Trigger (before inspiration)
LOOK FOR:
Ineffective trigger: Negative deflection during inspiration or expiration on pressure (Paw) waveform without accompanying positive deflection in flow (V) waveform.14,61 Double trigger: Within the same breath cycle, a second, stacked breath occurs after a very short inspiratory time on the Paw wave-form. The second breath within the cycle has a higher peak airway pressure and may exhibit an increased peak expiratory flow.14
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Ineffective trigger attempt: Patients with low respiratory drive do not reach the trigger pressure in the correct time. Other factors: low PCO2, high tidal volume and pH,36 sedation,38 high levels of assist (pressure support or assist control).11,35,36 Double triggering: Patient effort is greater than volume/flow delivery (inappropriate settings or sudden onset of increased patient demand),14,36 shortened ventilator inspiratory time and low ratio of PaO2 to fraction of inspired oxygen.36 Less sensitive trigger level setting.14 Poor breathing cycle dynamics: Patient does not receive a positive inflation after inspiratory delay time (due to excessive auto-PEEP, pressure drop across endotracheal tube, COPD).14 Ventilator unable to supply flow to pressurize the circuit.14
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Ineffective trigger attempt: Assess respiratory drive (neuromuscular/chemical) and correct as able. Evaluate for over-sedation and reduce sedative infusion rate.11, 61 Determine if patient is receiving too much assistance, may adjust pressure-support ventilation to decrease tidal volume.11 Double triggering: Adjust flow or volume settings to meet the patient’s demand.14 Consider source of hypoxemia and provide measures to optimize oxygen saturation. Consider sedation. Increase trigger sensitivity setting without causing auto-triggering, consider flow-based trigger (may reduce trigger delay <100 ms).11 Evaluate for auto-PEEP in the expiratory waveform.14 Add PEEP cautiously (moves the ventilator trigger level closer to the patient’s intrinsic airway pressure).14,35,61 Optimize use of bronchodilator to reduce PEEP.11 Consider use of flow-waveform triggering if auto-PEEP present.61 Adjust the slope setting by increasing Inspiratory time and/or flow.
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Flow (during inspiration)
LOOK FOR:
Concave appearance during inspiratory phase in the Paw waveform.14
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An increased patient respiratory effort causes ventilator to not match the patient’s demand.14 In volume-controlled ventilation: Incorrect peak flow setting for patient.14 In pressure-controlled ventilation: A patient’s flow demand and the rate of flow delivery.14
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Evaluate pressure time waveform to differentiate this from trigger dyssynchrony. Assess neural drive to breathe, determine causative factors (undersedation, agitation, disease) and manage appropriately. Volume-controlled ventilation: “Adjust peak flow setting until patient’s demand met, evidenced by pressure waveform resembling that of passive breathing”14(p213) Pressure-controlled ventilation: Finely adjust rise time (rate of valve opening) to maximize flow-pressure waveforms.14 Consider dual control modes14 Consider size and diameter of endotracheal tube
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Termination (inspiration terminated when cycling variable reached)
LOOK FOR:
Premature: A concavity in the Paw occurs with prolonged high flow during breath, followed by breath with rapid flow deceleration in convex shape.14 Delayed: A spike at end of the breath will occur on Paw, coinciding with rapid decrease in flow14 (patient attempts to exhale).
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Premature termination: During pressure-support ventilation: Caused by low levels of support, short time constant (rate of flow decay influenced by resistance and compliance factors of respiratory system29 and dynamic hyperinflation.62 During assist volume control: Caused by settings that yield short inspiratory time.62
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Delayed termination: May be more prevalent in patients with COPD62
Pressure-support ventilation settings that result in a long time constant (COPD, high pressure support level and low flow threshold for cycling off variable).62 Assist-control ventilation settings that result in long inflation time (high tidal volumes, low inspiratory flow).62
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Evaluate with flow and pressure time waveforms.14 May be difficult to see delayed termination during pressure support.62 Monitor for double triggering in premature termination and missed trigger efforts (ineffective trigger) during delayed termination.11,18 For premature termination: Consider decreasing the flow threshold in patients with acute respiratory distress syndrome.62 For delayed termination: Determine if auto-PEEP is present by using expiratory hold maneuver. If auto-PEEP is present, consult with team to consider adjusting trigger sensitivity, peak flow, flow pattern, inspiratory time, breath-termination criteria, expiratory time, or respiratory rate, depending on the ventilation mode.14 Use a sedation scale to consider sedation and neuromuscular relaxants to weaken patient’s effort.29 Monitor patients for ineffective triggering, especially if auto-PEEP is present.
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Expiratory (on expiration)
LOOK FOR:
Expiratory flow wave-form does not return to baseline.14
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Common phenomena because neural (patient) inspiratory time varies, which affects termination and timing of expiration. Premature and delayed termination29
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Monitor for auto-PEEP by using expiratory hold technique. Adjust trigger sensitivity, peak flow, flow pattern, pressure slope, inspiratory time, breath termination criteria, expiratory time or respiratory rate, depending on mode.14 Monitor for trigger dyssynchrony if auto-PEEP is present. If present, consider applying PEEP so as to equal the amount of auto-PEEP.13,14
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