Effect on echo examination on arrhythmias and conduction disturbances
Cardiac arrhythmias and conduction abnormalities pose a challenge to the echocardiographer, as they may impact on the accuracy in interpretation of cardiac disease presence and severity. Among all echocardiographic methods, Doppler echocardiography is most prone to the consequences of the lack of a normal sinus rhythm. This chapter will summarize the main aspects and pitfalls related to most common rhythm and conduction disturbances.
In normal individuals, the R-R interval shortens mostly at the expense of diastolic duration and filling, while the systolic ejection time is shortened only at very high heart rates (HR). In patients with abnormal left ventricular relaxation, both diastolic filling and systolic ejection time may be impared due to tachycardia, therefore its prevention represents an important therapeutic strategy in these subjects. For the echocardiographer, a shorter duration of cardiac cycle during tachycardia than in normal sinus rhythm (60 to 80/min) has the following caveats and implications:
- All echocardiographic recordings require a higher than normal temporal resolution, in order to avoid undersampling
- For measuring diameters, M-mode is preferred over the 2D method due to its higher temporal resolution, except when a proper alignment of M-mode cursor is not possible
- For measuring Doppler time integrals and intervals, a high sweep-speed of at least 100 mm/s is required (preferably 200 mm/s)
- Left ventricular volumes can be underestimated due to a limited duration of diastolic filling
- During very fast HRs, an accurate assessment of left ventricular function can be quite challenging due to the very short duration of heart cycle and of ventricular filling (often leading to an underestimation of ejection fraction). However, Tei index - a Doppler parameter of ventricular function - seemed to be less affected by the accelerated HR in fetal heart studies, as the ratio of time intervals used for its calculation corrects for tachycardia effects.
- Mitral E/A ratio is largely affected by even minor HR accelerations, due to an increase in A wave amplitude occuring progressively with the partial E-A fusion (Figure 1)(1). A Doppler E wave velocity measured at the beginning of A wave higher than 20 cm/s indicates the inaccuracy of E/A ratio and should prevent its use for diastolic function interpretation.
- The tachycardia-related changes in A wave timing and duration adversely affects the reliability of other diastolic function indices (i.e. E-wave deceleration time, Ar-A duration). However, other parameters may still be useful for assessing left ventricular filling pressure elevation during tachycardia (e.g. IVRT<70 ms, E/A>1 in presence of left ventricular systolic dysfunction, fused E-A/e’-a’ >8, systolic filling fraction from pulmonary vein flow ≤40% etc.)
- The assessment of stenosis severity is challenging at fast HR due to several reasons:
- - maximal valve opening is underestimated due to undersampling, particularly if 3D echocardiography is used to assess stenosis severity;
- - pressure-half time cannot be accurately measured on short R-R cycles at HRs >100 bpm etc.
- - for atrio-ventricular valves (native or prosthetic): Doppler velocities and mean pressure-gradients are overestimated and common cut-off values for stenosis severity may not apply at HRs higher than 80-100 bpm (2)
Cardiac rhythm and heart rate during echocardiographic examination should be routinely included in the final report, and the interpretation of echocardiographic parameters should take into account their expected changes accordingly (particularly when disease progression or inter-technique consistency are in question)
Supraventricular and ventricular ectopic beats disrupt the normal diastolic flow of the preceeding normal beat, and alter the haemodynamics also during the prolonged post-ectopic interval. The ventricular contraction following the post-ectopic interval is hyperdynamic as a result of the increased diastolic fillling (post-extrasystolic potentiation) (Figure).
- As a general rule, echocardiographic analysis (and image storage!) should avoid the cardiac cycles before and after the ectopic beats.
- During periods with bigeminal rhythm, the echocardiographic measurements may be less accurate than during sinus rhythm, while in trigeminal rhythm the beat after the long post-extrasystolic interval can be used as representative cycle.
- Occasionally, ectopic beats may provide insights otherwise not apparent during regular sinus rhythm:
- - when mitral E-A are fused (either by tachycardia or AV block), the longer interval after an ectopic beat may unmask an abnormal relaxation pattern
- - in severe aortic stenosis, a significant post-extrasystolic potentiation of peak systolic gradient reflects the presence of contractile reserve or, if higher than 70 mmHg, can serve as an index of stenosis severity (3)
- - a post-extrasystolic increase in ejection time (>20 ms) can discriminate between a dynamic suboartic stenosis and a fixed aortic stenosis and corresponds to the classical Brockenbrough-Braunwald-Morrow sign described in patients with hypertrophic cardiomyopathy.
Irregular cardiac cycle lengths determine a large beat-to-beat variability of echocardiographic measurements. The loss of atrial kick is reflected in Doppler flow patterns (absent A wave, pulmonary/hepatic Ar and TDI a’), but may be occasionally misinterpreted.
- As a general rule, echocardiographic analysis (and image storage!) should take into account at least 5 (up to 10) consecutive cycles for Doppler indices and 3-5 consecutive cycles for 2D measurements.
- Echocardiographic calculations that involve multiple image acquisition and quantitations from different cycle lengths are particularly flawed; careful cycle selection and matching for R-R intervals is crucial for biplane calculations (volumes and ejection fraction etc) or Doppler formulas (continuity equation, Tei index, regional contraction delay etc).
- For time intervals (pressure half-time etc), the R-R interval corresponding to an optimal HR (60-80 bpm) is representative (2); E-wave deceleration time should be measured only if ending before the beginning of the next QRS complex, otherwise may be falsely reduced.
- Occasionally, mid-diastolic filling waves (“L waves” in the setting of elevated filling pressures) or small waves (“F waves” in the setting of atrial flutter) can be taken for A waves, unless correlated with ECG aspect (Figures).
- Pulmonary S wave amplitude is reduced (as it depends on atrial relaxation) and D wave becomes dominant, irrespective of the left atrial pressure.
- Immediately after cardioversion to sinus rhythm, atrial stunning phenomenon may lead to erroneous interpretation of mitral flow pattern, such as restrictive mitral filling (Figure).
As a consequence, the assessment of left ventricular diastolic function and filling pressures during atrial fibrillation requires an individualized interpretation, other than recommended in sinus rhythm. Advanced methods (triplane and single-beat 3D echocardiography etc) are expected to facilitate the assessment of left ventricular size and function during atrial fibrillation.
Bradicardia and atrio-ventricular block
Sinus bradicardia is associated with prolonged ventricular diastolic filling, leading to increases in ventricular size, stroke-volume and E/A ratio. Valvular regurgitations become longer and thus may be overestimated by color Doppler in comparison with the examination during normal sinus rhythm, while atrio-ventricular valve stenoses can be underestimated on the basis of mean pressure gradient. In first-degree AV block, atrial contraction and relaxation occur prematurely during diastole, and A wave becomes superimposed on E wave (as during tachycardia). This leads to the following consequences:
- shortening of left ventricular filling (by earlier equalization of left ventricular and left atrial pressures);
- development of diastolic mitral regurgitation (atrial relaxation occurs early during ventricular diastole and, by reversing the normal pressure gradient that generates left ventricular filling, leads to an early and incomplete leaflet closure)(4).
Diastolic regurgitation of atrio-ventricular valves is a common finding in AV conduction disturbances; it should not be misinterpreted as an indicator of highly elevated intraventricular diastolic pressures occuring with normal AV activation sequence in acute aortic regurgitation or severe left ventricular dysfunction. Yet, in the presence of left ventricular dysfunction, the loss in normal temporal sequence between atrial contribution and ventricular filling due to AV conduction delay may precipitate heart failure.
In second or third-degree AV block, a highly variable pattern of mitral inflow can be found, depending on R-R cycle length and on the temporal relationship between atrial and ventricular activation (Figure).