The patients suffering from HTP are hypoxemic. They have very limited oxygen reserves due to decrease of the functional residual capacity (CRF). The apnea period, which follows the anesthetic induction, is often associated with a fast desaturation, even if a good pre-oxygenation was performed before. This desaturation causes an increase of the pulmonary vascular resistances with the hemodynamic consequences previously mentioned. A risk of hypoxic heart arrest is also present.