A 57-year-old man is undergoing open repair of an abdominal aortic aneurysm, and intraoperative transesophageal echocardiography is used for monitoring of volume status and diagnosis of ischemia.
The transmitral flow (TMF) velocities are recorded with pulsed wave Doppler (PWD). (Fig. 1) What is the cardiac rhythm?
Blood flow can be imaged by using spectral or color-flow Doppler echocardiography. Use of spectral imaging, in particular, can facilitate interrogation of the underlying cardiac rhythm. This may serve a critical function in the cardiac and even noncardiac operating rooms if the anesthesiologist suspects an arrhythmia is the cause of hemodynamic deterioration, and the electrocardiogram (ECG) tracing is not available, contains significant artifact, or is equivocal.1 For example, ineffectual atrial contraction, as occurs with atrial fibrillation, loss of external pacing, or conduction abnormality, may be considered the reason for hypotension. In this case, a specific treatment may be indicated, such as cardioversion or transvenous, epicardial, or transcutaneous pacing for enhancement of left ventricular (LV) preload or even increased contractility (Bowditch effect in the bradycardic patient).2 Whether or not the ECG tracing is unreliable, an accurate diagnosis may be made with Doppler interrogation of the TMF.
Normal Transmitral Flow
Diastolic blood flow across the mitral valve occurs when the left atrial (LA) pressure exceeds the LV pressure. The TMF can be recorded by using PWD, with a 1 to 3 mm sample volume positioned between the mitral valve leaflet tips, in the midesophageal 4-chamber or midesophageal long-axis views.3 Diastolic inflow occurs twice in a given cardiac cycle, during early (passive) filling (E wave) and during late (active) filling after atrial contraction (A wave) (Fig. 2). Both E and A waves are recorded below the zero baseline because blood is moving away from the transducer, and both waves are triangular in shape. In addition, the deceleration phase (downward slope) of the early TMF velocity lasts longer than the acceleration phase (upward slope), giving E wave the shape of a scalene triangle. This is in contrast to the isosceles shape of A wave, because the acceleration and deceleration of the late TMF velocity are always equal. The heights (peak velocity) of E and A waves and the slope of the deceleration phase of E wave relate to the underlying LV diastolic properties, which have been reviewed in the journal.4
Temporal Relation Between TMF and ECG
Knowledge of the temporal relationship between TMF and the ECG signal in sinus rhythm belies its utility in diagnosing arrhythmias, and the ECG tracing should always be displayed along with the spectral Doppler display. Typically, there is a 10 millisecond delay between the ECG electrical event and the Doppler velocity signal, depending on the ultrasound scanner.5 The T wave of the ECG signals electrical repolarization and LV myocardial relaxation. Thus, E wave occurs after, not coincident with, the T wave of the ECG. The ECG P wave signals atrial contraction; therefore, A wave occurs several milliseconds after, not coincident with, the ECG P wave.
Rhythm Analysis Using Transmitral Flow
Presence of an A wave indicates effective atrial contraction, a hallmark of all sinus rhythms, including those with atrioventricular (AV) prolongation (e.g., conduction block). When a distinct A wave is not obvious in the TMF, it may be fused with E wave or may be absent; 2 situations with unique diagnoses are discussed below.
When E and A waves are fused, the differential diagnosis includes normal sinus rhythm, sinus tachycardia, and AV block. In sinus tachycardia, diastole is abbreviated, and the sinus node depolarization that triggers the next cardiac cycle occurs quickly after the previous ventricular repolarization.6–8 Consequently, the ECG P wave and A wave occur earlier. In this case, atrial contraction may start during early filling, resulting in partial or complete fusion of E and A waves (Fig. 3A). A similar pattern is observed with AV block in which AV prolongation is present9 (Fig. 3B). In paced rhythms, A wave will only be present if there is effective atrial contraction. The presence of a pacemaker spike on the ECG represents electrical activity only, not atrial capture. When atrial pacing successfully triggers atrial contraction, it results in first-degree AV block with fusion of E and A waves10 (Fig. 3C).
Alternatively, fusion of E and A waves is also found in impaired ventricular relaxation, the first stage of diastolic dysfunction, when the early (LA–LV) pressure gradient has a delayed onset (Fig. 3D). Thus, impaired relaxation is an important confounding factor when assessing cardiac rhythm with TMF alone.4
With cardiac rhythms other than sinus, lack of atrial contraction results in the absence of A wave. With atrial fibrillation (Fig. 3E), A wave is absent, and E wave has variable velocity and configuration, reflecting the irregularly irregular diastolic interval. In junctional rhythms (Fig. 3F), where the atrial impulse originates in the AV node, atrial contraction may occur during ventricular systole, when the mitral valve is closed. In this situation, the pressure of LA contraction will not exceed LV systolic pressure; therefore, the mitral valve will not open, and A wave will be absent. With atrial flutter (Fig. 3G), there may be multiple, low velocity A waves dispersed throughout the cardiac cycle independent of E wave.
Cardiac rhythm and its associated TMF pattern are dynamic and can change either abruptly or throughout the course of surgery, depending on clinical circumstances. Diastolic time may be gradually shortened, and occasionally, atrial contraction can be lost completely. Figure 4 demonstrates dynamic rhythm changes in a patient during cardiac surgery.
In conclusion, analysis of TMF patterns by using PWD may provide useful information regarding cardiac rhythm when ECG analysis is challenging. Fusion of E and A waves and the absence of A wave have unique differential diagnoses and may lead to distinctive treatment pathways.
The rhythm of the patient presented in the index case is sinus rhythm with first-degree AV block. E
TMF and Cardiac Rhythm
- Obtain a pulsed wave Doppler tracing of transmitral flow.
- Look for E and A waves.
- If an A wave is present, then the left atrium is contracting.
- If only 1 wave is present, then E and A waves are fused or A wave is absent (e.g., atrial fibrillation).
- If multiple A waves are present at a high frequency, consider atrial flutter.
- The morphology of a single wave should be carefully examined to determine whether E and A waves are fused or A wave is absent.
- If the shape approximates an isosceles triangle, then E and A waves are likely to be fused.
- If the shape resembles a scalene triangle, then single wave is likely an E wave and A wave is absent.
Name: Brian L Alexander, MD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Brian L Alexander approved the final manuscript.
Name: Nikolaos J Skubas, MD, FASE.
Contribution: This author helped design and conduct the study, and write the manuscript.
Attestation: Nikolaos J. Skubas approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
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