Optimizing Posterior 12-Lead ECG Placement in Prone Patients

Are you familiar with all the practice modifications for proning a patient? One of these is to understand how to place ECG electrodes on the prone patient, and to understand how this placement alters the recorded 12-lead ECG and affects its interpretation. Data differs somewhat on what ECG changes to anticipate, so the results of five studies are summarized here.

Reasons for proning

Prone positioning is used for patients following spinal and orthopedic surgeries and surgeries involving the retroperitoneum, for patients with moderate to severe ARDS, and for those with COVID-19 experiencing hypoxic respiratory failure. Cardiac events, including myocardial injury and cardiac dysrhythmias, are reported in some patients with ARDS and those with COVID-19. Turning the patient back to supine when a 12-lead ECG is indicated is labor-intensive and would interrupt the therapeutic purpose of prone positioning. In some cases, supine position would be contraindicated. So it is sometimes necessary to place precordial (V leads) on the posterior chest wall, and it is important to understand how this placement impacts the 12-lead ECG in the prone patient.

Application of posterior electrodes

• There are some differences in the placement of posterior leads across the five studies, but the most common advice is to mirror the anterior placement:
• It is critically important to label the ECG, so all healthcare personnel know it was obtained using posterior placed leads. Follow your institution’s recommendation:
  • Labeling V leads is often pV(number), such as pV1, which is the approach used in this blog.
  • I’ve also seen V(number)p, such as V1p.
• Place pV1 to the right of the spine at the T7 spinous process, and place pV2 to the left of the spine at T7.
  • Recall that ribs are not attached to the spine at the same angle as to the sternum, and the 4th anterior intercostal space (where you would place V1 and V2 for a normal ECG) aligns with the T7 spinous process.
• Place pV3 to pV6 in the plane of the 5th anterior intercostal space, with pV4 at the mid-scapular line and V6 in the mid-axillary line. This placement aligns with the plane of the T8 spinous process. Place pV3 midway between pV2 and pV4; place pV5 midway between pV4 and pV6.
• Limb lead placement and labeling is unchanged from a standard 12-lead ECG.

Changes in the 12-lead ECG when V leads are placed on the posterior chest

Chieng et al studied 100 subjects, comparing 12-lead ECGs obtained in three positions: 1) using a normal supine 12-lead ECG; 2) using precordially placed leads when the subject was prone; and 3) using posterior placed leads when the subject was prone. All subjects were recruited from cardiology units, were not on a ventilator, and were not positive for COVID-19. The following changes in the 12-lead ECG were documented:

• A small increase in QTc duration was noted, as the mean QTc on supine ECG was 432 +/- 31 ms, with precordial lead placement in a prone position (437 +/-31 ms), and posterior lead placement in the prone position (436 +/- 34 ms).
• In leads V1 to V3, a qR morphology was seen in 90% of subjects and changes in T-wave polarity in 84% of subjects, when they were prone with posterior placed leads.
• In subjects with ischemia of the anterior wall, the ST-segment changes in V1-V3 that are observed when the subject is supine for a normal 12-lead ECG were no longer seen, and an R wave was seen when the subject was prone with posterior placed leads.
• In subjects with bundle branch block (BBB), it remained detectable when the subject was prone with posterior placed leads, but
  • Left BBB was seen as right BBB in 71% of subjects.
  • QRS narrowing with qR in V1 was seen in subjects with documented right BBB.
• ST-segment and T-wave changes in limb leads and dysrhythmia detection were largely unaffected when subjects were prone with posterior placed leads.

Daralammouri et al compared standard 12-lead ECGs in 40 healthy volunteers to ECGs obtained in the prone position with posterior placed V leads, with several different findings:

• Mean HR was higher in the prone position: supine (69.5 +/- 11.5 bpm) vs. prone (73 +/- 12.4 bpm)
  • Study authors identified this finding as due to a reduction in venous preload caused by inferior vena cava compression, and a rise in resistance to left ventricular filling due to increased intrathoracic pressure, leading to a lower stroke volume, which was assumed to lower the arterial pulse wave that inhibited baroreflexes and consequently enhanced nervous sympathetic activity and increased HR.
• QRS duration decreased from 92.8 +/- 12.6 ms supine to 84.9 +/- 11.9 ms prone.
  • They identified this finding as due to decreased heart size due to decreased preload and compression of the heart in the thoracic cavity.
• The mean QRS axis moved to the left when prone compared to supine (prone 40.5 vs. supine 49 degrees).
• The amplitude of the precordial QRS decreased from supine (7.42 +/-3.1 mV) to prone (3.68 +/- 1.7 mV), and new Q waves were noted in V1-3.
• QTc was longer supine (406 +/-18.8) than in prone position (385 +/- 64.8).

Kotruchin et al conducted a similar comparison of supine ECG with standard lead placement vs. prone position with posterior lead placement in 40 patients with COVID-19:

• QRS duration was 101 +/- 45 ms supine compared to 95 +/-14 ms prone.
• HR was 74 bpm supine vs. 72 bpm prone.
• Mean QTc was 450 +/-85 ms supine vs. 439 +/-36 ms prone.
• ST deviation of V1 & V2 was diminished in a prone position in 90% of patients.
• Diagnosis of dysrhythmias was identical for both methods.

Nguyen, Trohman and Huang presented a case study of a patient diagnosed with COVID-19 and subsequent ARDS requiring intubation and mechanical ventilation. They noted:

• Low amplitudes and prominent Q waves in V1-V3 when the patient was prone with posterior ECG lead placement.
• A shift in the vector of ventricular depolarization resulting in QS and/or low amplitude R waves in V1-V3 with prone position and posterior lead placement.

They subsequently reviewed traditional supine vs. prone ECG in 20 subjects with COVID-19, finding no significant difference in limb lead waveforms but a significant decrease in mean QRS amplitude in V1-V5 and a reduction in R-wave amplitude in V1-V4 when ECGs were obtained in the prone position.

Romero et al studied 85 subjects, of whom 45 were diagnosed with ARDS and 40 were healthy volunteers:

• No significant differences in HR, PR QTc or QRS axis were noted.
• QRS duration (92.2 +/- 15 ms) was seen when subjects were supine with anterior leads, but QRS duration was shorter (85.8 +/-17.5 ms) when subjects were prone with posterior leads.
• Lower QRS amplitudes were seen in V1-V5 in prone subjects with posterior leads, with no statistical difference noted in QRS amplitude for limb leads.
• New prominent Q waves were seen in the prone position with posterior leads. Q waves were found in 74.1% of subjects in V1 when prone with posterior leads, vs. only 10.6% of supine patients with anterior leads.
  • Q waves in V1 on the ECG for a supine patient are commonly considered to possibly indicate myocardial infarction.
  • The study authors advise that Q waves should be considered normal in V1-V2 prone, posterior, but their presence in V3-V6 should prompt further evaluation.
• T inversion or flat T waves were much more common in prone subjects with posterior leads, as seen in  V1 (supine 66% vs. prone 98%), V2 (supine 8% vs. prone 96%), and V3 (supine 7% vs. prone 45%).

Implications for 12-lead ECG interpretation when V leads are placed on the posterior chest wall on prone patients

• The 12-lead ECG is unreliable for the detection of anterior myocardial injury.
• Changes in ST-segment and T-wave abnormalities in limb leads are reliable indicators.
• Rhythm monitoring and BBB detection are reliable, although differentiation of left BBB vs. right BBB is not reliable.
• QTc may be longer in prone patients with posterior placed leads. It is recommended to obtain a baseline prone QTc and observe trends in QTc over time in the same position.

Summary of key points

• Follow the above guidance for the landmarks to use when placing V leads on the posterior chest in a prone patient.
• Clearly annotate any ECG tracing recorded with atypical body position (proning) and/or atypical lead placement (posterior lead placement).
• Cardiologist expertise in interpreting the 12-lead ECG is essential when ischemia or infarction is suspected.

What practice pearls can you share when performing 12-lead ECG using posterior placed leads on a patient in a prone position?