Valsalva maneuver and input impedance.
Authors: M O'Rourke
Journal: Circulation (1981)

To the Editor:Murgo and colleagues (Circulation 63: 122, 1980) elegantly characterized the changes in ascending aortic pressure and flow con- tour and in derived ascending aortic impedance that occur during the Valsalva maneuver.Their data support and extend the less detailed work of Kroeker and Wood,' Mills et al.2 and others.Murgo et al. showed that during Valsalva strain, ascending aortic pressure and flow waves come to resemble each other in contour, while derived ascending aortic impedance displays little fluctuation in frequency (after the initial steep fall from its zero frequency value) and so approximates the characteristic impedance at all fre- quencies above 1.5 Hz.They interpreted these findings to indicate marked reduction in wave reflection from a peripheral reflecting site.These interpretations must be reconciled with the findings of Mills et al.,2 who showed that during Valsalva strain, patterns of pressure and flow in the descending aorta (and in the brachio- cephalic artery) remain different from each other even though con- tour of pressure and flow waves in the ascending aorta become similar.In addition, analysis of waves published by Mills et al. indi- cates that brachiocephalic impedance is little altered during Val- salva strain, although ascending and descending aortic impedance are markedly altered.A satisfactory explanation of all data is available if one considers wave reflection not in terms of one peripheral site (as was the basis of Murgo's analysis), but in terms of separate upper and lower body reflecting sites, one representing the result of all arterial termina- tions in the upper part of the body and the other representing the result of all arterial terminations in the lower part of the body.This concept was originally introduced by McDonald3 and appears to ex- plain pressure and flow contour and impedance patterns in different arteries of different animals' 6 and of man."In terms of this con- cept, the Valsalva strain would be expected to reduce transmural pressure in the thoracic and abdominal aorta, and so, by reducing pulse wave velocity to lower body sites, to delay wave reflection from the lower body (while reflection from the upper body re- mained unchanged).If this were the case (and studies by Mills et al.2 and by Kroeker and Wood' suggest that it is), then the flattened as- cending aortic impedance modulus observed by Murgo et al. during Valsalva strain could be explained on the basis of different timing of wave reflection from upper and lower body sites.Such an explana- tion has been advanced" 7 to explain the flattened impedance modu- lus in the ascending aorta of dogs, and was actually suggested by Mills et al. for the changes that occur in man during Valsalva strain.Another feature of Murgo's data supports this explanation.The impedance data showed similar fluctuations in phase during Valsalva strain as under control conditions (albeit at lower fre- quencies) even though modulus values were almost constant with frequency.When wave reflection is absent or markedly diminished, impedance phase is almost zero at all frequencies.8 9These points are presented as a possible explanation of hemo- dynamic changes during Valsalva strain, which was not considered in the article published by Murgo et al. and which appear to recon- cile their findings with other data.