There are many physiological differences between mechanical and spontaneous breaths. During a mechanical breath the ventilator controls many of the primary variables responsible for breathing, for example inspiratory flow rate, inspiratory time, and the length of the respiratory cycle. Inspiratory gas is pushed into the chest and the ventilator performs all the work required to inflate lungs and chest wall. The tidal volume and pattern of gas flow will then be related to the ventilator and the mechanical characteristics of the lungs and chest wall. Gas exchange occurs during each breath and is principally related to ventilation-perfusion abnormalities and to true shunt. As intrathoracic pressure will be greater than atmospheric pressure during a mechanical breath, extrathoracic effects of this positive pressure occur, affecting blood flow to other organs.

The physiology of mechanical ventilation differs from spontaneous breathing in terms of lung mechanics, control of breathing, and gas exchange. The physiological effects will be more obviously altered by disease. Few deleterious effects occur in healthy subjects, but adverse effects may be pronounced in the critically ill.

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