π Guideline basis
Lang RM, et al. ASE/EACVI chamber quantification (JASE 2015;28:1β39); Silvestry FE, et al. ASE guidelines for the echocardiographic assessment of atrial septal defect and patent foramen ovale (JASE 2015;28:910β58); Nagueh SF, et al. ASE 2025 diastolic function update (JASE 2025;38:537β69); Hahn RT, et al. comprehensive TEE examination (JASE 2013).
The left atrium
Measurement
Left atrial volume is measured at end-systole (the frame before mitral valve opening), from dedicated LA-focused apical four- and two-chamber views. Foreshortening is the dominant error: maximise the LA long axis and the mitral annular width, tilt anteriorly to show the pulmonary veins entering the atrium, and ensure the two long-axis lengths agree within 5 mm. Exclude the appendage and the pulmonary veins from the trace.
Method | Formula | Note |
Biplane method of discs | Summation of stacked discs | The reference method |
Areaβlength | LAV = (8/3Ο) Γ (A_4C Γ A_2C) / L, L = the shorter long axis | Yields systematically different values from disc summation |
LAVi = LAV / BSA abnormal > 34 mL/mΒ²The ASE 2025 diastolic guideline reports different normal ranges by method (disc summation versus areaβlength) and by age, and states that LAVi is placed in the second stage of the diastolic algorithm precisely because its correlation with left atrial pressure is weak and it fails to track acute changes in pressure (Chapter 8).
The anteroposterior LA dimension from the parasternal long axis remains widely reported and should not be used to define LA enlargement β it measures a single axis of an asymmetrically remodelling chamber.
What LA enlargement means, and what it does not
LA volume reflects the cumulative, chronic effect of elevated filling pressure β the "HbA1c of diastolic function". It therefore cannot rise or fall over the hours across which ICU physiology changes, and a normal LAVi in a patient with acute severe mitral regurgitation is expected rather than reassuring (Chapter 20).
Causes of LA enlargement that are not elevated filling pressure: atrial fibrillation or flutter, significant mitral valve disease, athletic remodelling, anaemia and other high-output states, and heart transplantation with the biatrial technique. The ASE 2025 algorithm requires these to be excluded before LAVi is used as a marker of elevated LA pressure.
Left atrial strain
LA reservoir strain (LARS) is the deformation during ventricular systole as the atrium fills. LARS β€ 18% indicates elevated LA pressure in the 2025 ASE algorithm β specific but insensitive in preserved-EF patients, and explicitly not to be used in atrial fibrillation, significant mitral regurgitation, transplant recipients, patients with normal EF and GLS > 18%, or suspected atrial stunning. LA stiffness is estimated as E/eβ² divided by LARS.
Left atrial appendage
Not assessable transthoracically. On TEE, the appendage is imaged in multiple planes (typically 0Β°, 45Β°, 90Β°, 135Β°) because thrombus can hide in a single lobe.
Finding | Significance |
Thrombus | Discrete, echodense, with a defined border, distinct from the pectinate muscles; contraindicates cardioversion |
Spontaneous echo contrast ("smoke") | Stasis; a thromboembolic risk marker, graded qualitatively |
Emptying velocity < 20 cm/s | Associated with thrombus and with thromboembolic risk |
Coumadin ridge | The muscular ridge between the appendage and the left upper pulmonary vein, with a bulbous "Q-tip" tip; a classic thrombus mimic (Chapter 3) |
Pectinate muscles | Normal trabeculations; continuous with the wall |
TEE exclusion of appendage thrombus is required before cardioversion of atrial fibrillation of more than 48 hours' or unknown duration, unless the patient has been adequately anticoagulated or is haemodynamically unstable (Chapter 40).
The right atrium and the interatrial septum
RA volume index is graded in the ASE 2025 right heart guideline: normal < 33 mL/mΒ², mild 33β38, moderate 39β44, severe > 44 mL/mΒ² (Chapter 9).
The interatrial septum as a pressure transducer
This is one of the most useful and least used observations in critical care echocardiography. The thin fossa ovalis membrane bows toward the chamber of lower pressure, giving a direct, continuously available comparison of left and right atrial pressure β obtainable from the subcostal four-chamber view on TTE or the mid-oesophageal bicaval view on TEE.
Septal position | Interpretation |
Neutral | LAP β RAP |
Bows toward the right atrium | LAP exceeds RAP (by roughly 5 mmHg) |
Bows toward the left atrium | RAP exceeds LAP (by roughly 5 mmHg) β RV failure, tamponade, pulmonary hypertension |
Transient mid-systolic left-ward bowing only | A normal finding; sustained bowing is what matters |
The ASE 2025 diastolic guideline lists interatrial septal position among the LVAD-specific indicators of filling pressure (Chapter 35), and it retains value in every ICU patient in whom the standard indices are excluded β which, given the algorithm's exclusion list, is a large fraction of them.
Patent foramen ovale and atrial septal defect
Anatomy
A patent foramen ovale is a flap valve, not a hole: the septum primum overlaps the septum secundum and separates only when right atrial pressure exceeds left. It is present in roughly a quarter of the adult population and is usually irrelevant. An atrial septal defect is a true tissue deficiency β secundum (most common), primum, sinus venosus, or coronary sinus type.
Why the PFO matters in the ICU
Scenario | Mechanism |
Refractory hypoxaemia disproportionate to lung disease | Right-to-left shunting when RA pressure exceeds LA pressure. High PEEP, acute cor pulmonale, pulmonary embolism, and RV infarction all create the gradient |
Platypnoeaβorthodeoxia | Positional hypoxaemia worse upright, from positional realignment of IVC flow toward the septum |
Paradoxical embolism | Systemic embolism from a venous source; also a route for air embolism from central lines |
Failure to improve with increasing PEEP | Raising PEEP raises RA pressure and can worsen the shunt β a diagnostic clue |
A patient with hypoxaemia that worsens as PEEP is increased should prompt a bubble study.
The agitated saline bubble study
Step | Detail |
Preparation | 9 mL saline + 0.5β1 mL air, Β± a small amount of the patient's blood to stabilise the bubbles; agitate vigorously between two syringes via a three-way tap |
Injection site | Right antecubital or a right-sided central line β left-sided injection can produce a false negative in a persistent left superior vena cava, and IVC-directed flow is preferentially aimed at the fossa in some patients |
Imaging | Apical four-chamber (TTE) or mid-oesophageal bicaval (TEE); record continuously from injection |
Provocation | Essential. Valsalva with release timed to the arrival of contrast in the RA; cough. In a ventilated patient, positive pressure abolishes the transient right-to-left gradient β use a brief airway pressure release, or a sustained inflation followed by rapid release, to reproduce the physiology |
Interpretation | Bubbles appearing in the left atrium within 3β6 cardiac cycles of full right atrial opacification indicate an intracardiac shunt. Appearance after 5β6 cycles suggests an intrapulmonary shunt (arteriovenous malformation, hepatopulmonary syndrome) |
Quantification | Semi-quantitative bubble counts in the LA are used to grade shunt size |
A negative study without adequate provocation does not exclude a PFO, and this is the commonest technical failure in the ventilated patient.
Shunt quantification
Qp/Qs = (CSA_RVOT Γ VTI_RVOT) / (CSA_LVOT Γ VTI_LVOT)Qp/Qs | Interpretation |
β 1.0 | No net shunt |
> 1.5 | Haemodynamically significant left-to-right shunt |
< 1.0 | Net right-to-left shunt |
The calculation requires accurate RVOT diameter β harder to measure than the LVOT and subject to the same squared-error problem (Chapter 11). It is invalid in the presence of significant pulmonic or aortic regurgitation.
Post-infarction ventricular septal defect
Colour Doppler demonstrates a high-velocity systolic jet crossing the septum, left-to-right, with RV volume overload and a Qp/Qs typically well above 1.5. Location follows the infarct: apical after LAD occlusion, basal-inferior after RCA occlusion. In a shocked post-infarct patient, putting colour across the septum takes seconds and changes the entire management pathway (Chapter 30).
Note that the jet velocity reflects the LVβRV pressure gradient: as the patient deteriorates and systemic pressure falls, the velocity falls and the jet becomes less conspicuous β the opposite of intuition.
Iatrogenic shunts
Origin | Notes |
Post-TEER atrial septal defect | Created by the transseptal access; usually small. The ASE 2025 diastolic guideline describes using CW Doppler across it to estimate LAP: LAP = 4VΒ² + RAP, with an interatrial left-to-right velocity > 1.7 m/s usually indicating elevated LAP. Not reliable in biventricular failure |
Post-ablation, post-appendage-occlusion, other transseptal procedures | Same principle |
Post-TAVI ventricular septal defect | Rare complication of annular injury |
ICU-Specific Limitations
Confounder | Effect | Response |
Positive-pressure ventilation | Abolishes the transient right-to-left gradient; false-negative bubble studies | Use a release manoeuvre |
Rising PEEP | May create or worsen a right-to-left shunt | A diagnostic clue rather than a confounder |
Atrial fibrillation | LA enlargement is expected and does not indicate elevated pressure; LARS invalid | Do not use LAVi or LARS as filling-pressure markers |
Poor apical windows | LA volumes unmeasurable; appendage never assessable | TEE |
Foreshortening | Underestimates LA volume | Dedicated LA-focused views |
Falling systemic pressure with a VSD | Reduces jet velocity and conspicuity | Look with colour at a low Nyquist; do not rely on jet appearance |
π Critical pitfall: Reporting "no PFO" from a bubble study performed without provocation in a ventilated patient. Positive pressure prevents the shunt from opening, and the study is uninterpretable without a release manoeuvre.
π Critical pitfall: Using a normal left atrial volume to exclude significant mitral regurgitation. In acute severe MR the atrium has not had time to remodel (Chapter 20).
π Critical pitfall: Mistaking the coumadin ridge for appendage thrombus, or pectinate muscles for a mass. Both are normal anatomy with stereotyped appearances.
- π‘ Clinical pearl: In a hypoxaemic patient who gets worse as PEEP is increased, do a bubble study. A right-to-left shunt through a PFO is a recognised, treatable and repeatedly missed cause.
- π‘ Clinical pearl: The interatrial septum gives a free, continuous comparison of left and right atrial pressure, and it works in patients where every guideline-endorsed filling-pressure index is excluded.
- π‘ Clinical pearl: In the deteriorating post-infarct patient, sweep colour across the entire interventricular septum from base to apex in both the parasternal and apical views. A basal-inferior VSD after an inferior infarct is easy to miss from a single apical view.
References
- Lang RM, Badano LP, Mor-Avi V, et al. Recommendations for cardiac chamber quantification by echocardiography in adults. J Am Soc Echocardiogr 2015;28:1β39.
- Silvestry FE, Cohen MS, Armsby LB, et al. Guidelines for the echocardiographic assessment of atrial septal defect and patent foramen ovale. J Am Soc Echocardiogr 2015;28:910β58.
- Nagueh SF, Sanborn DY, Oh JK, et al. ASE 2025 diastolic function update. J Am Soc Echocardiogr 2025;38:537β69.
- American Society of Echocardiography. Guidelines for the echocardiographic assessment of the right heart in adults. J Am Soc Echocardiogr 2025.
- Hahn RT, Abraham T, Adams MS, et al. Guidelines for performing a comprehensive transesophageal echocardiographic examination. J Am Soc Echocardiogr 2013;26:921β64.