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TTE vs TEE vs POCUS in Anesthesia

MD Anaesthesia Exam - Detailed Long Answer

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I. INTRODUCTION

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II. PRINCIPLES OF ULTRASOUND (Common to All Modalities)

• Frequency: Echocardiography uses 2-12 MHz. Higher frequency = better axial resolution but less penetration. Lower frequency = deeper penetration but poorer resolution.
• Transducer: Phased-array transducers with piezoelectric elements; function as both transmitters and receivers.
• Resolution components:
  • Spatial resolution - ability to distinguish two closely spaced structures (subdivided into axial, lateral, elevational)
  • Temporal resolution - ability to capture rapid motion
  • Contrast resolution - ability to differentiate tissue densities
• Imaging modes: M-mode (motion mode), 2D B-mode imaging, Colour Flow Doppler (CFD), Spectral Doppler (PW and CW), Doppler Tissue Imaging (DTI), and 3D/4D imaging.
• Artifacts arise when the assumptions of image formation are violated (straight-line propagation, constant attenuation, single reflection).

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III. TRANSTHORACIC ECHOCARDIOGRAPHY (TTE)

A. Definition and Probe Placement

B. Standard Windows and Views

C. Advantages of TTE

• Non-invasive and risk-free
• No special patient positioning or anesthesia required
• Can be performed preoperatively and postoperatively without restrictions
• Preferred modality outside the operating room and in the ICU
• Useful for preoperative cardiac function and volume assessment
• Permits IVC assessment for fluid responsiveness

D. Disadvantages and Limitations of TTE

• Image quality limited by acoustic windows (obese patients, COPD with hyperinflation, dressings, drains, mechanical ventilation, surgical drapes)
• Cannot be used intraoperatively without interfering with the surgical field
• Structures behind the esophagus (e.g., left atrial appendage, descending aorta) are poorly visualised
• Operator-dependent; highly variable image quality
• Limited views in the intubated, supine surgical patient

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IV. TRANSESOPHAGEAL ECHOCARDIOGRAPHY (TEE)

A. Definition and Historical Basis

• 1950s: First M-mode ultrasound of cardiac structures
• 1970s: First practical TEE reported (1976); first intraoperative use soon after
• 1980s: 2D transducers mounted on flexible gastroscopes; intraoperative monitoring established
• Mid-2000s: Real-time 3D (RT-3D) TEE entered clinical use

B. TEE Probe Levels and Image Acquisition

C. Standard Examination Protocols

• ME 4C → ME 2C → ME LAX → ME ascending aorta long-axis → ME ascending aorta short-axis → ME AV SAX → ME RV inflow-outflow → ME bicaval → TG SAX → descending aorta short-axis → descending aorta long-axis

D. Levels of Training and Certification

E. Advantages of TEE

• Superior image quality - no acoustic window issues, posterior cardiac structures (left atrium, pulmonary veins, left atrial appendage, descending aorta) are excellently visualised
• Does not interfere with the surgical field - the main reason it is the intraoperative modality of choice
• Continuous, real-time monitoring during cardiac and non-cardiac surgery
• Can detect regional wall motion abnormalities (RWMA) as earliest sign of myocardial ischemia
• Guides valve repair and replacement surgery (mitral valve repair "surgeon's view" via 3D)
• Assesses de-airing of the left heart post-cardiopulmonary bypass
• Evaluates ventricular filling and guides volume therapy
• Diagnoses aortic pathology (dissection, atherosclerotic plaques before aortic cannulation - important for stroke prevention)
• Guides transcatheter procedures (TAVR, TEER/MitraClip, LAA closure, transseptal puncture)

F. Disadvantages and Limitations of TEE

• Semi-invasive procedure - requires probe passage through the oropharynx into esophagus
• Requires patient cooperation or general anesthesia/deep sedation
• Cannot directly image the anterior mediastinum or ascending aorta (air-filled trachea obstructs)
• Requires specialised training and credentialing
• Costly equipment and maintenance

G. Contraindications to TEE

• Perforated viscus
• Gastroesophageal strictures, masses, or diverticula
• Tracheoesophageal fistula
• Recent upper GI surgery
• Active oral, hypopharyngeal, or gastroesophageal hemorrhage

• Previous radiation therapy to neck or mediastinum
• Previous esophageal or gastric surgery
• History of upper GI hemorrhage
• Esophageal varices
• Cervical spine instability or atlantoaxial joint disease
• Severe thrombocytopenia
• Anticoagulation

H. Complications of TEE

• Esophageal injury (most common): mucosal lacerations, intramural hematomas, thermal injury - seen at endoscopy in >80% post-procedural guideline TEEs but mostly clinically silent
• Odynophagia - most common symptomatic complication; relevant for post-op respiratory complications
• Minor: oral bruising/laceration, sore throat, chipped teeth
• Serious (rare): esophageal or gastric perforation, major hemorrhage - often fatal; occurs in up to 1 in 3,000 in cardiac surgery TEE series
• Overall complication rate: 0.18-2.8%; major complication rate: 0.2-1.4%

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V. POINT-OF-CARE ULTRASOUND (POCUS)

A. Definition

B. Scope of POCUS in Anesthesia

• Cardiac (FoCUS - Focused Cardiac Ultrasound)
• Pulmonary (pneumothorax, pleural effusion, B-lines for pulmonary edema)
• Abdominal (FAST - Focused Assessment with Sonography in Trauma; abdominal free fluid)
• Vascular (DVT, arterial access, CVC placement)
• Ocular (optic nerve sheath diameter for ICP estimation)
• Airway (subglottic diameter estimation, ETT confirmation, cricothyroid membrane identification)
• Neuraxial (spinal/epidural landmark identification)
• Genitourinary, skin and subcutaneous tissues

C. Cardiac POCUS / Focused Cardiac Ultrasound (FoCUS)

• Goal-directed
• Problem-oriented
• Limited in scope
• Simplified technique
• Time-sensitive and repeatable
• Qualitative or semiquantitative
• Performed at the point of care by clinicians
• Answers questions in a yes/no format

D. FoCUS vs Limited TTE vs Comprehensive TTE

E. FATE Protocol (Focus Assessed Transthoracic Echo)

• Position 1: Subcostal 4-chamber (pericardial effusion, chamber dimensions)
• Position 2: Apical 4-chamber (all 4 chambers, ventricular function)
• Position 3: Parasternal long-axis and short-axis (LV function, wall motion)
• Position 4: Pleural scanning bilaterally (effusions, consolidation)

• Pericardial effusion/tamponade
• Dilated RA + RV (right heart failure, PE)
• Dilated LA + LV (left heart failure)
• LV hypertrophy (AS, systemic hypertension, HOCM, myocardial deposits)

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VI. COMPREHENSIVE COMPARISON: TTE vs TEE vs POCUS

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VII. INDICATIONS SUMMARY FOR ANESTHESIA PRACTICE

ASA/SCA Guidelines for Perioperative TEE (2020 Update):

1. TEE should be used in all adult open-heart and thoracic aortic surgical procedures (absent contraindications)
2. Should be considered in coronary artery bypass surgery
3. In noncardiac surgery: indicated for persistent unexplained life-threatening circulatory instability
4. In critical care: when diagnostic information expected to alter management cannot be obtained by other means
5. Consider case-by-case in small children (unique risk profile)

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VIII. POCUS IN SPECIFIC ANESTHESIA SCENARIOS

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IX. QUANTITATIVE ASSESSMENTS COMMON TO ALL MODALITIES

1. Ejection Fraction (EF) - visual estimation (qualitative POCUS/FoCUS) vs. biplane Simpson's method (comprehensive TTE/TEE)
2. Stroke Volume (SV) - VTI × cross-sectional area of LVOT (SV = VTI × CSA); cardiac output = SV × HR
3. Filling pressures - E/A ratio, E/e' ratio, LA size
4. Wall Motion Score Index (WMSI) - for ischemia; 16 or 17 segment model
5. Valvular gradient - CW Doppler (Bernoulli equation: ΔP = 4V²)
6. Regurgitation severity - vena contracta width, PISA/EROA, jet area
7. IVC collapsibility index - > 50% collapse with respiration suggests fluid responsiveness (IVC < 2.1 cm)
8. Strain imaging (STE - Speckle Tracking Echocardiography) - global longitudinal strain (GLS) for subclinical LV dysfunction

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X. EPIAORTIC AND EPICARDIAL ULTRASOUND (Special Forms)

• Epiaortic Ultrasound (EAU): A sterile-sheathed probe placed on the ascending aorta by the surgeon to detect atherosclerotic plaques before aortic cannulation - the ascending aorta is not visualised by TEE (tracheal shadow). Detects plaques and guides cannula/clamp placement to reduce embolic stroke risk.
• Epicardial Echocardiography: Probe placed directly on the epicardium in a sterile sheath. Used when TEE is contraindicated (esophageal pathology) and intraoperative imaging is still required.

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XI. THREE-DIMENSIONAL (3D) ECHOCARDIOGRAPHY

• Narrow sector: Small pyramidal volume, high temporal/spatial resolution
• Wide sector: Larger region of interest
• Full volume (multiple-beat gated): Largest sector, highest quality, gated to R-wave over consecutive heartbeats

• Better volumetric quantification (LV and RV volumes)
• Surgeon's view of the mitral valve - facilitates repair planning
• Guidance of transcatheter procedures (TAVR, MitraClip, transseptal puncture)
• LAA morphology

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XII. KEY POINTS FOR MD EXAM

1. TEE is the intraoperative standard because it provides superior image quality without interfering with the surgical field; used in all open-heart and thoracic aortic surgery.
2. TTE is preferred outside the OR (ICU, pre-op assessment) because it is non-invasive and avoids TEE's esophageal risks.
3. POCUS is goal-directed and problem-specific - answers yes/no questions rapidly at the bedside; it is an extension of physical examination, not a substitute for formal echocardiography.
4. FoCUS ≠ Limited TTE ≠ Comprehensive TTE - these are distinct examinations with different scopes, training requirements, and output.
5. Ascending aorta is the blind spot of TEE (tracheal air shadow) - requires epiaortic ultrasound.
6. LAA, descending aorta, posterior LV structures = TEE is superior.
7. Contraindications to TEE must be memorised (GI pathology is the main category); when TEE is contraindicated intraoperatively, epicardial echo is the fallback.
8. Complication rate of TEE: overall 0.18-2.8%; mortality up to 1:3,000 in cardiac surgery.
9. Stroke volume calculation via LVOT VTI is possible with both TTE and TEE.
10. 3D TEE provides the surgeon's en-face view of the mitral valve - useful for repair guidance.

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• Miller's Anesthesia, 2-Volume Set, 10th Edition, Chapter 33 (Perioperative Echocardiography)
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9th Edition, Chapter 27
• Morgan & Mikhail's Clinical Anesthesiology, 7th Edition, Chapter 5
• ASA/SCA Practice Guidelines for Perioperative TEE (2010, 2020 updates)
• ASE/SCA Guidelines for Comprehensive TEE Examination, J Am Soc Echocardiogr 2013;26(9):921-964
• Via G et al. International evidence-based recommendations for focused cardiac ultrasound, J Am Soc Echocardiogr 2014;27(7):683