Tradeoffs between PEEP tidal volume and FiO2

In a given clinical situation, the potentially harmful effects of tidal volume, FiO 2, and PEEP at their respective values must be considered (SJuteky 1994). For instance, when a tidal volume of 900 ml and a PEEP of 15 cmH2O at a fractional inspired oxygen (Fio2) of 1.0 result in arterial blood gas values of pH 7.43, Paco2 = 5.7 kPa (43 mmHg), and Pao2 = 7.5 kPa (56 mmHg), and an airway plateau pressure of 53 cmH2O, the tidal volume should first be lowered to decrease plateau pressure at the (acceptable) risk of an increasing Paco2 and a decrease in arterial pH.

The trade-offs between PEEP and Fio2 may be more difficult to justify and to decide upon. In a situation such as that described above, a test at an Fio 2 of 0.9 could be performed, with careful continuous monitoring of pulse oximetry and a further blood gas analysis after 15 min. It must be stressed that a less than normal Pao2, pH and Paco2 are usually well tolerated by patients with normal reserves in other vital organ functions, and that potential damage to lung tissue by mechanical ventilation and a high FiO2 may be as important as discrete tissue hypoxia for ultimate survival.

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