Читать книгу Emergency Medical Services - Группа авторов - Страница 165

There are two basic modes of mechanical ventilation, volume control and pressure control, with multiple variations and combinations of these. Common modalities include assist control (AC), synchronized intermittent mandatory ventilation (SIMV), adaptive support ventilation (ASV), and pressure support (PS). The key to understanding these modes is recognizing that the time of the respiratory cycle (i.e., respiratory rate), tidal volume, flow rate, and pressure developed in the airways are all interdependent and affected by the individual patient’s airway physiology. In each of these modes, different combinations of these variables are controlled by the machine, and the patient’s respiratory function determines the uncontrolled variables.

In AC mode, the ventilator delivers a set tidal volume with each breath. A default respiratory rate is set, but the patient may trigger breaths above that default rate. In AC mode, the machine will deliver the full set tidal volume on either a patient‐ or machine‐triggered breath. SIMV is very similar to AC, and, in fact, in patients without spontaneous respiratory effort the two are effectively identical. The major difference is that in SIMV the machine does not deliver the full set tidal volume in response to a patient‐triggered breath, but rather allows the patient’s effort to determine the volume of the breath. In SIMV mode, the ventilator will synchronize ventilator‐triggered breaths with patient‐triggered breaths, assuring that the set rate is met or exceeded. In both AC and SIMV modes, care must be taken to monitor the airway pressures developed during the respiratory cycle. In contrast, PS mode delivers a set inspiratory pressure above a baseline PEEP with each patient‐triggered breath. The patient’s respiratory drive determines the rate and the patient’s lung compliance and airway resistance determine the tidal volume developed. ASV combines several modes of ventilation in an adaptive manner dynamically to adjust levels and modes of support to the patient’s requirements.