Mechanical Ventilation

• Set sequence that delivers a pre-defined breath...machine controls timing (when inhalation starts) and size (when inhalation stops)
• 

• Control: (independent variable)
• -Volume
• Control the flow for a target amount of time
• 
• -Pressure
• Control the pressure for a target amount of time
• **Pressure control is especially good for stiff lungs: won't overdistend the alveoli
• 

• -Dual: control pressure but adjust by computer. "Pressure regulated volume control"
• 

• -Air pump
• -Inhalation valve solenoid  - reduces (controls) the pressure on inspiration
• - Exhale valve solenoid - safety mechanism (release valve) for if the pt coughs or obstructs

• All controlled by a microprocessor
• 

• Trauma to the alveoli caused by over-distension (or pressures >>> physiologic)
• 

• Avoid over-distension by:
• -Tidal volume around 6ml/kg
• -PIP less than 30cm/H2O (peak inspiratory pressure)

Those who get lower VT have improved survival and discharge status

• Continuous mandatory ventilation (aka assist control)
• Intermittent mandatory ventilation
• No mandatory ventilation (PEEP, CPAP, etc.)

• Set length of breath
• Set number of breaths/minute (respiratory rate)
• 

Ventilator responds to stimulus from patient (i.e. they begin to take breath) by giving a mandatory breath

• *Back up rate set as a safeguard (minimum number of respirations per minute) in case pt doesn't give stimulus
• 

• PEEP
• Positive end expiratory pressure
• -"Floor" pressure - independent of pt's breathing attempt
• -Often combined with supported breaths (extra pressure when ventilator senses pt's effort)

CPAP - continuous positive airway pressure without the supported breaths

• Indications:
• -The addition of positive pressure can help maintain open alveoli...
• ARDS - adult respiratory distress syndrome
• -thought to be a process of decreased surfactant production
• -smaller lungs with fewer gas-exchange alveoli
• -fewer inflating alveoli
• 

PEEP: increases the pressure to keep some of the "bad" alveoli open

• -Hysteresis -
• different volumes for the same pressure depending on whether the lung is being
• loaded or unloaded (inflating or deflating)
• 

• -LaPlace's law- 
• pressure within the alveoli is dependent upon the surface tension and the radius of the alveoli

• Pressure = (2 x surface tension)/radius
•