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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Atrial Pressures 



The recording of atrial pressures is beset with 

 considerable experimental difficulty, as is the case 

 with all fast changing phasic events in low pressure 

 s\ stems. The finer details of atrial pressure contours 

 are therefore often affected by artifacts from impacts 

 or vibrations which make it difficult to arrive at 

 accurate deductions concerning atrial flow dynamics. 

 Indeed the atrial pressure pulse contour depicted in 

 figure 14 is highly schematized. 



Atrial pressure begins to rise at the onset of atrial 

 systole (A wave). Since at this moment the atrium 

 and ventricle form a common cavity, the height which 

 the atrial pressure attains is influenced by the volume 

 distensibility characteristics both of the atrium and 

 of the ventricle, in addition to the rate of translocation 

 of the fluid from one part of the cavity into the other. 

 The resistance to flow through the normal atrio- 

 ventricular orifice is so low that it cannot be measured 

 with presently available techniques. 



Little is known about the synchronicity or asyn- 

 chronicity of the contraction of atrial muscle fibers. 

 The excitation wave spreading over the atrial walls 

 proceeds from the sino-atrial node. Therefore it can 

 be assumed that the contraction, which follows 

 depolarization after a brief interval, similarly pro- 

 ceeds in a wave. The concept of asynchronous atrial 

 muscle contraction is based on the premise that the 

 delay between depolarization and contraction is the 

 same for all atrial muscle fibers. It is not certain that 

 diis is the case. A contractile wave is difficult to 

 demonstrate conclusively and consistently by means 

 of slow motion pictures. 



The drop in atrial pressure, after the pressure has 

 reached a peak during atrial contraction, is probably 

 caused by the beginning of atrial muscle fiber relaxa- 

 tion. Again, it is not possible to state whether this 

 occurs synchronously or in a sequential order and to 

 state what effect diis process has on the flow dynamics. 

 From the configuration of the atrial pressure curve 

 one cannot necessarily infer the exact onset of atrial 

 muscle relaxation. Just as the ventricular pressure 

 curve can still rise although the rate of ejection already 

 declines (see fig. 15), the atrial pressure could well 

 start to decrease before or after the relaxation in the 

 atrial musculature actually begins. The convention of 

 calling the leveling off of the declining atrial pressure 

 curve the "end of atrial systole" is also arbitrary 

 [Opdyke el al. (126); Opdyke & Brecher (125)]. 



There is often a brief period (Z point) during which 

 the atrial pressure curve remains level after its decline 



from the systolic rise. This is the last moment at which 

 the atrial and ventricular cavities are probably still 

 in communication before complete closure of the 

 atrioventricular valves. The atrial Z point pressure 

 is almost equal to the ventricular end-diastolic 

 pressure because the rate of ventricular inflow has 

 become minimal at this moment. Therefore it is 

 fairly safe to take the Z point as a representative of 

 end-diastolic ventricular pressure. It is definitely more 

 accurate to use the Z point than the mean atrial 

 pressure, which depends upon numerous factors 

 unrelated to the end-diastolic ventricular pressure, 

 such as integration of artifacts and peaks at the A, C, 



V points ( 126). 



After the Z point the atrial pressure often rises 

 briefly and precipitously (C wave). This pressure 

 rise, frequently accompanied by vibrations, is ascribed 

 to the bulging of the atrioventricular valves into the 

 atrial cavity during ventricular isovolumetric con- 

 traction. Immediately following the sharply peaked C 

 wave, atrial pressure usually declines to a level 

 corresponding to atmospheric zero in an open-chest 

 preparation (see fig. 14), or to near-zero transmural 

 pressure in a closed-chest organism. It is believed that 

 this pressure drop is caused by the pull of the papillary 

 muscles on the atrioventricular valve leaflets and by 

 the descent of the atrioventricular junction which 

 suddenly enlarges the atrial cavity. The bottom of the 

 pressure drop is called the X point (or wave). There- 

 after atrial pressure rises slowly up to the V point (or 

 wave) located at the end of ventricular isovolumetric 

 relaxation. The pressure rise from the X point to the 



V point is probably caused by an inflow of blood 

 which distends the atrial walls. The atrial pressure 

 drop (V point or wave) after the opening of the 

 atrioventricular valves results from the rapid transfer 

 of blood into the ventricular cavity in which a lower 

 pressure prevails. While it is assumed that the actual 

 opening of the atrioventricular valves occurs at the 

 summit of the V wave, there is some debate whether 

 or not it occurs slightly afterward [see also Xixon 

 (120)]. 



A minor change in the conventional labeling of 

 atrial pressure tracings has been used by Kaplan (88). 

 Without mentioning the Z point, he refers to the 

 small decline in pressure which frequently follows 

 atrial systole, before the C wave, as the X wave. Then 

 he designates the pressure decline after the C wave 

 as the X 1 wave. There is no conspicuous advantage 

 to this svstem of notation. 



