Ventilatory Support: Egan's Chapter 40

Mechanical Ventilation

• Mechanical ventilation is an artificial support of lung function that uses positive pressure applied to the airway or negative pressure applied to the thorax or abdomen to create intrapulmonary ventilation.
• Mechanical ventilation can affect nearly every organ system in the body.

Indications for Mechanical Ventilation

• Apnea
• Acute ventilatory failure
• Impending ventilatory failure
• Severe hypoxia

Pressure Gradients

• Transairway pressure (Pta) is the difference between the pressure at the airway opening (Pawo) and the alveolar pressure (Palv).
• Transpulmonary pressure (Ptp) is the difference between the alveolar pressure (Palv) and the pleural pressure (Ppl).
• Transthoracic pressure (Ptt) is the difference between the alveolar pressure (Palv) and the body surface pressure (Pbs).

Spontaneous Breathing

• Negative pressure is created in the pleural space by the descending diaphragm, resulting in airflow into the chest.
• Air flow is directed to the dependent portions of the lungs.
• The same negative intrathoracic pressure augments venous return to the right side of the heart.
• Exhalation is passive, returning to the resting end-expiratory levels.

Positive Pressure Ventilation (PPV)

• PPV reverses the gradients of normal breathing, decreasing ventilation to the dependent portions of the lungs.
• PPV increases V/Q mismatch and intrathoracic pressure, decreasing venous return to the right heart and resulting in decreased cardiac output (CO) and blood pressure (BP).

Oxygenation with PPV

• PPV delivers an FiO2 of 100%.
• Increasing FiO2 improves hypoxemia in V/Q mismatch, hypoventilation, diffusion defect (+ PEEP), and shunt (+ PEEP).

Effects of Positive Pressure Ventilation

• Increased minute ventilation in hypoventilating patients, leading to retention of CO2 and hypercapneic respiratory failure.
• Increased alveolar ventilation, which has an inverse relationship to PaCO2.
• Increased V/Q ratio, resulting in an increased V/Q mismatch.
• Increased deadspace ventilation.

Cardiovascular Effects of PPV

• PPV compresses the intrathoracic veins, increasing CVP and decreasing venous return.
• PPV enhances the cardiac cycle in patients with LV dysfunction.
• PPV decreases LV afterload, potentially increasing cardiac output.
• PPV may decrease mean arterial pressure (MAP) and increase venous pressure, leading to diminished cerebral perfusion pressure (CPP).

Other Systems Affected by PPV

• Renal: PPV reduces urinary output by 30-50%.
• Gastic:
• Nutrition:
• Neurologic:
• Hepatosplanchnic:
• Airway:
• Sleep:

Complications of PPV

• Hazards of establishing the airway (intubation): teeth, lacerations, misplaced tubes
• Nasal tubes: infection
• Endotracheal tubes: infection, VAP, pressure sores, tracheal ischemia/edema
• Barotrauma
• Hypotension
• Decreased renal output
• Hyperventilation
• Suppression of the hypoxic drive
• Increased work of breathing (WOB)