Critically ill patients requiring mechanical ventilation are subject to a variety of complications and adverse effects associated with positive pressure ventilation. An awareness of the major physiological effects is important, as recognition, prevention and appropriate treatment of complications is critical to optimizing patient outcome. Pierson (1990) suggests that complications due to ventilation occur with greater frequency than is generally appreciated, and can be a response to suboptimal ventilatory management as a result of poor communication and lack of understanding. This may also adversely affect patient comfort, morbidity and outcome. This can be avoided by improved knowledge of both the function of ventilators and the adverse effects associated with mechanical ventilation. Intensive care nurses face the challenge of caring not only for critically ill patients but also for complex machinery. To improve the delivery of care to the patients, an understanding of the machines and their adverse effects is essential. This increases the nurses' confidence and allows them to focus on the patients and associated problems while maintaining safe and informed care. With the introduction of mechanical ventilation, major physiological changes occur, for example airway resistance and intrathoracic pressures are increased and lung mechanics are altered. The following article provides explanation as to how and why mechanical ventilation produces these changes, and highlights areas where they occur.