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Correspondence  |   May 2011
A High Significance Level after Analysis of Covariance in a Small-group Study?
Author Affiliations & Notes
  • Antonio Pesenti, M.D.
    *
  • *University of Milan-Bicocca, Monza (MB), Italy.
Article Information
Correspondence
Correspondence   |   May 2011
A High Significance Level after Analysis of Covariance in a Small-group Study?
Anesthesiology 5 2011, Vol.114, 1244. doi:10.1097/ALN.0b013e3182134f68
Anesthesiology 5 2011, Vol.114, 1244. doi:10.1097/ALN.0b013e3182134f68
In Reply:
We thank Bloomstone for his interest in our article1 and for his comments. We entirely agree with him when he states that the weaning process is complex, multifactorial, and highly variable; this is also outlined in the editorial that accompanied our article. Our original hypothesis was that oxygen consumption (V̇o2) would increase more in patients unable to sustain the weaning effort; this hypothesis was probably simplistic and did not account for some literature data, such as those published by Zakynthinos et al.  2 On the other hand, we relied on solid evidence in the literature3–6 showing how increased V̇o2during weaning would be associated with failure. Moreover, Bloomstone wisely underlines how V̇o2is linked to the complex interplay between peripheral extraction and delivery. Unfortunately, as we acknowledge in the discussion of our article, the lack of assessment of the hemodynamic changes in our patients stands as a relevant limitation of our work.
We appreciate Chen's deep attention in revising our data. In his sharp comment, he notes a paradox between the results of the Student t  test and those of the ANOVA. However, the two tests are difficult to compare because they are performed on different sets of data. In fact, Chen neglects the fact that, although the minimum V̇o2readings were compared as absolute values using the Student t  test, the ANOVA is performed after normalization of V̇o2by the minimum V̇o2reading of each patient. This normalization is expected to decrease the between-patient heterogeneity in the “absolute values” of V̇o2, causing the observed increase in statistical significance. Moreover, at variance from figure 2 of the original article,1 using ANOVA, the levels of pressure support are expressed as difference from the “resting” level of pressure support, rather than as absolute values; in other words, all patients are “aligned” on the x-axis, with the minimum recorded V̇o2corresponding to the same level of pressure support. We agree with Chen regarding the appropriateness of Bland– Altman analysis to evaluate the reproducibility of V̇o2measurement. Because this was not included in our original article, we report it herein: the mean difference between the minimum V̇o2value during the decremental pressure support trial and the V̇o2during the resting phase was 14 ml/min (95% CI, 61 to −33 ml/min).
Fig. 1.  Bland-Altman analysis showing the comparisons between two measurements of oxygen consumption (VO2) obtained respectively as the minimum value during the decremental pressure support trial and during the resting phase (VO2, min). For further details please see the original manuscript.1
Fig. 1. 
	Bland-Altman analysis showing the comparisons between two measurements of oxygen consumption (VO2) obtained respectively as the minimum value during the decremental pressure support trial and during the resting phase (VO2, min). For further details please see the original manuscript.1
Fig. 1.  Bland-Altman analysis showing the comparisons between two measurements of oxygen consumption (VO2) obtained respectively as the minimum value during the decremental pressure support trial and during the resting phase (VO2, min). For further details please see the original manuscript.1
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*University of Milan-Bicocca, Monza (MB), Italy.
References
Bellani G, Foti G, Spagnolli E, Milan M, Zanella A, Greco M, Patroniti N, Pesenti A: Increase of oxygen consumption during a progressive decrease of ventilatory support is lower in patients failing the trial in comparison with those who succeed. Anesthesiology 2010; 113:378–85Bellani, G Foti, G Spagnolli, E Milan, M Zanella, A Greco, M Patroniti, N Pesenti, A
Zakynthinos S, Routsi C, Vassilakopoulos T, Kaltsas P, Zakynthinos E, Kazi D, Roussos C: Differential cardiovascular responses during weaning failure: Effects on tissue oxygenation and lactate. Intensive Care Med 2005; 31:1634–42Zakynthinos, S Routsi, C Vassilakopoulos, T Kaltsas, P Zakynthinos, E Kazi, D Roussos, C
Shikora SA, Benotti PN, Johannigman JA: The oxygen cost of breathing may predict weaning from mechanical ventilation better than the respiratory rate to tidal volume ratio. Arch Surg 1994; 129:269–74Shikora, SA Benotti, PN Johannigman, JA
Shikora SA, Bistrian BR, Borlase BC, Blackburn GL, Stone MD, Benotti PN: Work of breathing: Reliable predictor of weaning and extubation. Crit Care Med 1990; 18:157–62Shikora, SA Bistrian, BR Borlase, BC Blackburn, GL Stone, MD Benotti, PN
Mitsuoka M, Kinninger KH, Johnson FW, Burns DM: Utility of measurements of oxygen cost of breathing in predicting success or failure in trials of reduced mechanical ventilatory support. Respir Care 2001; 46:902–10Mitsuoka, M Kinninger, KH Johnson, FW Burns, DM
Miwa K, Mitsuoka M, Takamori S, Hayashi A, Shirouzu K: Continuous monitoring of oxygen consumption in patients undergoing weaning from mechanical ventilation. Respiration 2003; 70:623–30Miwa, K Mitsuoka, M Takamori, S Hayashi, A Shirouzu, K
Fig. 1.  Bland-Altman analysis showing the comparisons between two measurements of oxygen consumption (VO2) obtained respectively as the minimum value during the decremental pressure support trial and during the resting phase (VO2, min). For further details please see the original manuscript.1
Fig. 1. 
	Bland-Altman analysis showing the comparisons between two measurements of oxygen consumption (VO2) obtained respectively as the minimum value during the decremental pressure support trial and during the resting phase (VO2, min). For further details please see the original manuscript.1
Fig. 1.  Bland-Altman analysis showing the comparisons between two measurements of oxygen consumption (VO2) obtained respectively as the minimum value during the decremental pressure support trial and during the resting phase (VO2, min). For further details please see the original manuscript.1
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