Ventilator settings and positive end expiratory pressure (PEEP)

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  1. Effect of PEEP on cardiac performance
    1. in normal lungs there is not much compression of the vessels by PEEP but in lungs with decreased compliance (e.g. pneumonia, pulmonary edema), PEEP compresses the heart and intrathoracic blood vessels
    2. decrease preload by compressing ventricles and decreasing the pressure gradient from extrathoracic to intrathoracic cavities
    3. decreased afterload by compressing the ventricles
    4. sum total: when ventricular filling is not compromised PEEP can increase cardiac output
  2. patterns of assisted ventilation
    1. standard method of positive pressure mechanical ventilation involves volume-cycled lung inflation; the patient can initiate each mechanical breath (assisted ventilation) but when this is not possible the ventilator provides machine breaths at a preselected rate (assist-control ventilation); I:E ratio must be between 1:2 to 1:4 and is varied by adjusting the amount of time it takes to put a volume of air in the lungs
      1. risks: hyperinflation and respiratory alkalosis
    2. problems with rapid breathing during mechanical ventilation with AC ventilation led to the intermittent mandatory ventilation (IMV); IMV delivers periodic volume cycled breaths at a preselected rate but allows spontaneous breathing between machine breaths; because each spontaneous breath does not trigger a machine breath, there is a reduced risk of respiratory alkalosis and hyperinflation injury; often used to wean patients off the ventilator
      1. risks: increased work of breathing (due to resistance of the tube) and tendency to reduce cardiac output
    3. pressure controlled ventilation
      1. because of the risk for ventilator induced lung injury from large inflation volumes, pressure controlled ventilation is used commonly; pressure controlled ventilation is pressure cycled breathing that is completely controlled by the ventilator with no participation by the patient; in volume cycled breathing, the inspiratory flow rate is constant throughout lung inflation wheras in pressure cycled breathing the inspiratory flow decreases exponentially during lung inflation to keep the airway pressure at the preselected value
        1. risk: tendency is for inflation volumes to vary with changes in the mechanical properties of the lungs
        2. good for patients with neuromuscular disease
    4. pressure support ventilation
      1. allows the patient to determine the inflation volume and respiratory cycle duration; keeps the inflation pressure constant; used to augment inflation volumes during spontaneous breathing or to overcome the resistance of the breathing circuit
    5. continous positive airways pressure (CPAP)
      1. spontaneous breathing in which a positive pressure is maintained throughout the respiratory cycle is called CPAP; decreases work of breathing; may keep patients from not needing to be intubated or to wean them
      2. often used in those with difficult work of breathing
  3. PEEP
    1. extrinsic PEEP is applied by placing a pressure limiting valve in the expiratory limb of the ventilator circuit
    2. tends to reduce cardiac filling but cardiac output is magnified; PEEP ventilation may reduce cardiac output by increasing the mean intrathoracic pressure – this is independent of the amount of PEEP thus 5 mm Hg of PEEP may due similar damage as 15 mm Hg
    3. distal airspaces tend to collapse at the end of expiration and this tendency is exaggerated when the lungs are stiff as in ARDS; PEEP prevents the alveoli from collapsing at the end of expiration and can open alveoli that have collapsed raising arterial PO2 allowing FIO2 to be reduced
    4. may increase pulmonary edema by increasing intrathoracic pressure and decreasing outflow of lymph
  4. weaning from a mechanical ventilator is best done through daily or multiple daily trials of spontaneous breathing
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