The MAPA is the reference method to estimate the PA during the SAHOS. The MAPA can be recommended over 48 hours: indeed, the PA falls at the beginning of every apnea then increases gradually up to a pressif peak arising at the time of the ventilatoire resumption. These variations arise under the influence of 4 stimulus: the désaturation in O2, the rise of the PaCO2, the increase of the respiratory effort and the microawakening of the end of apnea who are at the origin of a sympathetic stimulation.