Positive Pressure Ventilation

Tntermittent positive pressure ventilation, particu­larly when applied with positive endexpiratory pressure, improves blood oxygenation by increasing volume and surface area of the lung for gas exchange.However, changes of lung volume and increased airway pressure have important side effects on cardiac output and regional blood flow. The magnitude of these hemodynamic changes is largely dependent on Paw and pulmonary compliance as well as on baroreceptor reflexes and blood volume. To avoid these untoward effects, Oberg and Sjostrand proposed a mechanical ventilation technique using low tidal volumes at a high respiratory rate. However, to obtain an adequate pulmonary gas exchange, Paw during HFJV had to be similar to Paw during IPPY If lung inflation was moderate and the regulation of systemic arterial pressure by baroreceptors was not modified, systemic hemodynamic effects of HFJV and IPPV were similar for identical values of Paw. In contrast, HFJV was less detrimental on the cardiovascular system than IPPV in case of cardiocirculatory shock.

Transmission of intrathoracic pressure to the intra­cranial space occurs through the venous system or directly on the cerebrospinal fluid space via thoracic vertebral foramina. Impairment of cerebral blood outflow across valveless veins and an increased cer­ebrospinal fluid pressure can increase cerebral venous and intracranial pressures. Results of studies in animals and in man have, however, demonstrated that, com­pared to conventional mechanical ventilation, HFJV had a similar effect on intracranial pressure. buy cheap levitra

The specific relationship between cerebral venous pressure and ICP, and the mechanisms of interaction of these two pressures on CBF are poorly understood. The effects of intracranial hypertension and an in­creased cerebral venous pressure on CBF are attenu­ated by the CBF autoregulation, the reflex venoarte- riolar contraction, and the waterfall mechanism.Rapid fluctuations in ICP induced during IPPV may result from momentary variations in brain volume and may be associated with decreased CBF. However, animal studies have shown that CBF is not influenced by the respiratory variations of ICP and cerebral venous pressure for both ventilatory supports (IPPV, HFJV) at different values of ICP and Paw.

Since the effect of ventilatory modes on CBF has not yet been studied in humans, the purpose of this study was to compare the effect of HFJV and of IPPV on cerebral and systemic hemodynamics in patients presenting no CNS pathologic condition.