The Left Atrial Appendage in Humans: Structure, Physiology, and Pathogenesis

Nabeela Karim; Siew Yen Ho; Edward Nicol; Wei Li; Filip Zemrak; Vias Markides; Vivek Reddy; Tom Wong

Disclosures

Europace. 2020;22(1):5-18. 

In This Article

Mechanical Function of Left Atrial Appendage

The mechanical properties of the LAA are due to its contractile nature. Many studies have demonstrated its role in haemodynamic function[24] through modulation of left atrial pressures.[1] It was previously thought that the LAA was a passive structure; however, LAA contraction is in fact greater than that of the LA[25] and can be stimulated by dobutamine in both sinus rhythm and AF.[26]

Left Atrial Appendage Flow Patterns in Sinus Rhythm

The flow pattern in sinus rhythm is typically quadriphasic[1] (Figure 3); commencing with LAA contraction after the P wave on the electrocardiogram. This correlates with atrial systole; specifically, late diastolic mitral inflow. It is marked by a positive Doppler outflow signal and can be used to quantify LAA ejection fraction.[22] The next phase is marked by LAA filling, occurring in early systole, as a result of the combined effect of active relaxation and elastic recoil. It results in a negative flow signal on the Doppler trace.[23] Following this are low velocity outflow–inflow waves, referred to as systolic reflection waves.[23] They are due to passive flow during the remainder of systole, which can vary in number and amplitude.[23] The final phase is the early diastolic LAA flow, represented by low-velocity outflow signal[22] and coincides with the start of transmitral inflow.[1]

Figure 3.

LAA flow patterns: (A) LAA qudriphasic flow in sinus rhythm with labels 1–4 demonstrating the four phases of cycle. (B) LAA flow in atrial fibrillation demonstrating varying amplitude resulting in the typical saw tooth signal. (C) LAA flow in atrial flutter with a regularized saw tooth signal. LAA, left atrial appendage.

In normal hearts during sinus rhythm, typical contraction velocities range from 50 to 83 cm/s, with filling velocities from 46 to 60 cm/s.[12] During tachycardia, the quadriphasic aspect of LAA contractility is less apparent, with early and late diastolic emptying merging, and reflection waves disappearing.[23]

Left Atrial Appendage Flow Pattern in Atrial Fibrillation and Atrial Flutter

In AF, the LAA flow pattern has a saw-tooth signal (Figure 3), with varying amplitude.[23] Flow velocity (the combination of LAA contraction and filling), is lower compared with sinus rhythm.[27] Flow velocity is also influenced by ventricular rate, with the two having an inverse relationship.[1] When taking measurements, the flow velocity should be averaged over several cycles.[27]

In atrial flutter, the flow velocity is also saw toothed, but regularized with a greater wave amplitude than in AF[23] (Figure 3).

Left Atrial Appendage Flow Pattern in Conjunction With Other Variables

Increasing age results in a linear reduction in LAA flow velocity;[23] in those with preserved left ventricular function. Left atrial appendage contraction velocities declines by 4.1 cm/s and filling velocity by 2.0 cm/s for every 10 years of age from the age of 45 years.[27] Gender differences also exist, with males having higher flow velocity.[27] Raised CHA2DS2-VASc score is also independently associated with reduced flow velocity regardless of rhythm.[28]

Although left atrial size and body surface area does not influence LAA flow velocity, LAA size does.[27] Larger appendages are associated with a reduction in contractility, independent of underlying pathology.[27] Flow velocity decreases with increased left atrial strain[28] and raised left atrial pressure,[23,29] which improves upon heart failure (HF) treatment due to a reduction in left atrial filling pressures.[30] Left ventricular systolic and/or diastolic function also affects LAA contractility, with impaired left ventricular relaxation demonstrating a decline in early LAA diastolic flow.[27]

Mitral stenosis decreases flow velocity by impacting on both active and passive LAA emptying, independent of rhythm.[31] The degree of reduction is related to the degree of stenosis severity.[31] Interestingly, LAA function recovers after mitral valvotomy.[32]

In addition, flow velocity independently correlates with LAA morphology; the chicken wing subtype demonstrates higher flow velocity compared with cactus and cauliflower subtypes.[33]

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