Influence of Surrounding Structures on Parallel Volume Determinations by Conductance Catheter

Abstract

When left ventricular blood volume is evaluated by the conductance catheter technique, an important fraction of the injected current “leaks” into the surrounding tissues giving rise to a parallel volume (Vp). To study the influence of the thoracic structures on Vp, 5 anesthetized dogs were studied during (a) closed thorax, (b) open thorax, (c) reclosed thorax without pericardium, (d) open thorax without pericardium, (e) replacing the pericardium with a nonconducting bag. Intraventricular volume and pressure, surface ECG, and Vp (determined by saline dilution technique) were monitored during each condition. A conductance diagram was used to represent the behavior of the structures within the thoracic cage, including cardiac muscle, pericardium, and also other structures (OS) such as lung, skeletal muscle, and bone tissues. Under conditions (a) and (b), Vp was not significantly different (p < 0.005); thus, changes in OS did not much modify Vp. When the pericardium was removed, the average Vp values for conditions (c) and (d) were smaller than those for (a) and (b) (p < 0.005). Apparently, because of good conductivity, the pericardium would favor the passage of current. Finally, when the fifth condition (e) was compared with (a) or (b), the differences were highly significant (p < 0.005), while the differences between (e) and either (c) or (d) were less significant and more variable (p < 0.05). Duncan's multiple comparisons by pairs confirmed the previous statements. The relative fraction of the total measured volume through the surrounding tissues (p%) systematically decreased as we moved from condition (a) to (e). Condition (e) decidedly forced a concentration of current lines through the myocardium. In conclusion, the pericardium is responsible for a significant fraction of the loss, which should be separated out from that due to the myocardium and OS. The latter would contribute to Vp with not more than about 10%