Mechanical ventilation ventilator variables

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Variables variables

The following are the various variables on a ventilator:

FiO2

• Fraction of inspired air that is oxygen

Tidal volume (Vt)

• Volume of breath delivered
• Target should be lung protective: goal of less than equal to 6 cc/Kg

Repiratory rate (f)

• Rate is set by ventilator
• f may be lower than respiratory rate if breaths are patient triggered

Positive end expiratory pressure (PEEP)

• Positive pressure applied during exhalation via resistor in exhalation port
• Prevents alveolar collapse
• Decreases shunting
• Increases oxygentaion via alveolar recruitment
• Improves compliance
• Allows severely obstructed patient to initiate breaths
• Cardiac effects include decreased preload by increased intrathoracic pressure and decreased venous return to the heart. Decreases afterload by decreasing cardiac transmural pressure
• Adjustable oxygen delivery

• Auto-PEEP or intrinsic PEEP is a phenomenon where there is inadequate exhalation time and the lungs are unable to completely empty before the next breath (breath stacking)
• Auto-PEEP will decrease preload and may decrease cardiac output especially if patient is hypovolemic. It also increases work of breathing because patient must overcome auto-PEEP to trigger ventilator. It may be detected if end expiratory flow is not equal to zero before the next breath
• Auto-PEEP can be decreased by the following measures:
  • Increase expiratory time
  • Decrease respiratory rate
  • Decrease tidal volume
  • Manage bronchospasm and secretions

Inspiratory time

• Normally inspiratory to expiratory ratio (I:E) is 1-2
• Can be used to alter flow rate
• Used in pressure-control mode

Inspiratory flow rate

• Increased flow rate leads to decreased inspiratory time and increased expiratory time. Therefore, may improve ventilation in obstructive lung disease.
• May affect respiratory rate and bronchodilation/bronchoconstriction

Peak inspiratory pressure (PIP)

• Dynamic measurement during inspiration
• Set in pressure-targeted mode
• Determined by airway resistance and lung compliance
• Increased PIP without increased plateau pressure (Pplat) implies increased airway resistance (e.g. bronchospasm, plugging)
• Decreased PIP implies decreased airway resistance or air leak in the system

Plateau pressure (Pplat)

• Static measurement at the end of inspiration, when there is no flow.
• Determined by repiratory system compliance (airway resistance is not a factor since there is no flow)
• Increased Pplat implies decreased lung or chest wall compliance (e.g. pneumothorax, pulmonary edema, pneumonia, atelectasis). It may also be due to increased PEEP or auto-PEEP.
• Pplat < 30 cm H2O decreases barotrauma (decreased Vt, decreased PEEP or increased compliance for example secondary to diuresus)