Reply to Letter: Agreed: “PRAM May be Useful to Assess the Cardiovascular Changes in Obese Patients During Laparoscopy”

To the Editor: I thank Dr. Forfori et al. for their critique of my commentary [1], but certain statements presented by them as fact must be clarified. First, notwithstanding an inherent reproducibility error of ±15–20% [2], pulmonary artery thermodilution cardiac output (TDCO) provides mean values at normal cardiac indices that agree with other reference standards. Fact: thermodilution is still the clinical gold-standard which all pretenders of equivalence must mimic. Other methods, if to be accepted as an equivalent or replacement for TDCO, must provide a small bias and reproducibility error, and limits of agreement within ±30%. In the face of this, Forfori et al. trivialize the studies of Nguyen et al. [3] and Artuso et al. [4], which both used TDCO as their test method to determine CO and the derived indices of cardiac index (CI) stroke volume SV, stroke index (SI) and systemic vascular resistance (SVR) and SVRI. Alleging equivalence to TDCO, the authors emphasize that their results for SV are similar to those of Dumont et al. [5], which are, on the mean, 64 and 72 mL, respectively. They go on to suggest that, when comparing populations of different BMI and BSA, absolute SV is preferable to reporting SV indexed to BSA (SV/m=SI). Thus, the authors intimate that normalization of SV (or CO) between varying patient populations is unwarranted. It is this authors’ opinion that, normalization of SV/CO by BSA provides the only practical method to assess and compare the adequacy of tissue perfusion within and between patient populations of diverse anthropometric dimensions; I did not invent this rather obvious convention. With reference to their alleged agreement with the SV results of Dumont et al., Bland–Altman method yields a difference between absolute SV values of only 12%. However, normalization by their respective BSAs (2.64 m for Balderi et al. [6] vs 2.41 m for Dumont et al.) produces a SI difference of 21%, which represents an unacceptable negative bias. It can therefore be stated that SI obtained by the PRAM method substantially underestimated the SI estimate found by TDCO. Thus, the authors ought to have stated that, for a beat-to-beat method measuring SV, comparing measurements of absolute SV and CO between different methods and patient populations is misleading. The authors must have known this to be correct because Balderi et al. [6], the subject of the commentary and reply, reported their results as SI and CI (Table 2 and Fig. 1). With respect to systemic vascular resistance index (SVRI), Forfori et al. are quite correct in showing that Artuso et al. incorrectly labeled the dimensions of what they report as SVR (dyne·s·cm). Using data provided by Artuso et al., the values given are actually those of SVRI (dyne·s·cm·m). For the SVRI reported on induction of anesthesia, the following computation pertains: {[(MAP-CVP)×80]/CO}×BSA={[(82.97–15.15)× 80]/9.23}×2.6=1,528 dyne·s·cm·m, which is less than half that reported by Balderi et al. Considering the statement that the SVR values reported by Balderi et al. are “only slightly different from Dumont et al.”, simple computation shows that, on the mean, the SVR of Balderi et al. is 40% higher. OBES SURG (2009) 19:131–133 DOI 10.1007/s11695-008-9743-0