Continuous 3-dimensional vectorcardiography, a valuable tool for online ischemia monitoring

Fibrinolysis(1995) 9. Suppl I : 114-120 © 1995PearsonProfessionalLtd

Continuous 3-Dimensional Vectorcardiography, a Valuable Tool for Online Ischemia Monitoring

S. Hauck, M. Friedrich, R. Dechend, D.C. Gulba, R. Dietz

S U M M A R Y. By using the complete 3-dimensional electrocardiographic information, continuous vectorcardiography offers better insights into ischemic events in the lateral and inferior parts of the heart than the 12-lead ECG. In addition, by using other components of the ECG curve, such as QRS-complex changes, it also provides more sensitive markers of ischemia than mere ST-segment deviations. ST-segment changes and QRS-complex changes are registered in the three orthogonal leads and each parameter is condensed into one variable, averaging the root square of the sum of the second power of the absolute values. The result is continuously updated and displayed as a trendcurve which offers realtime ischemia monitoring. This capability is valuable for both the non-invasive assessment of the acute ischemic burden during PTCA and for ischemia monitoring in the critical 24 h thereafter where the hazard of reocclusion is highest. Continuous vectorcardiography also serves as a powerful tool in the non-invasive assessment of infarct artery patency after thrombolysis and can help to identify patients with persistent occlusion and reocclusion after primary successful procedures who thereby qualify for rescue PTCA. By using 3D-vectorcardiography on-line ischemia monitoring in patients with unstable angina for risk stratification is also feasible.

Thrombolytic therapy has demonstrated marked improvements in the natural history of acute myocardial infarction. Although an impressive proportion of patients are found to have 'patent' infarct arteries after thrombolysis, this conventional assessment of efficacy in terms of angiographic 'snapshot' view does not satisfactorily reflect the dynamic processes of coronary arterial recanalization and reocclusion or the adequacy of myocardial perfusion. 1 After successful thrombolytic therapy for acute myocardial infarction, intermittent coronary occlusions are detected with serial angiography in 16-58% of the patients. 2-4 Furthermore, the thrombogenicity of the plaque may be enhanced by adjunctive angioplasty. This bares the potential that by immediate angioplasty the prognosis of these patients worsens. Therefore, selective application of coronary angioplasty or other adjunctive strategies to patients with suboptimal reperfusion after thrombolysis is critically dependent on the ability to

rapidly and non-invasively assess the adequacy of myocardial perfusion. In clinical practice, however, no realtime ischemia monitoring currently is available. Previous work has shown that neither the development of arrhythmia nor the occurrence of angina pectoris are good indicators of ischemia and more than 40% of ischemic events are missed. 5 Thus accurate non-invasive detection of myocardial perfusion would be valuable for the assessment of patients at particular risk, In principal, these requirements may be provided by the 12-lead ECG, however, for technical reasons the 12-lead ECG is only obtained at certain time intervals or when symptoms are already indicative for ongoing ischemia. Furthermore, the 12-lead ECG is of limited value in the detection of ischemia in the lateral and inferior parts of the heart. Bush et al6 demonstrated in 115 patients that in the perfusion area of the left circumflex artery with the standard 12-lead ECG up to 20% of ischemic events are missed during angioplasty (Fig. 1). These methodologic shortcomings may be overcome by continuous three-dimensional vectoreardiography. This method was introduced by Hodges et al7'8 and subsequently studied and further developed by Sederholm911 and GrOtum.12- 14 The registered vector loop contains

S. Hauck, M. Frledrleh, R. Dechend, D.C. Gulba, R. Dietz, Franz Volhard Hospital and Max Delbriick Center for Molecular Medicine, Rudolf Virchow University Hospitals, Free University Berlin, Wiltbergstr. 50, 13125 Berlin, Germany. Correspondence to S. Hauck. 114

Update in Thrombolysis1994

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calculationof ST-VM,(c) displayof trendcurve. the entire three-dimensional information including the lateral and the inferior walls of the heart. In addition to ST-segment deviations, QRS changes, which are more sensitive ischemia markers, can be used for ischemia monitoring. Furthermore, by condensing the information obtained into one variable that can be displayed in a trendgraph, continuous vectorcardiography would allow for realtime monitoring.

second power of the absolute values. The result will always be positive and it is called QRS-vector difference (QRS-VD) (Fig. 3b). The QRS-VD is displayed in a trendcurve from which continuous ischemia monitoring

METHODS The complete vectorloop is represented in the three orthogonal leads x, y and z. Ischemic changes may be represented as ST-segment deviations in these orthogonal spatial representations of the vector loop (Fig. 2a). These ST-segment deviations are combined and summed up with a simple equation. The result is called the ST-vectormagnitude (ST-VM) (Fig. 2b). The ST-VM is displayed in a trendcurve. Every 5 s, a new average ST-VM is calculated and continuously lined up in a trendcurve. During ischemia, ST-VM rises and returns back to normal after oxygen supply has been restored (Fig. 2c). Beyond mere ST-segment changes, the vector loop changes in size and orientation during short periods of ischemia and flips back to normal after restoration of the antegrade blood flow. This pattern is represented in the orthogonal leads mainly as changes of the QRS-complex, which are usually too small to be recognized in the standard ECG (Fig. 3a). The vector loop of the QRScomplex is averaged over a period of 2 min. Every 5 s, the vector loop is repeatedly averaged thereafter and compared with the reference complex. The ischemic drift of the vector loop is reflected in the orthogonal leads as QRS-complex changes. These differences are combined with an equation similar to the one used to calculate STVM changes, averaging root square of the sum of the

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116 Continuous3-D Vectorcardiographyfor Ischemia

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Fig. 4 (a) The detection of temporary coronary occlusion during PTCA was possible in 92.5% with QRS-VD and in 65% with ST-VM. This difference was highly significant. (b) The superiority of QRS-VD over ST-VM in the detection of temporary coronary occlusion was most pronounced in the perfusion area of the LCX. (e) In the 28 patients without a prior myocardial infarction, detection of temporary occlusion was possible in 100%. Detection of occlusion by ST-VM was possible in only 68%. This difference was significant. Neither QRS-VD nor ST-VM could detect coronary occlusion very well in the 12 patients with a prior myocardial infarction in the perfusion area of the dilated vessel. (d) Eight of the nine patients without a prior myocardial infarction who had no changes of ST-VM despite QRS-VD changes showed a good collateral flow into the perfusion area of the dilated vessel.

is feasible. During a short period of ischemia, QRS-VD subsequently rises and returns back to normal after restoration of oxygen supply (Fig. 3c).

CONTINUOUS VECTORCARDIOGRAPHY DURING PTCA 40 patients were studied with single lesion coronary artery disease, who underwent elective PTCA. 18 patients with severe stenosis of the LAD, 8 with stenosis of the LCX and 14 with stenosis of the RCA. In this study, accurate detection of the coronary occlusion by QRS-VD was possible in 92.5% of the patients. In contrast, STVM indicated the occlusion during inflation of the balloon in only 65% (Fig. 4a). The accurate detection of coronary occlusion by QRS-VD was especially superior to ST-VM in the perfusion area of the LCX (Fig. 4b). A prior myocardial infarction or good collaterals diminished the ability to detect coronary occlusion, however, with both methods QRS-VD and ST-VM (Fig. 4c,d). 15

These data are in accordance with the findings of Dellborg who also demonstrated the superiority of QRSVD over ST-VM in detecting temporary coronary occlusion during PTCA. 16

ON-LINE MONITORING AND 'FINGERPRINTING' The hazard of reocclusion after successful PTCA occurs predominantly within the first 24 h. Fortunately, the majority of reocclusions occur directly after the intervention while the patient is still in the cardiac catheterization laboratory when immediate Re-PTCA is feasible. Approximately 40% of the early reocclusions, however, occur within the next 24 h. After the first 24 h reocclusion hazard is negligibly low. 17 Furthermore, continuous ECG-monitoring has the potential to provide not only information about the acute ischemic burden during PTCA but may also serve as a powerful tool for the detection of subacute reocclusion after the intervention.

Update in Thrombolysis 1994

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It has been demonstrated by Krucoff et al that repeated coronary occlusions generally produce the same distinct ECG pattern as the previous one. These authors also demonstrated that reocclusion after a primarily successful PTCA is accompanied by the same pattern of ECG changes that was elicited during PTCA in 84% of patients.18 These findings suggest to take an ECG fingerprint during ischemia provoked by PTCA and to use this template as a reference to detect ischemic events in the

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time period thereafter. Using continuous 12-lead ECG monitoring in 282 patients undergoing coronary angioplasty, Krucoff et al were able to demonstrate the presence of one or more transient ischemic episodes during the first 20 h after PTCA in 23% of patients. Out of these, 40% of ischemic episodes were asymptomatic indicating that in one out of ten patients ischemia after PTCA is silent. 29% of the patients who had such a transient ischemic episode after PTCA suffered major com-

118 Continuous3-D Vectorcardiographyfor lschemia plications such as urgent bypass operation, death or myocardial infarction which adds to the necessity of such continuous ischemia monitoring. These transient ischemic episodes are missed, however, in 87% of patients when only conventional CCU monitoring devices are employed.19 On the other hand, continuous vectorcardiography offers not only realtime monitoring of the ST-segment but also of all the other components of the ECG curve, such as QRS-changes. Furthermore, representation of the lateral and the inferior parts of the heart is superior to the standard 12-lead ECG. Figure 5 shows an example of a fingerprint ischemia during PTCA and the subsequent monitoring with continuous vectorcardiography. By taking the fingerprint ischemia as the detection matrix, ischemia is unveiled 16 h after the angioplasty. The displayed ECG of this event shows the same characteristic changes as were present during angioplasty. This indicates acute impairment of coronary blood flow in the region of angioplasty. This patient was recatheterized, redilated and went further uneventfully until hospital discharge.

NON-INVASIVE ASSESSMENT OF VESSEL PATENCY AFTER THROMBOLYSIS Failed recanalization after thrombolysis has been associated with higher rates of in-hospital mortality and morbidity and with minimal recovery of left ventricular function as compared with patients in whom reperfusion was successful. 20'21 Therefore, in the attempt to make a proper estimate of the patients prognosis, detection of vessel patency after thrombolytic therapy in myocardial infarction is important since, in patients with persistent occlusion, the infarct-related artery may be eligible for additional measures including rescue angioplasty. Standard 12-lead ECG has a moderate diagnostic accuracy using > 50% or 20% recovery of summated STsegment deviation to identify patency.22'23 Superior accuracy of non-invasive determination of coronary artery patency has been reported using continuous 12-lead electrocardiographic monitoring of ST-segment changes. 24 In a prospective study of 144 patients treated with thrombolytic therapy, infarct artery patency was correctly predicted using continuous 12-lead ST-segment recovery analysis in all except 20 patients. 25 In a further prospective and blinded angiographically controlled study in 96 patients, we were able to demonstrate that continuous vectorcardiography allows correct identification of infarct artery patency in 83% of the patients and in 73% with persistent occlusion. The diagnostic accuracy was more than 80%. 26 These results are in accordance with those reported from other electrocardiographic techniques. 27-29

DISCUSSION Despite the progress in thrombolytic treatment and cardi-

ology in the development of realtime monitoring devices, progress has been scarcely made in the past. In todays cardiology, however, there is a need for a simple non-invasive method that gives on-line information about the extent of the ischemic burden of the myocardium. The 12-lead ECG, with its more than 50-year-old arbitrarily defined leads, still serves as the most commonly used non-invasive tool to assess the myocardial perfusion status. With this technique, on-line monitoring is often limited to only continuous arrhythmia monitoring. In order to offer the best possible treatment for the cardiac patient, however, modern cardiology is in demand for immediate information about the ischemic burden of the heart and the myocardium at risk. The development of continuous 12-lead ECG-devices or continuous vectorcardiography both seem to provide non-invasive information that enables the physician to obtain on-line information of the perfusion status of the heart. Today's gold standard for the assessment of vessel patency at a defined point in time is coronary angiography. This gold standard has been used for the evaluation of diagnostic accuracy of continuous electrocardiography during thrombolysis. In these trials TIMI II rated 'open arteries' frequently have been identified by continuous electrocardiography as 'probably occluded', indicating that these methods may discriminate between the viable and the non viable myocardium rather than the perfusion status. These findings may offer the theoretical advantage to even better assess the ischemic burden of the myocardium. Furthermore, the use of other parameters than the classical ST-deviations may offer a higher sensitivity for the detection of ischemia. QRS-changes during myocardial ischemia seem to be a more sensitive and more rapid indicator of temporary coronary occlusion than ST-segment changes. Hamlin et al demonstrated that, immediately upon coronary occlusion, the QRS-vector loop dilated. These findings were explained by the appearance of a 'periischemic block' that causes a delayed and unopposed ventricular activation of the ischemic area. 30 STsegment changes need an electrical injury current to be elicited and therefore do occur slightly later than QRS changes and only during the more severe ischemic episodes. Myocardial ischemia in the inferior and the lateral part of the heart is often difficult to recognize. This is due to the inadequate representation of these perfusion areas in the standard ECG. It may however also result from the fact that the left anterior descending artery supplies a larger myocardial muscle mass. The later explanation is in accordance with a postmortem study of Kalbfleish and Hort,31 who reported that the average percentage of ventricular myocardium supplied by the left anterior descending artery was 41.5%, by the left circumflex artery 22.5% and by the fight coronary artery 36%. Vectorcardiography thus seems to provide a better representation of these areas. Finally, Krucoff et a132 have demonstrated that multilead ST-segment recordings that are taken during PTCA

Update in Thrombolysis 1994

can serve as patient-specific and site-specific non-invasire markers for abrupt reocclusion. The reported incidences of reocclusion have varied widely, partly due to differences in the treatment regimens. Continuous ischemia monitoring with the determination of a reference ischemia during PTCA and subsequent monitoring after the intervention may offer the exciting possibility to detect early reocclusion even when occurring without clinical symptoms.

CONCLUSIONS By using the complete three-dimensional electrocardiographic information, continuous vectorcardiography provides not only better insights into the lateral and inferior part of the heart than the 12-1ead ECG but also provides other components of the ECG curve such as QRS-complex changes, which are more sensitive markers of ischemia than ST-segment deviations. By condensing the electrocardiographic information into one continuously updated variable, continuous vectorcardiography offers the potential for realtime ischemia monitoring. Thus 3Dvectorcardiography may provide a suitable tool for the non-invasive assessment of the acute ischemic burden during PTCA and ischemia monitoring in the critical 24 h thereafter, where the hazard of reocclusion is highest. Continuous vectorcardiography also may serve as a powerful tool in the non-invasive assessment of infarct artery patency after thrombolysis and may help to identify patients with persistent occlusion who are potential candidates for rescue PTCA.

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Continuous 3-D Vectorcardiography for Ischemia

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