Transoesophageal Echocardiography

GENERAL

• allow real time anatomical and physiological assessment of cardiac status
• probes (single, bi, omiplane and epivascular)
• very expensive equipment
• very expensive but reusable

INFORMATION OBTAINED

• anatomy (heart, valves, great veins, aorta)
• preload (volume status)
• contractility (left and right systolic and diastolic function)
• regional wall motion abnormalities
• abnormal masses (i.e. vegetations)
• pericardial/pleural/aortic collections
• pressure gradients (across valves and chambers) -> remember NOT pressures as you can’t measure pressures with sound waves!
• using Doppler -> Q

PHYSICS

• sound waves via piezoelectric crystals -> electrical current -> produces and senses vibrations
• remember that sound waves can get bounced and also that if you seen something -> you need to ensure its not artefact by looking in two views
• crystals produce 2-3 wavelengths (short burst) and then listens for a comparatively long time
• remember: time is proportional to distance
• speed of sound in soft tissue = 1540m/s
• 4-7 MHz
• frequency can be adjusted based on what you are looking for

v = f x lambda

v = velocity
f = frequency
lambda = wavelength

• resolution is directly proportional to frequency (but you loose tissue penetration = depth)
• attenuation of U/S signal is proportional to distance travelled and medium travelling through (air = worse, H2O = best )

MODES

• M, 2D, Doppler (spectral, colour flow)

2D-mode

• multiple beams across a single plane
• resolution can be altered by changing the number of crystals being recruited and narrowing the area being studied
• goals in examining in cardiac surgery = assessment of ventricular function, volume and area that is being repaired
• there are over 20 views — 5 that should be mastered by a novice
  – 0º = U/S ray coming out horizontal from probe
  – 90º = U/S ray coming out vertical from probe
  – 180º – U/S ray coming out horizontal from probe (opposite from 0º)
  – long axis = view structure side on
  – short axis = view structure end on
• movements of the probe:

1. advance/withdraw
2. turn left/right
3. ante/retrovert
4. rotate (0-180º)
5. flex left/right

M-mode

• M = motion
• an amalgamation of A and B modes which represent lines and dots depending on the echogenic density of the medium through which the U/S wave is traveling.
• M mode displays this in real time
• it provides the highest resolution across a specific line
• used with colour flow Doppler to define timing of an abnormal flow within the cardiac cycle
• allow precise measurement of changes in cavity size, wall thickness and wall motion
• also the best at looking @ anatomical structures (vegetations, thrombus and valve dysfunction)

See Figure 3 in Article

LEARNING VIEWS

Mid Oesophagus (30cm)

• Four Chamber view (0º) – ventricular function, valvular function and atrial abnormalities, Doppler interrogation of MV and TV, pericardium
• AV short axis (30º) – “Mercedes Benz” sign of AV, interatrial septum, coronary ostia, LVOT, PV
• Two Chamber view (90º) – left atrial appendage, inferior and anterior LV walls, MV and transmitral flow and coronary sinus
• Long Axis (130º) – LA, LV, AV, MV and proximal aorta

Transgastric (40cm)

• Short Axis Mid Papillary View (0º) – LV function, RWA, RV function, tamponade (can look at this view and get a lot of information
• kissing sign = decreased preload, collapsing ventricle = decreased after load, RWA = ischaemia, pericardial fluid = tamponade)

EXPERT VIEWS

Upper Oesophagus (20cm)

– Aortic Arch Long Axis (0º) – PA bifurcation, aortic root
– Aortic Arch Short Axis (90º) – aortic arch, PA, PV, left brachiocephalic vein

Mid Oesophagus (30cm)

– 5 chamber view (0º) – LA, LV, RA, RV and LVOT
– Right Ventricular Inflow Tract & Coronary Sinus (60-90º)

– Right Ventricular Inflow and Outflow Tracts (from 90º, rotate to right)
– Bicaval (80-110º) – RA, right atrial appendage, atrial septum, venae cavae and LA (PFO and ASD)

Deep Transgastric (45cm)

– long axis (0º and anteflex) – LOVT, AV, ascending aortic arch, aortic outflow velocities

And there are many others!

IMPROVING IMAGE QUALITY

• increase frame rate = frequency of refreshment of screen
• limit frame = increased definition
• decrease depth = increases quality
• increase gain = power output or strength of the signal (increases will increase artefact however)
• adjustment of dynamic range compression (like adjusting contrast level on a TV)
• adjusting frequency filter to eliminate noise
• the cine loop = allows digital acquisition of and storage of cardiac cycles which are then displayed on the screen (can use full screen, split or quad screen to look @ different views of the heart @ the same time)
• zoom = U/S limited to a specific depth
• B mode colour = changes the grey scale of 2D imaging to various hues of blue, purple, orange or yellow

DOPPLER

• The Doppler shift = frequency shift that take place when RBC is moving towards and then away from transducer
• towards = sound waves compressed -> frequency increases
• away = sounds waves expand -> frequency decreases

Doppler Shift (Fd) = observed frequency – original frequency

V = Fd x speed of sound / 2 x generated frequency x Cos ɵ

V = velocity of RBC
Fd = Doppler shift
Speed of sound = 1540m/s
2 = because Doppler shift occurs twice
Generated frequency = between 3.7-7 MHz
ɵ = the angle between the interrogating U/S beam and direction of blood flow (should ideally be <20º -> blood moving directly towards or away from probe)

Spectral Doppler

• blood flow velocity can be measured using a pulsed wave or continuous wave.
• a spectral tracing derived by Fourrier analysis (time vs spectral tracing)
• the amplitude of the spectral tracing (y-axis) is directly proportional to the measured RBC velocity
• BART = blue away, red towards
• patterns of blood flow can also be determined (laminar to turbulent)
• laminar = well defined flow with pale centre
• see diagram in notes
• Bernoulli equation: change in P = 4V2

PULSED WAVE

• sends a signal and waits for its return before sending another one
• good for looking at an area of interest that is at a specific depth
• cannot measure high velocity turbulent flow (c/o high frequency shifts)
• colour flow mapping = a form of pulsed wave Doppler -> good for illustrating regurgitation and abnormal flow (ASD, PFO, VSD)

CONTINUOUS WAVE

• two different transmitters transmitting and listening
• can measure high velocity flows
• disadvantage is that the transducers cannot differentiate which signals come from which depth -> final analysis = an average of all U/S signals generated by moving RBC’s

CLINICAL APPLICATION

Measurement of:

• pressure gradients
• flow velocity
• valve areas
• pressure half time
• regurgitant factions
• Q
• systolic function
• diastolic function

Valves

• stenotic = calculate gradients
• regurgitant = use colour flow mapping

LV Function

• end systolic distance < 4.5cm
• end diastolic distance < 7cm
• wall thickness < 12mm – transgastric short access to get impression – fractional shortening (diastolic – systolic diameter of LV)
• fractional area change (diastolic – systolic circumference change of LV) – more accurate
• 2 and 4 chamber area change in systole and diastole = EF

Diastolic Function

• relaxation requires energy (ATP)
• phases of relaxation = isovolumetric relaxation, early filling, diastasis (when LA passively fills LV and then stops), atrial contraction
• transmitral flow velocity
  -> normal = E > A
  -> abnormal = A > E
  -> pseudonormal = E > A (LA dilated to compensated for lack of LV relaxation)
  -> restrictive = E >>> A

Regional wall motion abnormalities

• heart divided into 16 parts than can be seen on TOE (+1 = apex which is not seen)
• all can be commented on
• normal
• mild RWMA
• severe RWMA
• akinesis
• dyskinesis (move in opposite direction to where it is meant to be)

ADVANTAGES OVER TTE

Decreased interference from bone or air (exception = left main bronchus -> distal ascending aorta)

• Valves – increased resolution (endocarditis, post surgery)
• Atrial structures – appendage, septum, pulmonary veins
• Thoracic aorta
• TTE technically difficult – post cardiac surgery and in ventilated with high PEEP

COMPLICATIONS

• gastrointestinal bleeding
• oesophageal rupture

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