ALVEOLAR-ARTERIAL GRADIENT FOR OXYGEN IN HEART-FAILURE

[Relationship between the alveolar-arterial oxygen gradient and PaO₂/FiO₂-introducing PEEP into the model]

OBJECTIVES

To determine whether the alveolar-arterial oxygen gradient (Grad[A-a]O₂) helps confirm the influence of PEEP on PaFi (PaO₂/FiO₂).

DESIGN

Observational study; we used linear regression to perform a multivariate study to improve the PaFi formula by taking PEEP into account.

SETTING

Tertiary hospital.

PATIENTS

We included all patients who were admitted to the intensive care unit, regardless of pulmonary damage.

VARIABLES

We recorded personal history, clinical judgment, intensive care data, severity scores on the first day and progression. Two calculated variables: PaFi and Grad(A-a)O₂.

RESULTS

A total of 956 patients were included: 63.9% men; median age 68 years. On the first day, 31.8% did not have mechanical ventilation (MV), 13.1% had non-invasive MV and 55.1% had invasive MV. PaFi values: 32.9% 0-200, 32.2% 201-300, and 34.8% >300. PEEP values: 0-5 69.8%, 6-10 27.5% and >10 2.6%. We observed a correlation (Pearson) between Grad(A-a)O₂ and PaFi of -0.84 (p<0.001). On performing multiple regression (dependent variable: Grad[A-a]O₂), the following variables were included in the model: PaFi, PEEP, APACHE IV and SOFA; coefficient of determination (R²) of 0.62 without PEEP and 0.72 with PEEP. We changed the PaFi formula, referring to it as PaFip (PaFi plus PEEP): Ln (PaFi/[PEEP+12]). Correlation index between PaFip and Grad(A-a)O₂: -0.9 (p<0.001). We performed linear regression (dependent variable: Grad[A-a]O₂) and used PaFip instead of PaFi. Only PaFi remained in the model, and was discretely complemented by APACHE IV; R²=0.8.

CONCLUSIONS

By adding PEEP to the PaFi model (PaFip), we clearly improve the latter, as reflected by a better goodness of fit.

Positive expiratory pressure plateau breathing in spontaneously breathing patients with myocardial infarction and pulmonary oedema

Kerr, F., Ewing, D. J., Irving, J. B., Sudlow, M. F., and Kirby, B. J. (1974).Thorax, 29, 690-694. Positive expiratory pressure plateau breathing in spontaneously breathing patients with myocardial infarction and pulmonary oedema. The respiratory effects of breathing with a positive expiratory pressure plateau (PEPP) was studied in 24 spontaneously breathing, alert, nonsedated patients with pulmonary oedema following myocardial infarction. When 20 patients breathed room air with PEPP a small rise in arterial oxygen tension (Pao₂) with a fall in alveolar to arterial oxygen gradient (A—ado₂) occurred. Arterial carbon dioxide tension did not change significantly. When PEPP was used in six patients after breathing 100% oxygen for 20 minutes there was no significant change in Pao₂, A—ado₂ or anatomical shunt (Q̇s/Q̇t). Expired air collection in a further six of the patients demonstrated that although PEPP produced an increase in tidal volume (Vt), alveolar ventilation (Va) fell slightly as a result of a decrease in respiratory rate, and oxygen consumption (Vo₂) did not change.

In 10 normal subjects functional residual capacity (FRC), measured continuously in a whole-body volume displacement plethysmograph, decreased by a small amount when these subjects breathed with PEPP.

Positive expiratory pressure plateau breathing in spontaneously breathing conscious patients with pulmonary oedema produced a small improvement in ventilation/perfusion matching not by an increase in FRC but by an increase in Vt. The increase in Vt probably altered the ventilation/perfusion relationships by a redistribution of inspired gas.