Auto-PEEP Syndrome

Auto-PEEP (intrinsic positive end-expiratory pressure) is end-expiratory alveolar pressure above the pressure set on the ventilator, because exhalation is incomplete before the next breath begins. Gas accumulates (breath stacking), and lung volume at the end of expiration rises. This article concentrates on the ventilated patient.

Causes

• Short expiratory time: high respiratory rate, large tidal volume, long inspiratory time, high minute ventilation
• Expiratory airflow obstruction: asthma, chronic obstructive pulmonary disease, bronchospasm, mucus plugs, small endotracheal tube, kinked tube and high-resistance expiratory filters or heat-moisture exchangers
• High lung compliance and expiratory valve problems

Consequences

• Raised intrathoracic pressure that reduces venous return and cardiac output, which causes hypotension and can cause pulseless electrical activity (frequent after intubation of a patient with severe asthma)
• Increased work of breathing, with inspiratory efforts that fail to trigger the ventilator
• Barotrauma and volutrauma
• False elevation of central venous and pulmonary artery occlusion pressures
• Misleading plateau pressure and compliance measurements

Severe cases are fatal within minutes. Complications: cardiovascular collapse, pneumothorax, ineffective triggering with patient-ventilator asynchrony, and hypercapnia.

• Hypotension and tachycardia soon after starting positive pressure ventilation or after vigorous manual bagging
• Visible inspiratory effort without a breath, and use of accessory and abdominal muscles during expiration
• Wheeze and prolonged expiration, with a hyperinflated chest
• Rising peak and plateau pressures, and rising PaCO2
• Pulsus paradoxus, neck vein distension and reduced breath sounds
• Red flags: sudden hypotension or cardiac arrest in an asthmatic or chronic obstructive pulmonary disease patient on a ventilator

1. Flow-time waveform on the ventilator: expiratory flow that does not return to zero before the next inspiration begins.
2. End-expiratory hold maneuver: occlude the expiratory port for at least 3 to 5 seconds at end-expiration in a passive (sedated or paralyzed) patient. The pressure rise above the set positive end-expiratory pressure is the auto-PEEP, and total PEEP is the plateau reading. Values above 5 cmH2O are significant, and above 15 cmH2O are dangerous.
3. Esophageal pressure measurement to quantify the effort needed to trigger breaths in spontaneously breathing patients.
4. Chest radiograph: hyperinflation, flattened diaphragms and a narrow heart. Echocardiography: small, hyperdynamic ventricles and a collapsed inferior vena cava.
5. Exclusion: tension pneumothorax, pericardial tamponade, massive pulmonary embolism, cardiogenic shock, anaphylaxis and hypovolemia.

Differentials: tension pneumothorax, tamponade, endobronchial intubation, endotracheal tube obstruction, anaphylaxis and sepsis.

1. Hemodynamic collapse: disconnect the patient from the ventilator for 30 to 60 seconds and press on the chest wall to expel trapped gas. If blood pressure recovers, auto-PEEP was the cause. Then reconnect with reduced rate and volume.
2. Resuscitation: IV 0.9% saline 500 mL bolus for hypotension (volume expands the venous return), and norepinephrine 0.05 to 0.5 micrograms/kg/min IV if needed.
3. Lengthen expiratory time:
   • Reduce respiratory rate to 8 to 12 breaths a minute.
   • Reduce tidal volume to 6 mL/kg predicted body weight (6 to 8 mL/kg).
   • Shorten inspiratory time and increase inspiratory flow to 60 to 80 L/min, giving an inspiratory to expiratory ratio of 1:4 to 1:5.
   • Accept permissive hypercapnia with pH of 7.20 or higher, and give sodium bicarbonate 50 to 100 mmol IV if pH falls below 7.15 with instability.
4. Reduce airflow obstruction:
   • Salbutamol 5 mg nebulized every 20 minutes for 3 doses (or 10 puffs of 100 micrograms through the circuit), with ipratropium 500 micrograms nebulized.
   • Methylprednisolone 1 mg/kg IV every 6 hours (maximum 125 mg) in asthma, or prednisolone 40 mg orally daily for 5 days in chronic obstructive pulmonary disease.
   • Magnesium sulfate 2 g IV over 20 minutes for severe asthma.
   • Ketamine 1 to 2 mg/kg IV for induction and sedation as a bronchodilating agent.
   • Aspirate secretions, use a larger endotracheal tube (at least 8 mm in adults), and remove the heat-moisture exchanger or resistive filter in favor of a heated humidifier.
5. Sedation and paralysis early: propofol 1 to 3 mg/kg/h IV, and cisatracurium 0.15 mg/kg IV bolus for severe asthma until the obstruction improves.
6. Spontaneously triggering patient with chronic obstructive pulmonary disease: set external positive end-expiratory pressure at 75 to 85% of the measured auto-PEEP (typically 5 to 10 cmH2O) to ease triggering. Do not use external PEEP in a passive patient with flow limitation.
7. Avoid vigorous manual bagging, and treat pneumothorax immediately if it develops.

Follow-up: daily end-expiratory hold and waveform checks until resolved, and spontaneous breathing trials once bronchospasm settles. Counsel patients with chronic obstructive disease on inhaler technique and on early review of exacerbations.