Left Atrial Function

	LA Function
	The left atrium functions as a conduit for oxygenated blood from the pulmonary veins, a storage reservoir for the blood during ventricular systole, and a contractile chamber to assist or boost ventricular filling during diastole. Atrial function occurs in four phases.
	Phase I - reservoir phase - begins with mitral valve closure until mitral opening. During this phase, blood flows from the pulmonary veins into the left atrium, increasing the atrial volume to a maximum just before mitral valve opening. The increased volume produces a gradual increase in pressure - the "v" wave.
	Phase II - passive atrial emptying - During this phase, atrial volume decreases rapidly producing a decreased pressure due to blood passively flowing into the left ventricle - the "y" descent.
	Phase III - Atrial systole - Atrial volume decreases and reaches its minimum during atrial systole. Peak atrial pressure ("a" wave) is reached in the phase.
	Phase IV - Atrial diastole - The atrial pressure falls and remains low up to mitral valve closure and ventricular contraction - the "x" descent.
	Atrial Phases I II III IV PCWP Waveform
	The ratio of left atrial volume change to left ventricular stroke volume in normal subjects is about 50%. The change in left atrial volume from end-systole to end- diastole does not represent the total volume of blood entering the left ventricle during ventricular diastole. During phase II & III (passive atrial emptying and atrial diastole), blood is also flowing directly from the pulmonary veins into the left ventricle. During phase IV (atrial contraction), some blood flows retrograde into the pulmonary veins.
	Spontaneous Respiration Effect
	Respiration causes variations in left atrial filling. Spontaneous respiration and positive pressure ventilation under anesthesia have different effects on left atrial filling. Due to an increased negative intrathoracic pressure during spontaneous inspiration, the pulmonary veins expand and blood "pools" briefly in them, which decreases blood flow into the left atrium. During expiration, the blood empties into the left atrium from the expanded pulmonary veins.
	Positive Pressure Ventilation Effect
	Under positive pressure ventilation, the physiology is reversed. Left atrial filling is augmented during mechanical ventilation inspiration.
	Mitral Valve Closure
	While it is classically taught that the pressure difference between the left atrium and the left ventricle cause the mitral valve to close, recent evidence points toward partial mitral valve closure being due to atrial contraction.

Previous

Next