Misinterpretation of the TEI-index

Abstract

EDITOR: We read the article by Drs Lutz, Giebler and Peters reporting their results with the TEI-index with great interest [1]. The article addresses the impact of alterations in preload on the TEI-index measured in a group of anaesthetized, mechanically ventilated patients undergoing coronary artery bypass grafting (CABG). We agree with the authors that the TEI-index can be useful in cardiac surgery. However, there are some points we wish to comment on. Firstly, the authors state that ‘baseline TEI-index in our patients was slightly higher than the reported normal, despite a normal global ejection fraction’. It has been shown, however, that global myocardial performance as assessed by the TEI-index may be severely impaired even if the ejection fraction is within the normal range [2]. Comparing patients with and without critical coronary artery stenosis (>70%), Dagdelen and colleagues found a significant difference in TEI-index between the groups, while the ejection fraction showed no differences. Therefore, the TEI-index may be even superior to the ejection fraction in assessing global myocardial performance. Interestingly, the values of Lutz and colleagues are in good agreement with the subgroup of patients with critical coronary artery disease. Unfortunately, the authors do not provide information on the degree of coronary artery disease in their patients, which would have been helpful in this respect. Secondly, changes in the TEI-index were compared with alterations of the various components of the TEI-index during a head-up tilt and a head-down tilt manoeuvre and it was stated that the alterations of the components could not explain the decrease of the TEI-index observed with head-down tilt. All of the values which make up the TEI-index calculation, however, were changed in a consistent manner, the isovolumetric contraction and relaxation time not reaching statistical significance. Since the TEI-index is closely correlated with cardiac output [3], a very simple explanation for the improvement in the TEI-index and all its compounds during head-down tilt is that the left ventricle increased its stroke volume to match the increased venous return. It is highly probable that the patients in the study by Lutz and colleagues after overnight fasting and induction of anaesthesia, were in a relatively hypovolaemic state, and that the head-down tilt manoeuvre therefore uniformly provoked an increase in stroke volume. As an indicator of global myocardial performance, the TEI-index therefore had to improve in order to reflect the increase in cardiac output (a major determinant of cardiac function). Unfortunately, the authors report neither stroke volume nor cardiac output during the manoeuvres performed (although easily obtained with transoesophageal echocardiography (TOE)). It should be noted, that a significant variation in preload undoubtedly occurs between different patients who are subjected to the same clinical manoeuvres [4]. Therefore, the effect of volume loading on stroke volume in different subjects would have been helpful to assess the impact of an increased preload on cardiac function. Finally, the authors report a sensitivity of the TEI-index to an acute increase in left ventricular preload and conclude that its usefulness is limited to situations when major increases in preload can be excluded. An index claimed to depict global myocardial performance, however, cannot be independent of loading conditions, since otherwise it would be of very limited clinical value. This basic statement is rooted in the Frank-Starling relationship which describes the fundamental principle of cardiac behaviour, i.e. that the force of contraction of the cardiac muscle is proportional to its initial length [5]. In a study in conscious dogs, the relationship between stroke work and either end-diastolic segment length or chamber volume was highly linear, thus indicating that the normal ventricle is capable of increasing its stroke volume to match any increase in venous return [6]. This is not, however, the case for ventricles that are in failure. Consequently, the TEI-index was unchanged after preload manipulations in patients after a previous myocardial infarction [7]. In contrast to the authors' statement, the sensitivity of the TEI-index to changing preload may be extremely useful, especially in situations where fluid responsiveness is unknown. An improved TEI-index may indicate that a volume challenge may be beneficial, while a worsened TEI-index after volume loading suggests the opposite. In conclusion, the study of Lutz and colleagues emphasizes that the TEI-index may be a useful parameter and an early indicator of left ventricular dysfunction in patients with critical coronary artery disease and normal systolic function. J. Renner P. H. Tonner B. Bein Department of Anaesthesiology and Intensive Care Medicine, University Hospital Schleswig-Holstein, Campus Kiel, Kiel, Germany