution during OPCAB, as explained above. Extrapolation of descending aortic blood velocities for cardiac output estimation needs the knowledge of the ascending/descending aortic blood flow ratio [8,17,18]. Recently, Leather and Wouters have found that lumbar epidural anaesthesia increases TABF by inducing a lower body vasodilatation and thus consequently changing the ascending/descending aortic blood flow ratio [19]. Moreover, little is known about the accuracy of the COTD method in OPCAB procedures where the heart gets twisted and rotated in un-physiologic positions. With anterior heart displacement, Mathison and colleagues found that the right heart cavities were compressed and lost their shape, although this was not associated with any right outflow obstruction or significant regurgitation [8].

Continuous monitoring of SVO2 is closely related to oxygen delivery by cardiac output to the tissues while haemoglobin level and oxygen consumption remain unchanged. We observed in this study that SVO2 changes were more related to a TABF decrease rather than other haemodynamic parameters especially during the short periods of heart manipulation.

The period of heart displacement is the most critical period of haemodynamic instability and can last for up to 60 min according to the number of coronary anastomoses. Avoiding a low perfusion state before heart displacement during OPCAB must be the rule in order to limit low TABF only to the periods of heart displacement. To correct haemodynamic disturbances during OPCAB, {alpha}-agonist vasoconstrictors are currently recommended to increase the coronary perfusion pressure and ß-agonist drugs are usually avoided in order to limit myocardial oxygen consumption. The vasoconstrictors increase MAP but can induce redistribution of the aortic blood flow decreasing the blood delivery to the kidneys and the gut [2,3,20]. Low doses of dobutamine (<5 µg/kg per min) would not have much effect on myocardial oxygen consumption, but increase cardiac output and probably improve both mesenteric and kidney blood delivery. This drug should be preferred especially in patients with renal insufficiency or those who are at risk of mesenteric ischaemia.

The measurement of TABF by the Doppler method ideally needs continuous adjustment by the measurement of the aortic cross-sectional area and the thoracic aortic blood mean velocity with the best angle between the aorta and the Doppler probe. The TAD and consequently the aortic cross-sectional area showed little change during the procedure. Locating the aorta by the probe was not difficult during heart displacement and needed only a minor degree of correction during marginal coronary anastomoses to obtain the maximum amplitude of the Doppler signal. The oesophagus and thoracic aorta run parallel for more than 2 cm between the 5th and 6th dorsal vertebrae [7]. This relation remains fairly constant during heart displacement and traction on the pericardium by the traction suture, and the change in angle between the oesophagus and the thoracic aorta must reach 30° to produce any observed false decrease in TABF. Therefore, we believe that the observed decrease in TABF in this study was not due to an error in our technique. Furthermore, the technique of TABF measurement that we used in this study has been previously validated by both our team (data presented but not published) and by others [21].

In conclusion, this study has shown that a significant decrease in TABF occurred during heart displacement without significant changes in the classic, commonly used, haemodynamic monitoring methods. The impact of this decrease in TABF needs to be evaluated by regional perfusion markers. The operating team need to be aware of this fact in order to improve the haemodynamic management of patients undergoing OPCAB surgery. Therefore, we suggest that this technique of TABF measurement can play a significant role in accurate assessment of intraoperative cardiac output and consequently in prevention of undesirable haemodynamic changes, even before they are picked up by conventional monitoring methods during OPCAB surgery.


   References
Top
 Abstract
 1. Introduction
 2. Patients and methods
 3. Results
 4. Discussion
 References
 

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