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Editor,—We read with great interest the article by Kaulitz et al where Doppler measurements of various venous flows were performed with simultaneous respiratory and venous pressure recordings.1 The main findings were: venous flow and pressure are dependent on respiration in total cavopulmonary connection (TCPC); and venous flow and pressures are cardiac dependent in atriopulmonary connection (APC).
While the authors should be commended for monitoring simultaneous flow and pressure data along with respiratory and cardiac monitoring, there were no new insights. Venous flow in the TCPC, by the nature of dissociation with the atrial hydraulic function, has been shown to be augmented largely during inspiration when the further decrease of the negative intrathoracic pressures “draws” inferior venous flow antegrade.2-4 While venous flow in APC will show less respiratory effect as it is coupled with the atria, it has also long been known to be related to the cardiac cycle.2 ,5 ,6
Important new information might have been gathered by the authors if more subtle pressure–flow velocity relations in TCPC and APC had been described. Theoretically, the relation between pressure and flow allows evaluation of pathway resistance or vascular impedance, properties of the Fontan circulation that may be crucial to its efficiency.
We also have some methodological concerns; the use of maximal velocity as an indicator of flow can be misleading. As flow is related to the integration of velocity and time, it is dependent on duration of flow. In other words, it is possible to have high maximal velocity but low flow rate if the Doppler velocity time integral is small. This is especially true when the spectral profile is not symmetrically parabolic. Furthermore, as the venous flow in both APC and TCPC exhibited retrograde patterns, the use of maximal antegrade velocity does not take this into account and therefore would not reflect the net or total flow during inspiration or expiration.
Finally, in the evaluation of the haemodynamics of the venous system, particularly in a Fontan circulation devoid of a ventricular hydraulic source, hydrostatic forces may play important roles. Recently we have examined the effect of gravity during an investigation of infradiaphragmatic venous return using Doppler ultrasonography to evaluate flow rates. Our preliminary results show, for example, that gravity exerts a significant influence on both the splanchnic and systemic inferior venous flow dynamics.7 There are many additional haemodynamic variables that may influence these Fontan circulations, and further studies must concentrate on these more subtle, but perhaps just as important, relations in the future.