![]() At the beginning of the breath, flow is delivered at a high rate but then tapers off over the course of inspiration, resulting in a decelerating shape of the curve (Fig. In pressure-targeted modes, the peak inspiratory pressure (PIP) and inspiratory time are set and flow is variable. The shape of the inspiratory limb of the curve depends on the mode of ventilation. The area under the curve represents the volume moved during the phases of breathing. Inspiratory flow is a positive value on the graph, whereas expiratory flow is a negative value. Gas flow in between the patient and the ventilator is represented by the flow versus time scalar. Any combination of the above variables can define the type of breath delivered by the machine. The delivery of the breath is stopped when a certain amount of time has elapsed, a goal amount of volume has been inspired, or the ventilator senses a decrease in the flow taken in by the patient. The two main target variables are either a specific inspiratory flow rate or a pressure goal. This is also referred to as the target variable. After the breath is started, the gas is delivered to the patient in a set pattern that is sustained throughout the course of inspiration. This variable is also referred to as the trigger. A breath can be started by either the patient (referred to as a supported or assisted breath) or by the machine (referred to as a controlled breath). A mechanical breath is classified based on three main variables-how the breath starts, how the breath is delivered by the machine, and how the breath is stopped. Before reviewing the graphics associated with mechanical ventilation, it is important to understand the concepts of how ventilators can deliver breaths. ![]()
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