RESPONSE TO OXYGEN BREATHING IN ALI/ARDS PATIENTS

	To the Editor:

Santos and colleagues (1) report that venous admixture measured on maintenance FI_O2 with the shunt fraction equation tends to increase on pure oxygen breathing. In addition, they showed that shunt measured by the multiple inert gas technique (MIGET) increases significantly.

It has to be taken into account that some authors using the MIGET reported no shunt increments measured in patients with acute respiratory failure after pure oxygen breathing (2, 3). These patients had mean S/T values on maintenance FI_O2 of 0.31 and 0.29, respectively, whereas the patients of Santos and colleagues (1) had a relatively low mean T/T value (0.20). On the other hand, S/T decrements on FI_O2 1.0 has been described in patients with severe respiratory failure (S/T > 0.50) (4, 5).

An explanation for these dissimilar responses to pure oxygen breathing could be the different rates of intrapulmonary shunt in the studied groups. Actually, a relationship between S/T on maintenance FI_O2 and S/T changes on pure oxygen has been reported (6). Thus, S/T on pure oxygen breathing could either increase, decrease, or even remain the same, depending on the original levels of S/T on maintenance FI_O2.

Although Santos and colleagues (1) demonstrated shunt decrements measured by MIGET after pure oxygen breathing in ALI patients with moderate abnormalities in gas exchange, it remains to be proven if the same is true in ALI/ARDS patients with severe abnormalities in gas exchange (e.g.: S/T > 0.05, Pa_O2/FI_O2 > 50 mm Hg).

Instituto Raúl Carrea, FLENI, Buenos Aires, Argentina

Alejandro C. Raimondi

Sanatorio Mater Dei, Buenos Aires, Argentina

1. Santos C, Ferrer M, Roca J, Henández C, Rodriguez-Roisin R. Pulmonary gas exchange response to oxygen breathing in acute lung injury. Am J Respir Crit Care Med 2000; 161: 26-31 [Abstract/Free Full Text].

2. Lampron N, Lemaire F, Teisseire B, Harf A, Palot M, Matamis D, Lorino AM. Mechanical ventilation with 100% oxygen does not increase intrapulmonary shunt in patients with severe bacterial pneumonia. Am Rev Respir Dis 1985; 131: 409-413 [Medline].

3. Lemaire F, Matamis D, Lampron N, Teisseire B, Harf A. Intrapulmonary shunt is not increased by 100% oxygen ventilation in acute respiratory failure. Bull Eur Physiopathol Respir 1985; 21: 251-256 [Medline].

4. Briscoe WA, Smith JP, Bercofsky E, King TKC. Catastrophic pulmonary failure. Am J Med 1976; 60: 248-258 [Medline].

5. Lamy M, Fallat RJ, Koeniger E, Dietrich HP, Ratliff JL, Eberhart RC, Tucker HJ, Hill JD. Pathologic features and mechanisms of hypoxemia in ARDS. Am Rev Respir Dis 1976; 114: 267-284 [Medline].

6. Quan SF, Kronberg GM, Schlobohm RM, Feeley TW, Don HF, Lister G. Changes in venous admixture with alterations of inspired oxygen concentration. Anesthesiology 1980; 52: 477-482 [Medline].

	From the Authors:

We are grateful for the comments on our article by Drs. Raimondi and Raimondi (1). We do agree that several studies have observed no increases in mixed venous admixture ratio in patients breathing 100% oxygen, hence suggesting no increments in intrapulmonary shunt. It is of note, however, that measurements of intrapulmonary shunt with inert gases more accurately reflect the presence of both nonventilated and low ventilation-perfusion alveolar units than those calculated using the traditional oxygen method. Because the criteria for acute respiratory distress syndrome (ARDS) were not always clearly defined (2); it might be possible that many of these patients had severe pneumonia instead of a clear-cut picture of acute lung injury (ALI) or ARDS. If this is the case, then basal inert gas intrapulmonary shunt should remain unchanged in patients with life-threatening pneumonia, as has been shown by us (3) and others (4), with shunt values close to or higher than those seen in our study (1). Mechanisms for this lack of increased shunt in response to oxygen breathing have already been suggested (1, 3, 4), including the potential different behaviors of pulmonary versus extrapulmonary ARDS (5).

Notwithstanding the above, it might still be possible that ALI or ARDS patients with extremely high values of intrapulmonary shunt (. 50% of cardiac output) could be less responsive to hyperoxia because of a more devastating underlying lung injury; however, this has not yet been proven. The two patients with the highest basal shunt (35% and 45% of cardiac output, each) from our study exhibited oxygen responses similar to patients with lower shunt levels.

Universitat de Barcelona, Barcelona, Spain

1. Santos C, Ferrer M, Roca J, Torres A, Hernández C, Rodriquez-Roisin R. Pulmonary gas exchange response to oxygen breathing in acute lung injury. Am J Respir Crit Care Med 2000; 161: 26-31 .

2. Lemaire F, Matamis D, Lampron N, Teisseire B, Harf A. Intrapulmonary shunt is not increased by 100% oxygen ventilation in acute respiratory failure. Bull Eur Physiopathol Respir 1985; 21: 251-256 .

3. Gea J, Roca J, Torres A, Agustí AGN, Wagner PD, Rodriquez-Roisin R. Mechanisms of gas exchange in patients with pneumonia. Anesthesiology 1991; 75: 782-789 [Medline].

4. Lampron N, Lemaire F, Teisseire B, Harf A, Palot M, Matamis D, Lorino AM. Mechanical ventilation with 100% oxygen does not increase intrapulmonary shunt in patients with severe bacterial pneumonia. Am Rev Respir Dis 1985; 131: 409-423 .

5. Gattinoni L, Pelosi L, Suter PM, Pedoto A, Vercesi P, Lissoni A. Acute respiratory distress syndrome caused by pulmonary and extrapulmonary disease: different syndromes? Am J Respir Crit Care Med 1998; 158: 3-11 [Abstract/Free Full Text].