Left ventricular outflow tract obstruction in echocardiography (differential)

Aortic valve

stenosis • Aortic stenosis (transthoracic echocardiography) - Ganzer Fall bei Radiopaedia

Carroll, D. Aortic stenosis (transthoracic echocardiography). Case study, Radiopaedia.org. (accessed on 20 Oct 2022) https://doi.org/10.53347/rID-63358 • CC by-nc-sa 3.0 (modified)

Left

ventricular outflow tract obstruction in echocardiography (differential) • Takotsubo cardiomyopathy (echocardiography) - Ganzer Fall bei Radiopaedia

Kwong Y, Takotsubo cardiomyopathy (echocardiography). Case study, Radiopaedia.org (Accessed on 03 Feb 2023) https://doi.org/10.53347/rID-29997 • CC by-nc-sa 3.0 (modified)

Radiopaedia • CC-by-nc-sa 3.0 • de

Left ventricular outflow tract (LVOT) obstruction describes a state in which the egress of blood from the left ventricle to the systemic circulation is impeded as it traverses the anatomic LVOT to the aortic arch. Echocardiography, particularly with the use of spectral Doppler, may be used to determine the etiology, associated anomalies, and assess the effect of the necessarily increased afterload on the left ventricle .

The differential diagnosis may be narrowed based upon patient demographics and is commonly divided based upon the anatomical structure involved, as well as the response of the associated gradient to the local hemodynamic milieu.

Pathology

Structural and functional asymmetry of the LVOT forms the structural substrate for LVOT obstruction. The anterior leaflet of the mitral valve and the interventricular septum border the anatomic left ventricular outflow tract through which a stroke volume is ejected with predominantly laminar flow. If the outflow tract is narrowed relative to the flow which it receives, the velocity of the fluid must increase, and the pressure must decrease. The latter may result in suction of the anterior leaflet of the mitral valve into the outflow tract, impeding flow, and the former may result in turbulent flow, both decreasing forward flow and increasing the amount of requisite energy to maintain flow .

Accordingly, the structure of the left ventricular outflow tract should not be considered in isolation, with incorporation of hemodynamic context essential in the definition of both the cause and significance of LVOT obstruction. Examples of variables which influence hemodynamics, and may therefore influence the presence or degree of obstruction include:

fluid administration
- increased preload
vasopressor administration
- increased afterload
Valsalva maneuver
- decreased preload during strain phase
inotrope administration
- increased contractility
amyl nitrate administration
- decreased afterload

Dynamic LVOT obstructions will be provoked and/or exacerbated by decreased preload, decreased afterload and increased contractility, and will be ameliorated by an increased preload.

Radiographic features

Echocardiography

Anatomical localization of the obstruction, in the absence of suggestive B-mode features, may be suggested by aliased flow when using color-flow Doppler, and more precisely located by defining flow velocity by location with pulsed wave doppler.

The left ventricular outflow tract should be viewed as part of a comprehensive transthoracic echocardiography protocol, with subsequent transesophageal echocardiography if the former is inadequate. Relevant transthoracic windows obtained and pertinent features assessed include;

parasternal window
- long axis (PLAX) view
  - important for the recognition of asymmetric septal hypertrophy (ASH) and systolic anterior motion (SAM) of the anterior leaflet of the mitral valve
  - measurement of LVOT diameter
    - normal range is between 1.6 and 2.4 cm
  - utilization of M-mode provides superior temporal resolution to B-mode, and may be used to clarify the relationship between the excursion of the mitral and aortic valve leaflets and the cardiac cycle
- short-axis view
  - visualization of all three aortic valve leaflets to exclude fusion or presence of a raphe
apical window
- five-chamber view
  - allows for near parallel doppler interrogation of the LVOT
  - continuous-wave Doppler will allow for measurement of peak velocity and mean velocity
    - peak velocity is calculated using the Bernoulli equation
    - mean velocity may be obtained using the velocity time integral (averaged point velocities during the ejection time), which requires tracing the spectral envelope
  - pulsed wave doppler allows localization of velocities
    - the sample volume should be placed just proximal to the aortic valve, ideally in the location where the LVOT was measured
      - the "closure click" of the valve should be visualized with minimum spectral broadening and no "opening click"

A differential diagnosis may then be formed by noting the location of the obstruction and structures affected, as well as the physiological context in which the obstruction occurs;

subvalvular obstruction
- fixed obstruction
  - subaortic stenosis
    - transesophageal echocardiography is superior for delineation of a subaortic membrane
    - commonly classified as muscular, fibrous, or mixed
    - best demonstrated in the apical three or five-chamber view, as either a circumferential echogenic ring or discrete protuberance
    - continuous-wave Doppler envelope will demonstrate an early or mid-systolic peak with does not change with dynamic maneuvers, in keeping with fixed obstructive physiology
- dynamic obstruction
  - hypertrophic obstructive cardiomyopathy
    - obstruction is in the anatomic left ventricular outflow tract
    - basal septal hypertrophy may narrow the tract structurally
    - the anterior leaflet of the mitral valve may demonstrate systolic anterior motion (SAM)
      - degree of obstruction increases throughout systole, thus the doppler envelope is late-peaking
        broad base of the envelope allows differentiation from intracavitary gradients, which are narrow and late-peaking
      - eccentric, posterior mitral regurgitation jet common
    - hyperdynamic states
      - acute myocardial infarction
        associated with asymmetric septal hypertrophy and low intraventricular volumes
        
        hyperdynamic basal anteroseptal and inferolateral myocardial segments
        
        associated regional wall motion abnormalities in a typical vascular distribution
      - septic shock
      - catecholaminergic excess
      - hypovolemia
    - takotsubo cardiomyopathy
      - apical dyskinesis with hyperdynamic basal segments
valvular obstruction
- aortic valve stenosis
  - degenerative/calcific
    - leaflets diffusely calcified with aortic root involvement
  - bicuspid aortic valve
    - eccentric coaptation of valve leaflets
    - ellipsoidal aortic valve orifice
    - diastolic leaflet doming
    - associated aortic regurgitation jet directed posteriorly
  - rheumatic
    - triangular aortic valve orifice
    - leaflet thickening, preferentially affecting cusp edges
    - restricted leaflet excursion
    - mitral valve commonly affected
supravalvular obstruction
- supravalvular aortic stenosis
  - segmental
    - typically appears as an echogenic ring at the sinotubular junction imparting an "hourglass" appearance to the descending aorta
  - diffuse
    - narrowing of the ascending aorta from the sinotubular junction

Siehe auch:

Echokardiographie