Eur J Cardiothorac Surg 1999;16:S112-S116
© 1999 Elsevier Science NL
Evaluation of myocardial metabolism and function during beating heart coronary surgery
A.A Lotto,
M Caputo,
R Ascione,
C.T Lloyd,
V Lucchetti,
G.D Angelini*
The Bristol Heart Institute, University of Bristol, Bristol Royal Infirmary, BS2 8HW, Bristol, UK
* Corresponding author. Tel.: +44-117-928-3145; fax: +44-117-929-9737 (Email: n.holloway-dee{at}bristol.ac.uk).
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Abstract
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Coronary artery bypass surgery on the beating heart either via a left anterior small thoracotomy (LAST) or a median sternotomy is becoming increasing popular world-wide. Concern still remains about the potential for a temporary regional myocardial ischaemia associated with the stabilisation and occlusion of the coronary during construction of the anastomosis. This review summarises the results of a series of studies intended to evaluate the effect of beating heart coronary revascularization on myocardial function, myocardial tissue injury and clinical outcome.
Key Words: Off-pump coronary surgery • Myocardial function
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1. Introduction
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Coronary artery bypass grafting (CABG) with cardiopulmonary bypass (CPB) and cardioplegic arrest carries a relatively low but well-recognised risk of myocardial damage. This risk, in terms of myocardial tissue injury and post-operative left ventricular dysfunction, is accepted in exchange for a bloodless and still surgical field [1,2].
In recent years there have been a number of reports on myocardial revascularization on the beating heart without CPB either through a left anterior small thoracotomy (LAST) [3–5] or a median sternotomy [6,7]. The early and mid-term results, in terms of graft patency, freedom from reintervention and angina [4,6,8] are encouraging. The techniques described for operating on the beating heart, via a median sternotomy and a LAST, often requires temporary occlusion of the target vessel during the construction of the anastomoses [9,10]. Little is known of the potential deleterious effects of temporary ischaemia and subsequent reperfusion on regional myocardial function and metabolism during beating heart coronary surgery.
Revascularization on the beating heart is a routine practice at our institution. In this review we report the results of a series of studies carried out to evaluate the effects of beating heart coronary surgery on myocardial metabolism and function.
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2. Assessment of myocardial injury and function
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2.1 Serum markers
Biochemical ‘markers', are often used as surrogates of clinical outcome and can be employed to quantify the severity of myocardial damage [11].
Serum markers offer a simple, accessible and accurate tool to diagnose peri-operative myocardial infarction and to identify more subtle forms of myocardial injury. The troponins (T, C and I) are a group of intimately related regulatory proteins located in striated muscle [12]. Troponin I is solely confined to the myocardium and has been shown to be a highly specific marker for the detection of myocardial injury [13]. In our studies, troponin I release was serially measured in systemic venous blood samples for a period of 48 h post-operatively.
2.2 Tissue markers
Sampling of myocardial tissue via a direct needle byopsy can be used to investigate changes in cellular metabolic function and structural changes associated with ischaemia and reperfusion. Ischaemia alone or ischaemia and reperfusion are associated with profound metabolic derangement and disruption of the cellular ionic homeostasis [14]. During ischaemia, the heart is deprived of oxygen and energy production is decreased and then concentration of several metabolites will be altered. These changes can be used as markers of ischaemia and reperfusion injury [15–17].
2.3 Function assessment
Intraoperative trans-oesophageal-echo (TOE) is a sensitive indicator of segmental wall motion abnormalities during myocardial ischaemia, and has the capability to detect impairment of global and regional myocardial contractility earlier than electrocardiography [18,19]. This technique allows the monitoring of global or regional wall motion related to a specific left ventricular territory supplied by a grafted coronary artery [20]. It is therefore particularly suitable for the assessment of left ventricular contractility during beating heart coronary surgery.
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3. Myocardial metabolism and function
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At our Institute interest in beating heart coronary artery surgery developed in early 1996. Our initial experience started with the LAST procedure. Having previously demonstrated that coronary surgery with CPB was invariably associated with a significant release of specific markers of myocardial cell injury (troponin T and I) [21], our first goal was to see if there was a comparable release of such markers during beating heart coronary revascularization. We therefore analysed troponin I release (before the operation, 4, 12, 24, 36 and 48 h post-operatively) in a group of 14 patients undergoing the LAST procedure and compared the results with a similar group of 14 patients undergoing conventional CABG [22]. All patients had preserved left ventricular function and were comparable with respect to preoperative characteristics. No patient had clinical or electrocardiographic evidence of perioperative myocardial infarction. Preoperative troponin I concentrations were undetectable in both groups. In the on-pump group, a significant rise in troponin I levels was observed 4 h post-operatively, peaking at 12 h and this remained elevated at 48 h following the operation (Fig. 1). In contrast, in the LAST group, troponin I levels were significantly lower, peaking at 4 h and declining rapidly thereafter (Fig. 1). From this first study we concluded that beating heart coronary surgery was associated with a reduced ischemic insult compared to conventional CABG, as demonstrated by the significantly reduced release of troponin I.
The temporary occlusion of the LAD necessary to perform the anastomosis during beating heart coronary surgery could nevertheless be associated with intracellular metabolic derangement. In a selected group of patients undergoing beating heart CABG through a median sternotomy we decided to evaluate myocardial metabolic changes associated with ischaemia/reperfusion in biopsies taken from the anterior wall of the left ventricle [23]. Samples were taken immediately before (control) construction of the LAD anastomosis (mean ischaemic time 12±1.2 min) and after 3 min of reperfusion following completion of the anastomosis. These samples were immediately frozen and analysed for intracellular ATP and ADP levels. No difference was observed in the levels of any of the measured markers over time (Table 1
). This data confirmed the findings of the previous study that myocardial damage does not occur during temporary occlusion of the LAD coronary artery for periods up to 14 min.
Despite these reassuring findings it is not uncommon to notice during construction of the anastomosis some dysfunction or reduced contractility in the area supplied by the target coronary artery. We therefore decided to assess any changes in left ventricular function during the conduct of the anastomosis using TOE. Moreover an intra-coronary shunt (Anastaflow Intravascular Shunt, Research Medical, Midvale, USA) became available to us which allowed to maintain blood flow in the native coronary artery during construction of the anastomosis.
Forty consecutive patients undergoing a single left internal mammary artery (LIMA) to LAD anastomosis on the beating heart via a median sternotomy were prospectively recruited to the use of an intracoronary shunt or surgery conducted with snare occlusion of the LAD [24]. TOE was used to monitor left ventricular global wall motion score index (WMSI) and regional wall function (WMS) in the LAD territory. Ejection fraction (EF%) was also calculated by multiplane. The two groups were similar with respect to sex, age, preoperative left ventricular function, extent of coronary disease, time of LIMA–LAD anastomosis. The wall motion score index was assessed before occlusion of the LAD (control), then 1, 3 and 6 min during construction of the anastomosis and after 5 min of reperfusion. The WMS in the LAD territory was assessed at 6 min during construction of the anastomosis and after 5 min of reperfusion. Ejection fraction was calculated pre-operatively, 5 min into the construction of the anastomosis and 5 min after reperfusion.
During construction of the anastomosis when a shunt had been used, there was no change in WMSI, WMS in the LAD territory and EF%, however, a significant decline was seen in the group of patients in whom the anastomosis was constructed without the use of a shunt (Figs. 2–4)
. The decline in WMSI and WMS started soon after LAD occlusion, continuing to fall at 3 min, and decreased significantly at 6 min during construction of the anastomosis. After 5 min of reperfusion, WMS in the LAD territory, WMSI and EF% returned to control values (Figs. 2–4).
From this study we concluded that the use of an intracoronary shunt prevents impairment in left ventricular function during construction of the LIMA–LAD anastomosis. It seems therefore advisable to use an intracoronary shunt in patients with unstable angina, poor left ventricular function or in cases in which a longer time to perform the anastomosis is anticipated. Furthermore the information derived from TOE has a definitive influence on anaesthetic and surgical decision making during beating heart coronary artery bypass grafting.
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4. Clinical trial
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On the basis of the encouraging metabolic and functional results of the previous mentioned studies, we extended the operative indications for beating heart CABG via a median sternotomy to patients with multivessel coronary artery disease. Twelve months ago we started a prospective randomised study which intended to compare the clinical outcome in patients undergoing myocardial revascularization with or without CPB.
In the preliminary phase of this study eighty patients were prospectively randomised into two groups: group A underwent conventional CABG with CPB and intermittent antegrade warm blood cardioplegia (IAWBC) arrest, whereas group B underwent off-pump CABG. Serum troponin I release was serially measured preoperatively, after the operation and up to 48 h post-operatively. Hemodynamic measurements as well as inotropic requirement, incidence of arrhythmias and post-operative myocardial infarction were also recorded. There were no significant differences between the two groups concerning age, sex and extent of coronary disease, left ventricular function and number of grafts. There were no deaths nor intraoperative myocardial infarctions in either group. Troponin I release was constantly higher in the on-pump (group A) and this was significant at 1, 4, 12 and 24 h post-operatively (Fig. 5). The incidence of postoperative arrhythmia (supraventricular tachycardia, atrial fibrillation) was significantly less in the off-pump compared to the on-pump group.
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Fig. 5. Perioperative troponin I release (with permission of the Eur J Cardiothorac Surg [25]).  , off pump; •, on pump. *P<0.05 on versus off pump at 1, 4, 12 and 24 h post-operatively.
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5. Discussion
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Since the introduction of beating heart coronary surgery, at our Institution we have conducted a series of clinical studies to evaluate the myocardial metabolic and functional effects of this surgical technique. In our first report we assessed systemic troponin I release in patients undergoing CABG on the beating heart. Troponin I release was constantly less than that measured after revascularization with cardioplegic arrest and CPB.
These findings lead us to investigate the intracellular metabolic changes from ventricular biopsies taken from patients undergoing off-pump myocardial revascularization. No ischaemic stress (indicated by changes in myocardial high phosphates and lactate levels) was detected in the hearts of these patients. These might indicate that the heart could tolerate temporary ischaemia and preserve aerobic metabolism probably by using the collateral circulatory supply. Furthermore the chronic nature of the coronary disease might contribute to protect myocardial tissue against metabolic ischaemic damage.
Whilst the biochemical markers of myocardial injury showed no changes during temporary coronary artery occlusion, significant impairment of left ventricular function and segmental wall motion contractility could be detected during beating heart coronary surgery. Nevertheless these changes were completely reversed upon reperfusion. This information, along with the biochemical data we have accumulated indicates that any adverse effects on the myocardium are rapidly normalised and are not followed by any permanent sequelae representing myocardial ischaemia or infarction. Another important finding of this study was the observation that the use of an intracoronary shunt prevented any deterioration of left ventricular function during construction of the anastomosis. The shunt effectively avoids ischaemia and at the same time allows operation in a bloodless field.
On the basis of these findings, we carried out a prospective randomised study with the aim to compare myocardial function in patients undergoing multivessel coronary artery revascularization with or without CPB via a median sternotomy.
Troponin I release was significantly lower in the off-pump group and this was associated with a significantly reduced incidence of arrhythmias. Inotropic requirements postoperatively were less in the off-pump group but this did not reach statistical significance.
In conclusion the results of our studies suggest that beating heart CABG is a safe and effective strategy which offers improved myocardial protection when compared with conventional CABG and cardioplegic arrest.
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Acknowledgments
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This work was supported by the Sir Siegmund Warburg's Voluntary Settlement and the British Heart Foundation.
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Footnotes
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Presented at the 2nd MITSIG International Symposium: Controversies in Cardiothoracic Surgery, Hong Kong, November 20–21, 1998.
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Abstract
1. Introduction
, LAST operation; 
, conventional on pump CABG.
