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Communications to the Editor |

Clinical Application of a Prognostic Model for Severe Community-Acquired Pneumonia FREE TO VIEW

Ravindra Mehta, MD; Maritza Groth, MD, FCCP
Author and Funding Information

Affiliations: Winthrop University Hospital, Mineola, NY,  Harborview Medical Center Seattle, Washington

Correspondence to: Maritza Groth, MD, FCCP, Director of Critical Care, Department of Pulmonary and Critical Care Medicine, Winthrop University Hospital, 222 Station Plaza North, Suite 400, Mineola, NY 11501; e-mail: mgroth@winthrop.org



Chest. 2001;119(1):312-313. doi:10.1378/chest.119.1.312
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To the Editor:

The study by Pascual et al (February 2000)1 is one more step toward defining prognostic factors in the subset of patients with severe community-acquired pneumonia who require mechanical ventilation. By reason of retrospective analysis, the authors conclude that a prediction model—based on extent of lung injury assessed by a hypoxemia index, age > 80 years, immunosuppression, medical comorbidities with a median prognosis for survival < 5 years, and number of nonpulmonary organ failures—classifies patients at the threshold of 95% mortality rate, with a positive predictive value of 100% and specificity of 100%. A few observations in the study merit further analysis. In the univariate analysis, survivors had a mean duration of hospital stay prior to mechanical ventilation of 0.8 days compared with 2.8 days (p = 0.016) for nonsurvivors. The logical query is that, in the absence of standardized criteria for mechanical ventilation, did timing of mechanical ventilation impact on prognosis? Because the prognostic model was based on assessment of parameters after the initiation of mechanical ventilation, the assessment of the hypoxemia index would depend on the timing of mechanical ventilation, with the possible corollary that earlier initiation of mechanical ventilation improves prognosis. This suggests that the clinician’s assessment of the timing of intubation has a great impact on prognosis. The importance of the clinician’s role is further emphasized by the disparity in prognosis in the patients who had survived an initial trial of mechanical ventilation and were later given a do-not-resuscitate/do-not-intubate status; in these instances, clinical assessment overestimated rates of mortality compared with the prognostic model.

The criteria for diseases with median prognosis for survival of < 5 years included HIV with CD4 < 200 and New York Heart Association class 3 congestive heart failure (CHF). With the advent of highly active anti-retroviral therapy2 and newer modalities of treatment of CHF, the prognosis for these patients has improved, and this would impact on the prognostic model in these subsets of patients. The comparable number of medical comorbidities (1.9 in each group) and their effects on survival are interesting. It is a frequent clinical observation that patients with advanced COPD, interstitial lung disease, CHF, and/or cystic fibrosis are difficult to wean and would need prolonged treatment with mechanical ventilation; however, this did not seem to be the case in this study. The number of patients with chronic respiratory disease is not clear. Conceptually, the hypoxemia index would already be compromised in these patients, and an added respiratory insult such as pneumonia could skew their prognosis adversely.

Finally, according to the model, as prognostication based on assessment of lung injury is done early in the first 24 h of mechanical ventilation, a poor prognosis implies possible cessation of intensive care. If a similarly accurate prognostication, with a noninvasive assessment of lung injury, could be made before initiation of mechanical ventilation therapy, it would translate into greater advantages for the clinician, patient, and family perspectives, because withdrawing ventilatory support is always more difficult than not initiating it.

References

Pascual, FE, Matthay, MA, Bacchetti, P, et al (2000) Assessment of prognosis in patients with community-acquired pneumonia who require mechanical ventilation.Chest117,503-513. [CrossRef] [PubMed]
 
The CASCADE Collaboration: Concerted Action on Seroconversion to AIDS and Death in Europe. Survival after introduction of HAART in people with known duration of HIV-1 infection. Lancet 2000; 355:1158–1159.
 
To the Editor:

In response to the comments by Drs. Mehta and Groth, the observation that survivors had a mean duration of hospital stay of 0.8 days before receiving mechanical ventilation, compared with 2.8 days for nonsurvivors, could be taken to imply that earlier initiation of mechanical ventilation at the first sign of significant respiratory failure in community-acquired pneumonia (CAP) could favorably impact prognosis. In addition to the finding above, our observation, that the hypoxemia index was lower in survivors vs nonsurvivors (37 vs 59, respectively)1 over the first 24 h of mechanical ventilation, supports one of the following: (1) that survivors were intubated earlier in the course of their respiratory failure, as Drs. Mehta and Groth have suggested; or (2) that overall, survivors had less severe lung injury as a group, independent of the timing of intubation. Because this was a retrospective study, it is difficult to determine which of these two factors was more dominant. Based on our data, therefore, we cannot make the claim that earlier treatment with intubation and/or mechanical ventilation for respiratory failure ultimately leads to a more favorable prognosis in patients with CAP.

Drs. Mehta and Groth also raise a valid point that, with the advent of highly active antiretroviral therapy and newer modalities for the treatment of congestive heart failure, the overall prognosis for such patients has improved. We would urge clinicians to take these factors into account in determining whether a patient fulfills criteria for medical comorbidity with median survival of <5 years.

Finally, we would caution against using this prediction model in patients who are not receiving mechanical ventilation. The rule is not derived from that particular subset of patients with CAP whose prognosis may be influenced more strongly by factors other than the degree of lung injury. Because lung injury overall may not be as severe in that subset, there may be reason to believe that nonpulmonary organ system failure, medical comorbidity, or other prognostic markers may have greater influence and that the weighting of such factors in a model for that population might differ substantially from those in our model. Second, because of the problem of entrained air around an oxygen mask, it is difficult to obtain an accurate assessment of hypoxemic lung injury in a patient who is not intubated; typically, the actual airway fraction of inspired oxygen (Fio2) in such patients is less than the Fio2 delivered in the mask. Thus, any calculation of the hypoxemia index (or other measures of lung injury) in that situation could lead to an overestimate of the degree of lung injury and a spurious overestimate of the risk of death in the patient not receiving ventilation.

References
Pascual, FE, Matthay, MA, Bacchetti, P, et al Assessment of prognosis in patients with community-acquired pneumonia who require mechanical ventilation.Chest2000;117,503-513. [CrossRef] [PubMed]
 

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References

Pascual, FE, Matthay, MA, Bacchetti, P, et al (2000) Assessment of prognosis in patients with community-acquired pneumonia who require mechanical ventilation.Chest117,503-513. [CrossRef] [PubMed]
 
The CASCADE Collaboration: Concerted Action on Seroconversion to AIDS and Death in Europe. Survival after introduction of HAART in people with known duration of HIV-1 infection. Lancet 2000; 355:1158–1159.
 
Pascual, FE, Matthay, MA, Bacchetti, P, et al Assessment of prognosis in patients with community-acquired pneumonia who require mechanical ventilation.Chest2000;117,503-513. [CrossRef] [PubMed]
 
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