// Fetal Cardiology · Acquisition Series · Part 1

Fetal PR Interval
Acquisition Guide

Views, angles, sample volumes, and the small things that change your number.

LV In/Out SVC/Ao PA/PV TDI Normative Data Method Bias
// 01 — Concept

A Surrogate, Not the Real Thing

Pulsed-wave Doppler gives you a mechanical proxy for the electrical PR interval. Understanding the gap between the two changes how you place your cursor.

True Electrical PR
P wave onset → QRS onset

What the ECG measures. Onset of atrial depolarisation to onset of ventricular depolarisation.

Not available in routine fetal practice

Mechanical PR (mPR)
A-wave onset → Aortic ejection onset

What Doppler gives you. Consistently longer than the true electrical PR by approximately 14–16 ms — a systematic, predictable difference, not random error.

Mechanical proxy — validated across multiple cohorts

Electrical PR vs mechanical PR surrogate

Figure 1. Relationship between true electrical PR (ECG) and mPR surrogate. The mPR consistently exceeds the electrical PR by approximately 14–16 ms. This is a systematic, predictable difference and must be kept in mind when interpreting values against clinical thresholds.

📐
The mPR is always longer than the electrical PR. This applies to every pulsed-wave Doppler method. At a clinical treatment threshold of 150 ms, a 10–15 ms acquisition error is not a technical footnote — it dictates treatment.
150 ms threshold — why precision matters

Figure 2. At a clinical treatment threshold of 150 ms, a 10–15 ms acquisition error is not a technical footnote. It dictates treatment.

// Method 01 — Default

LV Inflow / Outflow
MV/Ao Method

The most widely used technique with the largest normative dataset. Your starting point in every study.

Sample volume — 5-chamber view

Figure 3. Sample volume positioning at the anterior mitral leaflet hinge point within the LVOT. GA 25w2d. The gate must simultaneously capture mitral inflow and aortic outflow — not deeper, not at the leaflet tips.

Doppler trace — A-wave + AO

Figure 4. LV inflow/outflow Doppler trace. A-wave above baseline (atrial contraction), aortic ejection below. PR Interval 109 ms. GA 25w2d, 150 mm/s.

< 20°
Angle of insonation to LVOT long axis
Requires apex-up or apex-down fetal orientation
1

Get to a 5-chamber view

Sweep slightly cephalad from 4-chamber until the aortic root appears. Fetal position must be apex-up or apex-down. A lateral heart axis makes reliable acquisition impossible — wait or ask the mother to reposition.

2

Set sweep speed to 150 mm/s before sampling

This is the minimum required to visualise valve clicks and time waveform onsets with precision. Below 100 mm/s, onset timing is guesswork.

3

Place the sample volume at the mitral hinge point

Position at the anterior mitral leaflet hinge point within the LVOT. Not deeper (A-wave disappears), and not at the leaflet tips (outflow signal lost). Both A-wave and aortic ejection signal must be clearly visible.

4

Confirm E and A waves are separate

At heart rates above approximately 160 bpm, E and A waves fuse and the A-wave onset becomes unidentifiable. Stop and switch to SVC/Ao.

5

Measure onset to onset — average 3 beats

Cursor 1 at the foot of the A-wave. Cursor 2 at the leading edge of the aortic opening click. Average 3 consecutive artefact-free beats. Single-beat measurement is not acceptable.

6

Document FHR and method

The reference range you compare against is FHR and GA dependent. Document both alongside every mPR value. Never mix methods across serial studies in the same patient.

✓ Do
✓Apex-up or apex-down fetal orientation only
✓Sweep speed 150 mm/s minimum
✓Sample volume at mitral leaflet hinge point
✓Measure from onset of A-wave, not its peak
✓Average at least 3 beats
✓Record FHR with every measurement
✓Switch to SVC/Ao if E-A fusion occurs
✗ Don't
✗Measure at sweep speed below 100 mm/s
✗Place gate deep in LVOT — A-wave will vanish
✗Measure from A-wave peak instead of onset
✗Use lateral fetal position with heart perpendicular to beam
✗Continue when E and A waves are fused (FHR >160 bpm)
✗Mix methods across serial studies in same patient
✗Report a single-beat measurement
MV OPENING CLICK

Start of E-wave. Not used for PR measurement.

MV CLOSING CLICK

Follows atrial contraction. Best ventricular landmark — marks IVC onset. Correlation r=0.92 vs ECG (Bergman 2006). No fetal normative dataset yet.

AoV OPENING CLICK ← standard cursor point

Start of ejection. Leading edge of click base is your landmark. Click base spans ~8–15 ms at 150 mm/s — choose consistently and document it.

AoV CLOSING CLICK

Not used for PR measurement.

Annotated valve click trace at 150 mm/s
Image to be added
💡
Place cursor at the leading edge of the aortic click base. The literature does not prescribe this precisely — consistency is what matters clinically.
// Method 02 — Rescue / High FHR

SVC / Ascending Aorta
SVC/Ao Method

The go-to when E-A fusion occurs or LV in/out alignment is limited. Requires near-zero insonation angle and wall filter at minimum.

Colour anatomy — Ao, SVC, IVC

Figure 5. Colour Doppler showing SVC and ascending aorta (Ao) with IVC visible. GA 22w5d. The SVC runs parallel to the Ao over a short mediastinal segment, enabling simultaneous sampling.

SVC a-wave reversal + Ao ejection

Figure 6. SVC/Ao Doppler trace. SVC a-wave reversal (A, above baseline) marks atrial systole. Aortic ejection (Ao, below baseline) marks ventricular ejection. GA 22w5d.

Near 0°
Stricter than MV/Ao — the SVC a-wave is a low-velocity signal
If angle exceeds 30°, the a-wave may be entirely absent from the trace
1

Find the SVC/Ao plane

From the 3-vessel view, sweep slightly caudad, or use a near-sagittal approach when the fetus is spine-posterior. SVC runs to the right of and slightly posterior to the ascending aorta. Both must run parallel to the beam.

2

Wall filter to minimum — before sampling

This is the most common reason the SVC a-wave is absent. The default wall filter will suppress it. Drop to the lowest available setting before placing the gate. If the a-wave is absent, check filter first.

3

Widen gate to span both vessels

Use colour Doppler to confirm both SVC and aorta are within the gate. SVC flow (venous reversal) is above baseline; aortic ejection is below.

4

Identify the SVC a-wave reversal

A small brief reversal in SVC flow during atrial systole — above baseline and short in duration. It appears just before the aortic ejection waveform. If absent, filter is too high or angle is off.

5

Measure and average 3 beats

Onset of SVC reversal to onset of aortic ejection (leading edge of click). 150 mm/s minimum. Average 3 beats.

✓ Do
✓Set wall filter to minimum — always, before sampling
✓Verify near-zero beam-to-vessel angle with colour Doppler
✓Use as default when FHR exceeds 160 bpm
✓Check for a-wave visibility before measuring
✗ Don't
✗Attempt when angle exceeds 30° — a-wave will be absent
✗Forget to reduce wall filter before sampling
✗Assume absent a-wave means 1° AVB — check filter first
✗Compare SVC/Ao values to MV/Ao reference ranges directly

Similar mean values to MV/Ao

Kato et al. (2012) compared both methods against fetal magnetocardiography in 135 and 84 fetuses respectively. Overestimation of the electrical PR was virtually identical: 14.6% for LV in/out and 14.7% for SVC/Ao. Andelfinger et al. also found no significant time difference between the two methods. Switching from MV/Ao to SVC/Ao as a rescue method does not introduce a clinically meaningful numerical offset.

Independent of LV filling dynamics

The SVC a-wave reversal is driven by atrial contraction against venous return — not by LV diastolic filling. This is why E-A fusion does not affect it. At any heart rate, the SVC a-wave remains identifiable provided the angle and filter are correct.

// Method 03 — Position Independent

Pulmonary Artery / Vein
PA/PV Method

Validated by Carvalho et al. (Royal Brompton, 2007). Less limited by fetal position but technically demanding — and values run systematically higher than MV/Ao.

⚠
Critical difference: In the pulmonary vein, atrial systole produces a notch or velocity reduction in forward flow — not a retrograde reversal as in the SVC. The atrial timing marker is a small dip, not a wave below baseline. This makes identification more demanding.
Colour — Lt/Rt pulmonary veins + Ao

Figure 7. Colour Doppler showing left and right pulmonary veins and aorta (Ao). GA 25w2d. Use colour Doppler to confirm vessel identity before placing the sample gate.

PA/PV Doppler trace — 114 ms

Figure 8. PA/PV Doppler trace. PR 114 ms. GA 25w2d, 150 mm/s. Note values run 6–15 ms higher than MV/Ao — method-specific reference ranges are mandatory.

PA/PV vs LV In/Out: The Offset by Gestational Age

Data from Anuwutnavin et al. Prenat Diagn 2018. Mean values in milliseconds. The offset widens progressively across gestation — this is clinically significant at any threshold near 150 ms.

GA (weeks) LV In/Out mean PA/PV mean Difference Clinical relevance
16–19 110.8 ms 118.2 ms +7.4 ms 2nd trimester range
20–24 115.9 ms 122.0 ms +6.1 ms 2nd trimester range
25–29 117.1 ms 126.0 ms +8.9 ms Offset begins widening
30–34 118.1 ms 129.9 ms +11.8 ms 3rd trimester — clinically significant
35–38 121.8 ms 137.3 ms +15.5 ms Largest offset — highest risk of misclassification

Applying MV/Ao reference ranges to PA/PV measurements is not valid at any gestational age. At 35–38 weeks a normal PA/PV value of 137 ms would appear to approach the 150 ms threshold when compared against MV/Ao ranges.

// Method 04 — Advanced

Tissue Doppler Imaging
Fetal Kinetocardiogram

Not a first-line PR surveillance tool. Best used when rhythm analysis rather than PR number is the primary question.

⚠
Know the limitation before using: TDI-derived AV intervals have a higher coefficient of variation than pulsed-wave Doppler methods in some studies. Best correlation with fetal ECG was R² 0.15 for TDI vs R² 0.10 for MV/Ao (Nii et al., Heart 2006) — statistically better but clinically both scatter widely. Use for rhythm assessment, not for precise mPR threshold decisions.
Colour TDI apical view — LV

Figure 9. Colour TDI apical view showing left ventricle (LV). Requires true apical orientation with cardiac long axis parallel to the beam. Angle of insonation below 30°.

a' wave + s' wave trace — 116 ms

Figure 10. TDI Doppler trace. a' wave marks atrial wall motion (atrial contraction surrogate). s' wave marks ventricular systole. PR 116 ms. GA 23w0d, 150 mm/s.

Machine Settings for TDI
→PRF lowered to approximately ±15 cm/s to capture myocardial velocities
→Wall filter at minimum
→Gain reduced to suppress blood flow signals
→Sweep speed 100–150 mm/s
→Apical view mandatory — angle below 30°
→Dedicated TDI preset required — not available on all systems
What TDI Measures

Atrial landmark: late diastolic atrial wall motion (A' wave). Ventricular landmark: isovolumic ventricular contraction spike (IVC spike). The interval mirrors the PR interval.

Lower bias vs ECG than MV/Ao in theory — but greater variability in practice. Supplementary role only for routine mPR surveillance.

// Non-Negotiables

Machine Settings
Before You Sample

Most measurement errors originate from wrong machine settings, not wrong probe position.

Machine settings infographic

Figure 11. Non-negotiable system parameters for accurate acquisition.

⚡
Sweep Speed
150 mm/s
Minimum to visualise valve clicks and time onset precisely. Below 100 mm/s, timing is guesswork.
🎚
Wall Filter
Minimum
Critical for all venous methods. Default settings suppress the SVC and PV a-wave signals entirely.
🎯
Sample Volume
1–3 mm
Small enough for spatial specificity. Large enough to capture both signals simultaneously.
📐
Insonation Angle
< 20°
Classic teaching standard. Near-zero preferred for SVC/Ao. A lateral heart position makes this unachievable.
📊
Beats Averaged
≥ 3
Minimum 3 consecutive artefact-free beats. Single-beat measurement is not clinically acceptable.
📝
Log Always
FHR + Method
The mPR number without concurrent FHR, GA, and method is uninterpretable. All three are mandatory.
// Reference Data

Normal Values by
Gestational Age

Anuwutnavin et al. Prenat Diagn 2018. Mean (SD) in milliseconds by method and gestational age group.

16–19 wk
20–24 wk
25–29 wk
30–34 wk
35–38 wk
LV In/Out
110.8
(±5.8)
115.9
(±5.4)
117.1
(±7.4)
118.1
(±7.4)
121.8
(±8.7)
SVC/AA
109.9
(±8.0)
114.8
(±6.0)
116.9
(±7.8)
117.8
(±8.1)
127.3
(±13.5)
PA/PV
118.2
(±8.4)
122.0
(±7.5)
126.0
(±8.0)
129.9
(±9.2)
137.3
(±11.4)
LV In/Out (MV/Ao)
SVC/AA
PA/PV
// Method Bias

How Each Method Compares
to the Electrical PR

All pulsed-wave Doppler methods overestimate the true electrical PR. Degree and reproducibility vary. Data from Bergman 2006 and Nii 2006.

The artefact trap — mid-surveillance method switching

Figure 12. Because PA/PV values run 6–15 ms higher than MV/Ao across gestation, switching methods mid-surveillance without accounting for this offset creates the illusion of PR prolongation where none exists.

Method Correlation vs ECG Mean Bias Reproducibility Rating Clinical Role
LV In/Out r = 0.82 +18.7 ms*
Good Default first-line
SVC/Ao r = 0.85 +14.7%†
Good High FHR / rescue
PA/PV ~r = 0.82 +6–15 ms‡
Moderate Position fallback
TDI R² = 0.15 +8.0 ms
Advanced Rhythm analysis

*Bergman 2006, neonatal ECG validation (n=22). †Kato 2012, fetal magnetocardiography (n=84). ‡Gestational age dependent — 6–9 ms in 2nd trimester, 12–15 ms in 3rd trimester (Anuwutnavin 2018). Nii et al. Heart 2006 (n=196 fetal echos).

// Clinical Utility

Method Selection
and Indications

Choosing Your Method

Situational routing flowchart

Figure 13. Situational routing for method selection — assess FHR and fetal position first, method follows.

→

Apex-up or apex-down?

Use LV In/Out (MV/Ao) as default.

Default method
→

FHR above 160 bpm or E-A fusion?

Switch to SVC/Ao. Reduce wall filter first.

High FHR rescue
→

Both limited by fetal position?

Try PA/PV. Apply method-specific reference ranges.

Position fallback
!

Non-negotiable at every study

Document method, FHR, and GA. Never compare values from different methods across serial studies without accounting for systematic differences.

Always

When Is mPR Measurement Indicated?

Routine mPR measurement in all fetal echos is not recommended by any current guideline. Its role is specific.

Indication Status
Anti-SSA/Ro positive pregnancy
Weekly from 16–26 weeks		 Standard
Fetal arrhythmia characterisation
AV interval essential for rhythm diagnosis		 Standard
Complex arrhythmia — AV relationship
SVC/Ao or TDI preferred		 Standard
Fetal growth restriction
Studied — not clinically adopted		 Not adopted
Channelopathy family history
No guideline recommendation		 Niche
Routine all-fetal screening Not indicated
💡
The specificity of the indication is what makes acquisition precision matter. In anti-Ro surveillance, a 10–15 ms error at the probe is a potential treatment decision.
// One-Page Summary
BEFORE YOU PROBE

Sweep speed 150 mm/s · Wall filter to minimum · Confirm fetal position · Note GA and FHR

MEASURE FROM

Foot of A-wave onset to leading edge of aortic opening click. Average 3 beats minimum.

SWITCH METHOD IF

FHR above 160 bpm · E-A fusion · Lateral heart · Ambiguous A-wave onset

ALWAYS REPORT

Method used · FHR · GA · Reference range applied · Beats averaged

References

  1. Anuwutnavin S et al. Prenat Diagn 2018;38:459–466
  2. Bergman G et al. Ultrasound Obstet Gynecol 2006;28:57–62
  3. Carvalho JS et al. Heart 2007;93:1448–1453
  4. Fouron JC et al. Heart 2003;89:1211–1216
  5. Kato Y et al. Fetal Diagn Ther 2012;32:109–115
  6. Nii M et al. Heart 2006;92:1831–1837
  7. Phoon CKL et al. Congenit Heart Dis 2012;7:349–360
  8. Sonesson SE. Acta Obstet Gynecol Scand 2016;95:697–704
  9. Swiercz G et al. J Clin Med 2025;14:7522
  10. Wojakowski A et al. Ultrasound Obstet Gynecol 2009;34:538–542
  11. ASE Fetal Echocardiography Guidelines. J Am Soc Echocardiogr 2023