Letters to the Editor: Letter to the Editor
To the Editor
We read with great interest the study by Schmidt et al1 about the influence of renal replacement therapy (RRT) on transpulmonary thermodilution (TPTD). Measuring the mean transit time (MTt) and the downslope time (DSt) is very relevant, and to our knowledge, it is the first time that these parameters are studied in such context. The authors found both MTt and DSt to be increased with RRT. Among possible mechanisms to explain the observed changes, authors excluded real hemodynamic changes with a convincing rationale. But they also disregarded turbulence in blood flow during RRT. We think that it is the most relevant mechanism to explain both decreased cardiac index (CI) and increased MTt and DSt because of the slow flow of thermal indicator, the consequences of which are amplified by associated recirculation and would have been best derived with curve form analysis.
In this study, the injection lumen is located between the arterial lumen and the venous lumen (catheter tip; Mahurkar 12F Triple Lumen High Pressure Catheter, Covidien Deutschland GmbH, Neustadt, Germany). Consequently, the thermal indicator is injected into a turbulent blood flow area. It has been shown that the perturbation of blood flow by the insertion of a venous catheter produces turbulences at the tip of the catheter, which further increases the residence time at that position.2 This phenomenon slows the cold bolus flow in the vena cava during TPTD measurement and delays the peak temperature change at the arterial detection point, hence, the increased MTt. Staier et al3 have shown that in patients with severe mitral regurgitation (situations in which indicator bolus flow is disturbed), the precision of cardiac output measured by TPTD versus thermodilution through pulmonary artery catheter was significantly reduced in patients with predominantly severe mitral regurgitation compared with the situation after valve repair.
Indicator recirculation could also help explain increased DSt. Recirculation phenomena observed with TPTD (for example, when part of the cold indicator translocates into the pulmonary extravascular space in states of pulmonary edema and eventually reenters intravascular space) are supposedly minimized by mathematical processing, such as extrapolation of early portions of thermodilution curves by PiCCO algorithms (Pulsion Medical Systems, Munich, Germany). However, Hillis et al4 have shown that aortic or mitral regurgitations (situations in which indicator bolus flow is disturbed) may produce an abnormal prolongation of the transpulmonary dilution curve, enabling recirculation before the end of dilution curve downslope. Giraud et al5 also observed that early recirculation is associated with an increase in DSt in patients with a left-to-right intracardiac or vascular shunt.
Distortions of the dilution curve resulting from the aforementioned phenomena could therefore have led to an increased area under curve, hence a decreased measured CI, beyond extrapolation abilities of underlying algorithms. Close proximity between lumens of the dialysis catheter and injection lumen in this particular study could explain why authors found a decrease in CI when compared with previous studies in which dialysis catheter and bolus injection sites were located in different vena caval territories.
Philippe Ariès, MDClermont-Tonnerre Military Teaching HospitalBrest, FranceFrench Military Health Service AcademyEcole du Val-de-GrâceParis, Francephil.firstname.lastname@example.org
Thomas Leclerc, MDBurn center, Percy Military Teaching HospitalClamart, FranceFrench Military Health Service AcademyEcole du Val-de-GrâceParis, France
1. Schmidt S, Westhoff T, Schlattmann P, Zidek W, Compton F. Analysis of transpulmonary thermodilution data confirms the influence of renal replacement therapy on thermodilution hemodynamic measurements. Anesth Analg. 2016;122:14741479.
2. Friedrich P, Reininger AJ. Occlusive thrombus formation on indwelling catheters: in vitro investigation and computational analysis. Thromb Haemost. 1995;73:6672.
3. Staier K, Wilhelm M, Wiesenack C, Thoma M, Keyl C. Pulmonary artery vs. transpulmonary thermodilution for the assessment of cardiac output in mitral regurgitation: a prospective observational study. Eur J Anaesthesiol. 2012;29:431437.
4. Hillis LD, Firth BG, Winniford MD. Comparison of thermodilution and indocyanine green dye in low cardiac output or left-sided regurgitation. Am J Cardiol. 1986;57:12011202.
5. Giraud R, Siegenthaler N, Park C, Beutler S, Bendjelid K. Transpulmonary thermodilution curves for detection of shunt. Intensive Care Med. 2010;36:10831086.