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Không đạt được mức ngưỡng của lưu lượng máu về phía trước, áp lực động mạch chủ, và do đó áp lực tưới máu mạch vành là luôn xác định là tiên đoán kết quả không thuận lợi dựa trên cả hai nghiên cứu thực nghiệm và lâm sàng | 198 G.Ristagno A. Gullo W.Tang M.H.Weil increases in intrathoracic pressure which account for decreases in cardiac filling and therefore decreases in forward blood flow during lung inflation. Failure to achieve threshold levels of forward blood flow aortic pressure and consequently coronary perfusion pressure are consistently identified as predictive of unfavourable outcomes based on both experimental and clinical studies 24 25-28 . To obtain shorter interruptions of chest compression during CPR the 2005 guidelines mandate compression ventilation ratios of 30 2 in lieu of 15 2. Although secure clinical proof of ultimate benefit of these revised com-pression ventilation ratios has not yet been published experimental studies have provided evidence that more frequent ventilations did not improve outcomes 29 30 . However increasing the compression ventilation ratios increased pulmonary blood flow and end-tidal CO2 without compromise of arterial oxygen content or acid-base balance 30 . Only more recently have we fully appreciated that the cardiac output and therefore pulmonary blood flow produced by chest compression during CPR is actually less than one-third of normal physiological levels. Accordingly fewer ventilations are required to maintain optimal ventilation perfusion ratios. Even more important gas exchange may be sufficient in the absence of external ventilation. Precordial compression itself provides sufficient gas exchange for the small pulmonary blood flow especially if high flow oxygen is passively delivered into the airway 31 32 . Spontaneous gasping provides another and probably important source of pulmonary gas exchange during CPR 33 34 . We now also recognize that earlier guidelines overestimated the tidal and minute volumes required during conventional CPR and failed to appreciate the adverse effects of interruptions of chest compression and descreased venous return 35 . Ventilation has indeed become of much lesser importance except in asphyxial .
