Continuous Cardiac Monitoring: Lead Selection, ST and QT

Monitoring shows rhythm, ST-segment and QT changes only when the chosen lead displays them and each alarm is checked against the patient. Time-critical when a rhythm or ischemic change could be life-threatening.

Continuous electrocardiographic (ECG) monitoring watches the heart's electrical activity through a few electrodes. It can catch rhythm trouble, ischemia (ST-segment change) and QT prolongation between 12-lead tracings. The AHA 2017 scientific statement on ECG monitoring in hospitals covers those three areas, plus alarm management and documentation.

How It Works

• Lead view: a monitor shows one or a few leads, which is not the 12-lead picture. Treat any monitor finding as a prompt to check a 12-lead.
• Rhythm: the monitor gets rate and regularity by detecting QRS complexes. Small or wide QRS complexes can be missed or miscounted.
• ST segment: every ST change is measured against a baseline. Lead choice, baseline and signal quality decide whether the reading is accurate.
• QT: the monitor estimates QT and QTc. Wide QRS complexes or noise make those estimates less reliable.
• Alarms: limits set off the tones and displays. In one 31-day study of 461 ICU patients, 88.8% of 12,671 annotated arrhythmia alarms were false positives. The audible burden came to 187 per bed per day. Alarm safety is a Joint Commission National Patient Safety Goal.

Where It Fits

Monitoring detects. The 12-lead ECG confirms. STEMI decisions rest on 12-lead criteria and the territory leads. A monitor trend alone does not make that call. In cardiac arrest, follow the current AHA ACLS or PALS algorithm for pads, rhythm checks and shocks. The monitor does not change those steps. For ST criteria, see the ST-T wave page. For QT thresholds, see the QT interval page.

Indications

• Arrhythmia: symptomatic or unstable rhythm, syncope, palpitations, or a rhythm that is changing
• Ischemia: suspected or known acute coronary syndrome. Continuous ST monitoring can pick up silent ischemia. No randomized trial has shown that it improves outcomes (AACN)
• QT: QT-prolonging drug therapy, electrolyte disturbance, bradycardia or pauses. The AHA 2017 statement addresses appropriate QT monitoring in select populations
• After a procedure or during care: after cardioversion, during pacing, sedation or transport, per local protocol

Absolute Contraindications

Nothing absolute stops monitoring when it is indicated.

Relative Contraindications & Cautions

• Skin damage under electrode sites: broken skin or burns. Use another site and document
• Noisy or unreadable signal: movement, shivering or poor electrode contact. Fix the signal before trusting the trend
• Wide QRS (bundle branch block or ventricular pacing): it sets off false rate alarms and makes the standard ST criteria invalid. See the ST-T wave page

Consider an Alternative

• 12-lead ECG: when an ST, QT or specific diagnosis is the question. See the 12-lead ECG acquisition page
• Pacing: bradycardia that needs treatment is a separate decision. See transcutaneous pacing

Team Roles During the Procedure

• Operator: places the electrodes and selects the lead
• Alarm manager: answers each alarm at the bedside and checks the patient
• Recorder: documents lead, alarm limits, baseline and any 12-lead obtained

V1 Electrode

1. Find the 4th intercostal space: count down from the sternal angle.
2. Place the electrode: at the right sternal border, just right of the sternum.
3. Confirm the position: over the chest wall, not on the sternum or breast tissue.

V5 Electrode

1. Locate V4: the 5th ICS at the midclavicular line.
2. Place V5: on the anterior axillary line, level with V4.
3. Women: lift the breast and place the electrode on the chest wall at the landmark, not on breast tissue or in the fold beneath it.

Equipment

Sizing

Use the device's adult or pediatric electrode setup. For children see equipment sizing.

Patient Positioning & Preparation

Supine if possible. If the patient cannot lie flat, sit them upright with back support and note the position. Clean, dry skin and firmly stuck electrodes make a better tracing. Record a baseline 12-lead before any ST or QT decision.

Monitor Settings

QT-Prolonging Drugs to Check on the Monitor

The risk is torsade de pointes. The AHA and ACCF 2010 statement on preventing it in hospital settings covers it. Here are the common drugs to watch.

Phase 1: Prepare

1. Confirm the reason for monitoring: rhythm, ischemia or QT, and what result will change care.
2. Check the monitor: lead set, alarm limits and whether ST or QT analysis is on.
3. Prepare the skin: clean and dry it, clip hair, and avoid broken skin and device pockets.

Phase 2: Apply

4. Place the limb electrodes: at the wrists and ankles, or at proximal or torso sites if access is limited or motion is a problem. Torso sites can shift the QRS axis and baseline ST level, so note the set used and confirm any ST change on a standard 12-lead.
5. Place the chest electrode: V1 at the 4th ICS, right sternal border, or V5 on the anterior axillary line, level with V4, per monitor and protocol.
6. Check the signal: QRS complexes visible, low noise, and no lead-off alarm.

Phase 3: Verify & Set Up

7. Select the rhythm lead: choose the lead with the clearest P waves and QRS complexes, and switch if the QRS is small.
8. Set individualized alarm limits: and document why they differ from defaults.
9. Obtain a 12-lead for ST or QT questions: use it as the baseline and the reference.
10. Document: lead, alarm limits, baseline and the first rhythm read.

Primary Confirmation

Check the pulse against the monitor. The displayed rate has to match it. Any ST or QT finding gets confirmed on a 12-lead ECG before action. A monitor rate that does not match the pulse is a sensing or artifact problem until proven otherwise.

Secondary Confirmation

• Lead check: P waves and QRS amplitude are clear in the chosen lead
• 12-lead: the ST pattern or QTc is confirmed on a tracing taken during the same period
• Pulse oximetry or arterial waveform: pulse matches the displayed rate

Ongoing Monitoring

• Each alarm: look at the patient, then the strip
• After every move: recheck the leads and the baseline
• After a QT-prolonging drug: repeat the 12-lead when the QTc trend changes or a second drug is added
• Skin under electrodes: check per local protocol and move the electrode if the skin is damaged

Pediatrics

• Electrodes: use the pediatric electrode setup for the device; see equipment sizing
• Heart rate: use age-matched references; see vital signs
• QTc: the adult thresholds above are not age-specific. Use age-specific limits with a pediatric reference
• ST: pediatric ST elevation is often non-ischemic. Read it against age-based references

Implanted Devices and Paced Rhythm

• Paced rhythm: wide QRS complexes can set off false alarms and get miscounted. Check the pulse
• Device guidance: follow the device clinic's and local protocol's electrode placement guidance

Immediate Aftercare

Once the leads are on, check the lead, the alarm limits and the baseline. If ST or QT was the reason for monitoring, record a 12-lead.

Ongoing Maintenance

• Electrodes: replace per protocol or when contact fails
• Skin: check under each electrode and move a damaged site
• Alarm limits: recheck after any change in rhythm, drug or condition

Transport Considerations

• Motion: movement and vehicle vibration create artifact. Recheck the leads after every move
• Alarm load: a moving patient and a busy vehicle mean more false alarms. Look at the patient first
• Drugs: when a QT-prolonging drug goes in during transport, recheck the QTc on a 12-lead

Pearls

• Confirm before acting: an ST or QT finding on the monitor needs a 12-lead
• Look at the patient: 88.8% of arrhythmia alarms were false in one ICU study. The real event is the one you can miss
• Switch leads deliberately: a small QRS in a lead means beats get undercounted
• Baseline first: an ST change needs a baseline from a stable tracing to be read against

Common Pitfalls

• Default alarm limits: individualize and document
• Trusting an ST trend after a lead change: reset the baseline first
• Assuming a quiet monitor means a normal QT: after a new drug, check the QTc on a 12-lead
• Silencing alarms at the bedside: pause, look at the patient, then respond

Skill Maintenance

• Practice lead placement on a partner and compare the strip with a 12-lead
• Practice setting an ST baseline on sample strips
• Practice recognizing a wide QRS, a small QRS and a noisy trace
• Debrief missed or delayed alarm responses using the unit's alarm log