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If a mechanical ventilator malfunction is suspected, remove the patient from the ventilator immediately and begin manual ventilation with a resuscitation bag.
Mechanical ventilators include waveform graphics, which are graphic representations of changes in pressure, flow, and volume within a ventilator circuit. Acquiring an understanding of these graphics takes time and practice. Respiratory therapists (RTs) caring for patients who are critically ill need to monitor patient–ventilator interactions at the bedside. RTs must be able to assess waveform graphics to determine patient–ventilator synchrony. The challenge for RTs is to provide ventilatory support that is synchronized with the patient's effort and drive.
The use of ventilator flow and pressure waveforms allows detection of asynchrony. Asynchrony, which occurs frequently, is associated with significant patient discomfort and distress and poor clinical outcomes (e.g., duration of mechanical ventilation, intensive care unit, and hospital stay). It can occur during all phases of ventilated breaths and all modes of ventilation.undefined#ref1">1
Common waveform graphics include pressure-time, flow-time, and volume-time waveforms. Pressure-volume and flow-volume loops are also commonly used (Figure 1) (Figure 2). A ventilator-assisted or ventilator-controlled breath can be divided into these parts: breath initiation, commonly termed triggering; breath delivery; breath termination, commonly termed cycling; and mechanical exhalation, which occurs when the patient is not receiving any assistance from the ventilator.
Understanding ventilator-derived waveforms requires a basic knowledge of pulmonary physiology. The RT should be able to interpret the waveforms that reflect how the pulmonary system is responding to changes in the ventilator prescription. Application of common therapeutics is implementable and traceable using graphic analysis.3 Patient–ventilator asynchrony can occur in both volume and pressure modes of ventilation, and the inspiratory time is critical to establishing the correct mean airway pressure waveforms. The range of inspiratory time should be between 0.6 and 1 second, depending on the patient's pulmonary mechanics.2 Excessive inspiratory times lead to excessive overdistention of the alveoli and air trapping.
Waveform graphics are often used to determine if a patient is being overventilated or underventilated and if he or she has inadvertent or intrinsic positive end-expiratory pressure (PEEP). The graphic for auto-PEEP (Figure 2) would show the breath not returning to baseline before the subsequent breath is given. Waveform interpretation can help RTs determine if the patient has increased or decreased lung compliance and airway resistance. Waveform interpretation has also been used to determine if a patient has lung function improvement after the administration of a bronchodilator.
Waveform graphics also assist the RT in determining if the inspiratory time is set appropriately for the patient, which is important in improving patient–ventilator synchrony. The flow and volume graphic should represent a smooth transition from inspiration to expiration. If the flow graphic shows a drop-off that does not go smoothly to baseline, it indicates an inspiratory time that is too short. If the volume graphic is flat at the top instead of rising to a peak and then declining, this indicates that the inspiratory time is too long for the patient. A beak appearance at the top of the pressure-volume loop indicates pressure in excess of volume benefit (Figure 1).
Rationale: Rising peak inspiratory pressure can indicate a change in compliance or the need for suctioning.
Rationale: Patients receiving pressure ventilation can show an increase or decrease in exhaled volumes with a change in compliance. If compliance decreases, volumes decrease; if compliance increases, volumes increase.
Rationale: Patients receiving volume ventilation can show an increase or decrease in pressure with changes in compliance. If compliance decreases, pressure increases; if compliance increases, pressure decreases.
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