Echo Reference — Diastolic Function & Haemodynamics
Right Atrial Pressure Estimation
IVC diameter and collapsibility algorithm for right atrial pressure estimation. Includes the standard three-tier classification, secondary indices for indeterminate cases, and special considerations for positive pressure ventilation, athletes, and hypovolaemia.
IVC-Based RAP Estimation
The IVC diameter and its respirophasic variation (collapsibility with sniff or quiet inspiration) are the primary echocardiographic method for estimating right atrial pressure. The IVC is imaged from the subcostal view in its long axis, and diameter is measured approximately 1–2 cm from the cavoatrial junction.
RAP Classification
| IVC Diameter | Respiratory Variation | Estimated RAP |
|---|---|---|
| ≤ 2.1 cm | Collapses > 50% with sniff | 3 mmHg (0–5) |
| > 2.1 cm | Collapses < 50% with sniff | 15 mmHg (10–20) |
| > 2.5 cm | Collapses < 50% with sniff + spontaneous echo contrast or colour flow reflux into IVC | > 20 mmHg (Very High) |
| Indeterminate — see criteria below | 8 mmHg (5–10) | |
Indeterminate Scenarios (RAP 8 mmHg)
When the IVC diameter and collapsibility do not fit neatly into the low or high RAP categories, an intermediate value of 8 mmHg is assigned. The most common indeterminate scenarios are:
| IVC Diameter | Respiratory Variation | Result |
|---|---|---|
| ≤ 2.1 cm | Collapses < 50% with sniff | Small IVC, poor collapse → 8 mmHg |
| > 2.1 cm | Collapses > 50% with sniff | Dilated IVC, good collapse → 8 mmHg |
Measurement technique: Measure the IVC in its long axis from the subcostal view, approximately 1–2 cm from the cavoatrial junction, perpendicular to the vessel wall. Use M-mode or 2D cine loop. For the sniff manoeuvre, ask the patient to take a brief, sharp nasal inspiration (not a deep breath). Measure at end-expiration for the maximum diameter.
Secondary Indices for RAP Estimation
When IVC assessment is indeterminate or technically limited, the following secondary indices can support RAP estimation. These should be used to upgrade or downgrade the IVC-based estimate — not as standalone measures.
| Secondary Sign | Suggests Low RAP (3 mmHg) | Suggests Elevated RAP (≥ 15 mmHg) |
|---|---|---|
| Hepatic vein Doppler | Systolic (S) dominance | Diastolic (D) dominance or systolic reversal |
| Tricuspid E/e' ratio | < 6 | > 6 |
| RA size | Normal | Dilated |
| Interatrial septum | Bows toward RA (normal) | Bows toward LA or fixed midline position |
| RV dilatation / dysfunction | Absent | Present — especially with TR |
Integration approach: The ASE recommends using secondary indices to refine the IVC-based estimate in indeterminate cases or when clinical suspicion is discordant with the IVC findings. For example, a small IVC with poor collapse (IVC-based RAP = 8 mmHg) in a patient with hepatic vein systolic reversal and dilated RA would be upgraded to 15 mmHg.
Special Considerations
| Scenario | IVC Behaviour | Implication |
|---|---|---|
| Positive pressure ventilation | IVC is typically dilated and non-collapsing | Standard IVC criteria are unreliable. Hepatic vein Doppler and other secondary signs should be used. Do not automatically assign RAP 15 mmHg based on IVC alone. |
| Young athletes | IVC may be physiologically dilated (> 2.1 cm) with preserved collapse | Athletic remodelling — does not indicate elevated RAP. Confirm with normal RA size, normal hepatic vein pattern, and clinical context. |
| Hypovolaemia / dehydration | Small, hyperdynamic IVC with near-complete collapse | RAP is genuinely low. The IVC assessment is reliable in this setting. |
| Severe TR | IVC may be chronically dilated with blunted respiratory variation | IVC dilatation reflects chronic volume loading. Hepatic vein systolic reversal is a more specific indicator of elevated RAP in severe TR. |
| Chronic pulmonary hypertension | IVC often dilated with poor collapse | Typically reflects genuinely elevated RAP. Secondary indices usually concordant. |
| High abdominal pressure (obesity, ascites) | IVC may appear compressed or have exaggerated respiratory variation | May underestimate RAP. Consider secondary indices. |
Clinical Context
Why RAP Matters
RAP is a critical component of non-invasive haemodynamic assessment. It is added to the TR-derived gradient to estimate PASP (PASP = 4v² + RAP), used in assessing right heart filling pressures, and influences the interpretation of hepatic and portal venous flow. Errors in RAP estimation directly propagate to PASP and can shift a patient between PH probability categories.
Accuracy limitations: IVC-based RAP estimation has moderate accuracy compared to invasive measurement (concordance approximately 60–75% in published studies). It works best at the extremes (clearly low or clearly high RAP) and is least reliable in the intermediate range. When RAP significantly impacts a clinical decision (e.g. PH probability assessment, ICD candidacy based on PASP), invasive confirmation should be considered.
Measurement Pitfalls
| Pitfall | Impact |
|---|---|
| Measuring too far from cavoatrial junction | IVC narrows distally — measuring below the hepatic veins overestimates collapse and underestimates diameter |
| Oblique imaging plane (not true long axis) | Foreshortens the IVC diameter and may create a false impression of increased collapse |
| Deep inspiration instead of sniff | A sustained deep breath can cause exaggerated collapse even with elevated RAP. The sniff should be brief and sharp. |
| Not distinguishing IVC from aorta | In obese patients or suboptimal views, the abdominal aorta can be mistaken for the IVC. Confirm by tracing the vessel to the RA. |
| Ignoring clinical context | A single IVC measurement without secondary indices or clinical correlation is insufficient. Always integrate IVC findings with hepatic vein Doppler, RA size, and the overall haemodynamic picture. |
Hepatic Vein Doppler Patterns
Hepatic vein Doppler is the most useful secondary index for RAP estimation. The normal hepatic vein waveform has two forward-flow phases (systolic S wave and diastolic D wave) and two brief reversals (atrial contraction A wave and venous V wave). The S/D ratio and the presence of systolic reversal are key to RAP assessment.
| Pattern | Waveform | RAP Interpretation |
|---|---|---|
| Normal | S > D (systolic dominance), small A reversal | Normal RAP. Dominant systolic forward flow indicates unimpeded RA filling during ventricular systole. |
| Blunted / S < D | S ≈ D or S < D (diastolic dominance) | Mildly to moderately elevated RAP. Reduced systolic forward flow suggests impaired RA compliance or early volume overload. |
| Systolic reversal | S wave reversed (flow away from heart during systole) | Significantly elevated RAP. Highly suggestive of severe TR but not exclusive — can also be seen with RV dysfunction or restrictive physiology. |
Atrial fibrillation: In AF, the A wave is absent and the S/D ratio becomes less reliable. The S wave is often blunted even at normal RAP. In AF, focus on whether there is frank systolic reversal (strongly suggests elevated RAP and/or severe TR) rather than the S/D ratio alone. The hepatic vein pattern in AF should always be interpreted alongside other secondary indices.
Systolic reversal specificity: Hepatic vein systolic reversal is highly suggestive of severe TR but is not entirely specific. It can also occur with severe RV dysfunction, restrictive cardiomyopathy, and constrictive pericarditis. Interpret in the context of the TR severity assessment and overall clinical picture.
Impact of RAP Error on PASP Estimation
PASP is calculated as 4v² + RAP, where v is the peak TR velocity. Because RAP is added directly, any error in RAP estimation shifts the PASP by the same amount. This can be clinically significant — particularly around PH probability thresholds.
| TR Velocity | 4v² | RAP 3 mmHg → PASP | RAP 8 mmHg → PASP | RAP 15 mmHg → PASP |
|---|---|---|---|---|
| 2.8 m/s | 31 mmHg | 34 mmHg | 39 mmHg | 46 mmHg |
| 3.0 m/s | 36 mmHg | 39 mmHg | 44 mmHg | 51 mmHg |
| 3.4 m/s | 46 mmHg | 49 mmHg | 54 mmHg | 61 mmHg |
Worked example: For a TR velocity of 3.0 m/s, the gradient is 36 mmHg. With RAP 3 → PASP 39 mmHg. With RAP 8 → PASP 44 mmHg. With RAP 15 → PASP 51 mmHg. That is a 12 mmHg range from the same TR jet — enough to shift a patient from "low" to "intermediate" PH probability on the ESC/ERS algorithm. RAP estimation accuracy directly impacts clinical decision-making.
Compounding errors: TR velocity measurement is itself subject to error (incomplete envelope, non-parallel alignment). Because velocity is squared in the modified Bernoulli equation, even a small TR velocity error (e.g. 2.8 vs 3.0 m/s = 5 mmHg gradient difference) compounds with RAP uncertainty, making the total PASP estimate potentially unreliable by 15–20 mmHg. When PASP drives a clinical decision, consider invasive confirmation.
When IVC Cannot Be Visualised
In some patients (obesity, poor subcostal window, recent abdominal surgery, wound dressings), the IVC cannot be adequately imaged. This should be documented explicitly rather than omitting RAP from the report.
Approach when IVC is not assessable: Rely on secondary indices (hepatic vein Doppler, tricuspid E/e', RA size, interatrial septum position, RV dilatation) to estimate RAP. If secondary indices are also unavailable or discordant, this should be stated explicitly in the report. In some cases, an intermediate value (e.g. 8 mmHg) may be assumed for PASP calculation, but this should be interpreted with caution and flagged as an assumption — it is not a guideline-endorsed default.
Reporting guidance: When IVC is not assessable, the report should state: "IVC not visualised — RAP estimated from secondary indices" or "IVC not assessable — RAP assumed at 8 mmHg for PASP calculation (interpret with caution)." Transparency about RAP certainty is essential for the interpreting physician.
References
- Mukherjee M, et al. Guidelines for the Echocardiographic Assessment of the Right Heart in Adults and Special Considerations in Pulmonary Hypertension: Recommendations from the American Society of Echocardiography. J Am Soc Echocardiogr. 2025;38(3):141–186.
- Rudski LG, et al. Guidelines for the Echocardiographic Assessment of the Right Heart in Adults: A Report from the American Society of Echocardiography. J Am Soc Echocardiogr. 2010;23(7):685–713.
- Lang RM, et al. Recommendations for Cardiac Chamber Quantification by Echocardiography in Adults. J Am Soc Echocardiogr. 2015;28:1–39.