General:
Important – forms the majority of requests for imaging.
Need to utilise multiple measures alongside subjective assessment by eye.
Assessment includes:
- LV dimensions
- Shape
- Wall thickness
- Cavity size
- LV mass
- LV systolic function
- Global
- Regional wall motion abnormalities – covered separately.
- LV diastolic function – covered separately.
- LVOT morphology (e.g. HOCM).
- LV masses or thrombus.
LV size:
Linear measures:
Measure IVS, LVID and LVPW in both end diastole and end systole at level of mitral valve leaflet tips.
Best taken in PLAX view.
Must be perpendicular to wall. Can use M-mode if perpendicular.
Index for BSA – can use Mosteller Formula to calculate BSA:
BSE (m^2) = Sq/height(cm)x weight (kg)/3600
Volumetric measures:
Modified Simpson’s biplane works on the principle that the LV cavity can be considered as a stack of elliptical discs of differing sizes from base to apex.
Measured in A4C and A2C in end diastole and end systole.
Should use both views if possible as assumption made that discs are elliptical and therefore by using two planes it becomes more accurate.
Accept if both measures are within ~5%.
Area-length method can be used to estimate LVEDV when endocardial border not clearly visualised.
Inaccurate – makes major assumptions/simplifications about shape of LV.
Two steps:
- PSAX – mid LV (PMs), freeze frame in end diastole and perform planimetry around endocardial border. This is LV cross-sectional area in cm^2.
- A4C – measure length of LV.
Use following formula:
LVEDV = 5 x area x length / 6
Repeat for LVESV.
Can adjust both for BSA.
LV mass:
Mass increases with LVH.
Full LVH assessment includes:
- Description of overall appearance of LV – concentric/asymmetrical hypertrophy.
- LV dimensions
- Calculation of LV mass
- Assessment of LV systolic & diastolic function
- Search for underlying cause – AS/coarctation etc.
- Exclude outflow tract obstruction/HCM.
Linear measures calculate mass in grams by ****including IVSd, LVIDd and LVPWd in the equation:
Volumetric measures include using area-length formula and Simpson’s biplane measures.
Area-length formula requires the following steps:
- PSAX mid-LV planimetry of the endocardial border (as above) to calculate endocardial CSA in cm^2.
- Planimetry of the epicardial border using same method. Epicardial CSA.
- Mean wall thickness (t) using following calculation:
- Measure LV long axis in A4C end diastole.
- Mass in grams is found using the following formula:
Simpson’s biplane measure relies on good endocardial and epicardial definition.
Requires the following steps:
- A4C and A2C measurement of LVEDV.
- Repeat process but use planimetry to trace the epicardial border to measure total LV volume.
- Myocardial volume is LV volume – LVEDV.
- Multiply myocardial volume by myocardial density (1.05g/ml) to calculate LV mass.
- Index for BSA.
Pitfalls:
Linear measures – small inaccuracies become greatly magnified because of cubing of LV dimensions in linear mass equation.
Cant use linear measures if distorted anatomy e.g. isolated areas of hypertrophy.
Care needs to be taken to trace epicardial and endocardial border accurately.
LV Systolic Function:
Heart failure affects 1-2% of adult population, and >10% of >70s.
Can be classified as:
- Systolic and/or diastolic
- Acute or chronic
- Left and or/right sided
- High output (e.g. thyrotoxicosis)/low output.
Many causes.
Ejection fraction is the most common measure for assessing systolic function and should be viewed as a load dependent measure of left ventricular work, combining:
- Myocyte work determined by underlying and current power capacity.
- Fluid balance.
- Cardiovascular drugs.
- Overall clinical state.
- Heart rate and rhythm.
Predominant plans are radial thickening and longitudinal shortening – 30% longitudinal.
Load independent measures exist, e.g. strain rate imaging, but these are not yet reliably performed at the bedside/in time frame relevant to critical care.
Should always use more than single measure.
Fractional shortening:
Percentage change in LV dimensions between diastole and systole.
Normal between 25-43%.
Calculated from the LVIDd and LVIDs:
FS = (LVIDd – LVIDs) / LVIDd x 100
Fractional area change:
Trace endocardial border at papillary level in PSAX in diastole and systole.
(LVAd – LVAs) / LVAd x 100
MAPSE:
Mitral annular plan systolic excursion – assessment of linear LV performance assessing descent of lateral mitral annulus using A4C.
Not useful in isolation.
Normal value 12mm.
Ejection fraction can be preserved in absence of normal longitudinal function.
Ejection fraction:
Measured using LVEDV and LVESV:
EF = (LVEDV – LVESV) / LVEDV x 100
Most commonly performed using the Simpson’s biplane method as documented above.
Should report as a range of values given possible inaccuracy.
Foreshortening overestimates EF.
Can also be calculated using linear rather than volumetric measures using:
EF = (LVIDd^3 – LVIDs^3) / LVIDd ^3 x 100
Highly prone to inaccuracies due to cubing of values. Does not take into account RWMAs.
Can also perform using area-length equation documented above.
Stroke distance:
Average distance travelled by the blood during each heartbeat. AKA VTI.
Measured in cm.
A5C – PW doppler to measure VTI of outflow in LVOT just proximal to annulus. Trace around doppler signal to get VTI.
Normal – 18-22cm
Stroke volume:
Quantity of blood ejected into aorta by the LV with each heart beat.
Multiple steps involved:
- PLAX – measure diameter of LVOT in cm at the level of AV annulus proximal to cusps. Machine will calculate CSA – CSA LVOT = 0.785 x (LVOT diameter)^2
- A5C – measure VTI at LVOT at same point the LVOT was measured. Trace doppler signal.
- SV can be calculated from following equation
SV = CSA LVOT x VTI LVOT
Normal value is 60-100ml/beat.
Relies on good alignment of Doppler angle with direction of flow..
Assumes LVOT flow is laminar and LVOT is circular.
Must measure LVOT diameter as accurately as possible.
Stroke volume index – adjusted for BSA.
Multiplying by heart rate = cardiac output.
Rate of ventricular pressure rise (dP/dt):
With normal function, rate of rise in ventricular pressure during systole is rapid. If function is impaired, dP/dt starts to fall.
Requires presence of MR. Cannot be used in acute MR. Misleading if increased afterload (AS or htn).
A4C – CW doppler of MR.
Set sweep speed high to spread out trace.
Mark points where regurgitant jet velocity reaches 1m/s and 3m/s .
At 1m/s the pressure gradient will be 4mmHg (Bernoulli equation – gradient = 4xV^2). At 3m/s the gradient will be 36mmHg.
Change in gradient is therefore 36 – 4 = 32.
dP/dt in mmHg/s is therefore 32 / time interval (s)
Tissue doppler imaging:
TDI measured in A4C at base of LV.
Does not provide comprehensive information on LV function as only represents maximal velocity at specific region of myocardium.
Measure S’ in cm/s
Normal ranges for basal lateral and medial walls vary according to age, but values of <6cm/s for septal and 8cm/s for lateral walls usually represent abnormally low systolic function.
Tei index:
Integrating transmitral PW doppler measurements with transaortic flow times – can calculate Tei index.
Devised by Tei Chuwa in 1995 as an index of myocardial performance, both systolic and diastolic performance are integrated to provide overall picture of LV function.
- Transmitral doppler signal obtained at tips of MV leaflets in A4C to measure duration of transmitral flow.
- Similar measurement made at the base of the LVOT below the AV leaflets to measure ejection time.
- Compare ratio of these two time intervals to generate index which has been shown to be relatively pre-load independent and of useful prognostic value in cardiology cohorts.
Practical use not clear in critical care.
Myocardial performance index (MPI) = outflow time – ejection time / ejection time
= IVCT + IVRT / ET
Normal = 0.39 +/- 0.05
For memory – can just consider it as useless time/useful time.
Should be indexed for HR if outside 70-100bpm:
MPI (indexed for HR) = MPI x 75 / HR
Possible to use TDI to measure all values in one cardiac cycle instead of two separate PW windows.
Mitral valve E-point septal separation:
EPSS is distance between maximal anterior movement of the AMVL and the septum.
Distance increases with worsening function.
Normal value <6mm.