Cardiology

12 Lead ST Monitoring

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What is ST segment analysis?

The sequential electrical activation of the heart muscle results in P-, QRS- and T-waves in the ECG. The P-wave represents atrial depolarization, the QRS- complex represents ventricular depolarization, and the T-wave represents ventricular repolarization. The ST segment, the phase between the ventricular depolarization and repolarization, is usually isoelectric and slightly slanted upwards.

Fig 1. ST-segment indicating ischemia

In coronary artery disease the blood flow to the heart muscle may be compromized. If oxygen delivery is not sufficient, ischemia results in the myocardium. The ischemic tissue cannot maintain the membrane potential. This is seen by displacement of ST segment: downwards in subendocardial injury and upwards in subepicardial injury. The larger the ischemic area, the greater this deviation. The most commonly accepted criteria for evidence of myocardial ischemia is ST segment deviation greater than 1 mm (0.1 mV).

Why 12-lead ST monitoring?

Early detection of ischemia can help in the institution of timely lifesaving intervention. Persistent ST segment elevation is a sign of acute myocardial injury: the more leads involved, the more detail can be provided about the injury.

ST changes in different leads reflect the location of ischemic area. A method of increasing the sensitivity in order to detect ischemic events is to add more ECG leads. If lead V5 is used alone the sensitivity is found to be 75 % but if leads V4 and V5 are used concurrently, the sensitivity increases up to 90 %. The conventional combination of leads II and V5 give a sensitivity of 80 %. However, if leads II, V4 and V5 are combined the sensitivity increases to 96 %. Sensitivity of close to 100 % is achieved if leads V2 and V3 are added to the above given lead combination.*

Fig 2. 12 lead ECG view

* London MJ: "Intraoperative Myocardial Ischemia: Localization by Continuous 12-Lead Electrocardiography", Anesthesiology 1988; 69: 232-41

How is ST segment analyzed?

The ST segment begins at the point where the QRS ends (J-point). Diagnostic criteria of ST segment changes have been defined to be measured at 60 ms after the J-point. For monitoring purposes it is important that the measurement point is accurate to obtain a reliable trend of ST changes.

The sophisticated algorithms of Datex-Ohmeda monitors search for the J and isoelectric (ISO) points. The system learns the ECG and stores the reference QRST complex. The algorithm sets the ISO and J points. These points can also be adjusted manually. The QRS analysis classifies each beat by using several criteria and rejects distorted complexes from the ST calculation.

ST is analyzed for all measured leads. With a 10-lead trunk cable Datex-Ohmeda monitors analyze and trend the ST segment changes continuously for 12 leads. With a 5-lead trunk cable continuous ST segment analysis and trending is done for 7 leads, and with a 3-lead trunk cable for 1 lead.

In the ST number field 3 user selectable ST values are shown. The 4th ST value shows the largest absolute ST value.This helps the clinician to notice if ischemia is spreading to new areas.

Fig 3. ST number field

Track the ischemic changes in QRST complex

The ST level changes can be referred at the start-up of the ST monitoring. The monitor will analyze and store reference QRST complexes of all leads,which can be easily compared to the following complexes. It is important to have the capability to store and interchange reference QRST complexes. This allows the clinician to see the effect of treatment on the patient.

With the Datex-Ohmeda monitor it is possible to store and select up to seven reference QRST complexes during monitoring. The clinician may review the ST changes during each course of therapy (e.g.through the entire thrombolysis and revascularization procedures).

In addition to trending ST values,the Datex-Ohmeda solution is also capable of trending the shape of QRST complex. QRST complexes are automatically saved for all leads every 10 minutes. These complexes can be reviewed by simply moving the cursor on the ST trends. Furthermore, the user may print the stored complexes to track the progress of the ST level over a selected time period.

Localizing the ischemic area

Localizing the area of ischemia on the 12-lead ECG is a clinical necessity. The leads of the 12-lead ECG are grouped according to the regions of heart they view. One criterion for identifying myocardial ischemia is ST elevation of 1 mm or more observed in two contiguous ECG leads.

Fig 4. ST View

The Datex-Ohmeda solution provides viewing, printing and alarm setting capabilities for anterior, inferior and lateral lead groups. Lead grouping combined with clear concise trend analysis allows for accurate estimation of patient ischemic status. The additional feature is the ischemic burden for lead groups.

Fig 5. ST Alarm View

Ischemic burden

Instead of the single term ischemic ST level, the duration of the ischemic events are considered to provide most valuable information while assessing the severity of cardiac ischemia.

In the Datex-Ohmeda monitoring solution,the sophisticated ischemic burden display gives valuable information about the development of ischemia. In the ST trend the user may select the thresholds over which the monitored ST levels should not rise. If the monitor detects ST levels over the threshold, the area will be shaded by a specific color. This allows easy and accurate visualization in the duration of ischemic events.

Fig 6. Ischemic Burden View

Practical Aspects In ST Segment Monitoring

Other factors affecting the ST segment?

Prinzmeta’s Angina is a temporary, reversible coronary artery spasm, which may or may not be associated with chest pain. It is indicated in the ECG as ST elevation, which resolves in minutes. Acute Pulmonary Embolism may be associated with ST elevation in leads monitoring the right side of the heart. In Pericarditis ST elevation is frequently seen, but is diffuse and involves multiple leads.

Filtering

The ST segment changes represent the low frequency changes in the ECG. Datex-Ohmeda monitors feature a special filter for ST analysis: STfilt (0.05 -30/40 Hz), which ensures accurate ST segment analysis, even in the most demanding environments. It filters out the high frequency noise and artifacts, but captures the low frequency ST segment changes without distortion. Because of possible distortion of the ST segment, ST monitoring is disabled when using the monitor filter.

Fig 7. Effect of filtering on the ST segment.

Pacemaker Patients

The ST segment of paced beats may not be diagnostic for ischemia or infarct. Datex-Ohmeda monitors exclude all ventricular paced beats from their ST-analysis.

Last updated: 1 September 2001Created
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