In some cases, mechanical ventilation is used as an adjunct to other forms of therapy. For example, it is used to reduce cerebral blood flow in patients with increased intracranial pressure.

Mechanical ventilation also is used frequently in conjunction with endotracheal intubation for airway protection to prevent aspiration of gastric contents in otherwise unstable patients during gastric lavage for suspected drug overdose or during gastrointestinal endoscopy. In critically ill patients, intubation and mechanical ventilation may be indicated before the performance of essential diagnostic or therapeutic studies if it appears that respiratory failure may occur during those maneuvers.

The correct answer is A. You answered B.

The answer is A.  Mechanical ventilation can be delivered in many different modes, which refers to the manner in which the breaths are triggered, cycled, and delivered. The most common mode of mechanical ventilation is assist-control. This mode is volume cycled and flow limited. An inspiratory cycle may be triggered by patient effort or, if there is no patient effort within a specific time, by a timer within the ventilator. Each breath, whether triggered by patient effort or the ventilator, delivers an operator-specified tidal volume. This mode is frequently the initial mode of mechanical ventilation when a patient is intubated as it ensures a known minute ventilation in the absence of respiratory effort. Common difficulties with the use of assist-control ventilation are asynchrony with the ventilator and tachypnea, which can lead to respiratory alkalosis and/or generation of dynamic hyperinflation. This occurs when a patient triggers an inspiratory cycle before a full exhalation occurs. Commonly known as auto-PEEP, dynamic hyperinflation can lead to decreased venous return and a fall in cardiac output. It may also lead to barotrauma, including pneumothorax and pneumomediastinum. Intermittent mandatory ventilation (IMV) is a mixed mode of ventilation. IMV is primarily a volume-cycled and flow-limited mode of ventilation and is most often delivered in a synchronized mode (SIMV). The ventilator will deliver a mandatory number of breaths at a specified tidal volume. If a patient breathes only at the set respiratory rate, SIMV is essentially the same as assist-control ventilation. When a patient breathes above the set respiratory rate, the spontaneous breaths may be unassisted or assisted with a pressure support mode of ventilation. SIMV was commonly used as a weaning mode of ventilation, but clinical trials have shown that trials of spontaneous breathing lead to a shorter duration of mechanical ventilation and more rapid extubation. Pressure control ventilation (PCV) is time triggered, time cycled, and pressure limited. This mode is often used in individuals who have preexisting barotrauma or for postoperative thoracic surgery patients. It is also used in ARDS to limit ventilator-induced lung injury. In PCV, there is no prespecified minimum tidal volume or minute ventilation. Rather, a specified pressure is set during inspiration by the operator, and the tidal volume and inspiratory flow rate are dependent upon lung compliance. Pressure support ventilation (PSV) is patient triggered, flow cycled, and pressure limited. PSV requires a patient to spontaneously initiate respiration because no machine-delivered breaths are given. PSV provides graded assistance through application of an inspiratory pressure that augments spontaneous respiration. The pressure support falls to a specified expiratory pressure when the flow falls below a certain rate. PSV is often combined with SIMV to augment the spontaneously generated breaths. PSV also is used in ventilator weaning as it is generally well tolerated.