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J Thorac Cardiovasc Surg 2006;131:504-505
© 2006 The American Association for Thoracic Surgery

Letter to the Editor

Reply to the Editor

U. Fakler, MD, J. Hess, MD

German Heart Center, Department of Pediatric Cardiology and Congenital Heart Disease, Technische Universität München, Munich, Germany

Reply to the Editor:

With great interest we read the comments of Berger and Bergstra, and we appreciate their remarks concerning our article. 1 Go We have to confirm with regret that there is a typographical error in the published version of our article. The correct version of the LaFarge/ Miettinen formula 2 Go is as follows:

For females: VO2/BSA = (138.1 – 17.04 x ln(age) + 0.378 x HR) (mL/min)/m2

For males: VO2/BSA = (138.1 – 11.49 x ln(age) + 0.378 x HR) (mL/min)/m2

However, we used this correct version for the analysis of our data, so the data and figures we presented are correct.

The difference between the populations and the regimen of general anesthesia and relaxation might explain the different findings of assumed oxygen consumption (VO2) values.

Secondly, Berger and Bergstra questioned whether the determination of VO2 with the Deltatrac II system (Datex, Engström, Helsinki, Finland) is acceptable as a reference method.

Behrends and colleagues 3 Go showed an acceptably accurate determination of VO2 using the Deltatrac II system compared with mass spectrometry and wet gas spirometry in an in vitro model for ventilated neonates: mean bias –3.8 % (SD 5.7 %).

Weyland and colleagues 4 Go described a laboratory validation in a pediatric setting comparing the Deltatrac system with mass spectrometry and wet gas spirometry. They found a mean bias of –3.2 % (SD 11.5 %), including the following conditions: FIO 2 (inspired oxygen fraction) 0.21 to 0.8, FIO 2-FEO 2 (expired oxygen fraction) 0.01 to 0.05, VE (expired volume) 300 to 6000 mL/min, VT (tidal volume) 8 to 300 mL, Paw (airway opening pressure) 10 to 60 mbar, and relative humidity 10% to 60%. 4 Go These wide ranges of respiratory variables might explain the wider ranges of the standard deviation.

Bauer and colleagues 5 Go performed an in vitro validation and clinical testing for ventilated preterm infants. In vitro validation was done by methanol burning during intermittent positive-pressure ventilation (IPPV) with two commonly used ventilators: Sechrist IV 100B (Sechrist Industries, Anaheim, Calif) and the Infant star software version 83 (Nellcor Puritan Bennet Inc, Carlsbad, Calif). They found a mean difference between VO2 measurements using the Deltatrac II system and VO2 determined by the methanol burning method from –2% to 2 % (max SD ± 5%) depending on the ventilator and peak inspiratory pressure they used.

For clinical testing, they developed a breath collector to avoid gas leakage around uncuffed endotracheal tubes or during nasal continuous positive airway pressure (CPAC). This test arrangement simultaneously sampled expired air expelled at the ventilator and escaping via the tube leak from the infant's mouth and nose and allowed VO2 measurements that are not affected by tube leaks.

They studied 15 preterm infants during IPPV and 10 preterm infants during nasal CPAP and determined a mean difference of VO2 of 6.5 (SD 1.5) mL · kg–1 · min–1 during IPPV and 6.6 (SD 2.2) mL · kg–1 · min–1 during nasal CPAP.

In conclusion, these studies prove that the Deltatrac II system is a feasible and reliable device to measure VO2 in ventilated infants if air leaks can be excluded.

We determined air leak by measuring inspiratory and expiratory tidal volume using the features of the ventilator (Siemens Servo Ventilator 900 D; Siemens, Erlangen, Germany). As described in the Methods section, we had to exclude 12 patients because of air leakage of more than 5%.

Finally, Berger and Bergstra correctly pointed out that a shunt ratio can be calculated using oxygen saturation measurements. A determination of pulmonary or systemic blood flow is necessary to calculate absolute shunt volumes.


   References
Top
 References
 

  1. Fakler U, Pauli C, Hennig M, Sebening W, Hess J. Assumed oxygen consumption frequently results in large errors in the determination of cardiac output. J Thorac Cardiovasc Surg 2005;130:272-276.[Abstract/Free Full Text]
  2. LaFarge CG, Miettinen OS. The estimation of oxygen consumption. Cardiovasc Res. 1970;4:23-30.[Abstract/Free Full Text]
  3. Behrends M, Kernbach M, Bräuer A, Braun U, Peters J, 0land W. In vitro validation of a metabolic monitor for gas exchange on ventilated neonates. Intensive Care Med 2001;27:228-235.[Medline]
  4. Weyland W, Weyland A, Fritz U, Redecker K, Ensink FB, Braun U. A new paediatric metabolic monitor. Intensive Care Med. 1994;20:51-57.[Medline]
  5. Bauer K, Ketteler J, Laurenz M, Versmold H. In vitro validation and clinical testing of an indirect calorimetry system for ventilated preterm infants that is unaffected by endoracheal tube leaks and can be used during nasal continuous positive airway pressure. Pediatr Res 2001;49:394-401.[Medline]




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