Ventilator Flashcards
What is the equation of motion of the respiratory system?
Muscle pressure + ventilator pressure = (volume / compliance) + (resistance x flow) The patient’s respiratory muscles and ventilator sum to = the trans-respiratory pressure This combined pressure differential b/w the thorax and atmosphere (or ventilator) causes volume and flow to be delivered to the patient Compliance and resistance are characteristics of the system and are ASSUMED to remain constant during single breaths under normal conditions.
Define compliance
Compliance = ΔV/ΔP change in volume divided by the corresponding change in pressure.
Give an example of a disease state that has decreased respiratory compliance.
acute respiratory distress syndrome (ARDS)
Give an example of a disease state with increased resistance to airflow.
asthma exacerbation
Define peak inspiratory pressure
the resultant highest pressure during inspiration while in volume-control mode
Which ventilator control mode will result in variable flow rates?
Pressure control As the lung expands, compliance decreases. As a result flow decelerates during the respiratory cycle –> variable tidal volumes whenever resistance changes (inflating/deflating abdomen during laparoscopic surgery)
List the parameters that can be controlled by the ventilator in delivery of a positive pressure breath.
airway pressure tidal volume flow duration
How does volume control ventilation work and what parameters are set by the clinician?
delivers a predetermined flow over a specific time to achieve a preset tidal volume Usually the clinician sets the tidal volume and respiratory rate. May set the inspiratory flow and inspiratory time, the product of which determines the actual tidal volume (tidal volume = mean inspiratory flow x inspiratory time)
Describes the two most common waveforms in volume control ventilation
constant flow and decelerating or “ramped” flow
Using a decelerating flow waveform –> a lower peak inspiratory pressure compared to using a constant flow waveform
How is airway pressure during volume control ventilation determined?
both airway resistance and elastic recoil of the lungs and the chest wall
Airway pressure due to elastic recoil
- increases as lung volume increases, regardless of flow
Airway pressure due to airway resistance
- dependent on flow
Using a decelerating flow waveform –> a lower peak inspiratory pressure compared to using a constant flow waveform
With a volume-control ventilation, how would decrease in lung compliance affect tidal volume and airway pressure?
Tidal volume not affected
Compiance dec –> inc airway pressure
When is volume control ventilation particularly useful?
Because tidal volume is constant in the face of changes in respiratory system compliance useful in intraoperative situations where chest wall compliance may change
- upper abdominal surgery (restriction of diaphragmatic excursion by retractors)
- laparoscopy (restriction of diaphragmatic excursion by the pneumoperitoneum).
How does pressure control ventilation work and what parameters are set by the clinician?
delivers a variable flow –> maintain the airway pressure at a set level for a set duration –> curvilinear, decelerating flow pattern
clinician sets:
- driving pressure (the increase in airway pressure above PEEP during inspiration)
- inspiratory time.
What factors determine the tidal volume in pressure control ventilation?
- ***Driving pressure*** (the amount by which airway pressure is increased during inspiration).
- Compliance of the respiratory system (CRS).
- Airway resistance.
- Duration of the breath (inspiratory time).
Changes in airway resistance affect how QUICKLY volume is delivered, but may not change the final volume if there is sufficient time for proximal airway pressure and alveolar pressure to equilibrate (i.e. flow = 0)
Describe the differences between pressure-control and pressure-support
- Pressure-support is always patient-triggered, while pressure-control may be patient-triggered or machine-triggered
- Pressure-support is flow-cycled (end inspiration is determined by inspiratory flow), while pressure control is time-cycled (end of inspiration determined by time)
