Residency · Residency · Internal Medicine

Acute Respiratory Failure and Mechanical Ventilation Basics

Definitions and Classification

Acute Respiratory Failure

Acute respiratory failure occurs when the respiratory system fails to maintain adequate gas exchange.

Type 1 (hypoxemic) failure is defined by a PaO2 below 60 mmHg on room air and results from V/Q mismatch, shunt, diffusion impairment, or low inspired FiO2. Common causes include pneumonia, ARDS, pulmonary embolism, pulmonary edema, and interstitial lung disease. Type 2 (hypercapnic) failure is defined by a PaCO2 above 45 mmHg with respiratory acidosis and results from alveolar hypoventilation -- either decreased minute ventilation or increased dead space. Common causes include COPD exacerbation, neuromuscular disease (myasthenia gravis, Guillain-Barre syndrome), chest wall disorders, drug overdose (opioids, sedatives), and obesity hypoventilation syndrome. Type 3 (perioperative) failure results from a combination of atelectasis and reduced functional residual capacity. Type 4 (shock-related) failure occurs when hypoperfusion of respiratory muscles leads to ventilatory failure.

Key Physiologic Concepts

The alveolar gas equation (PAO2 = FiO2 times (Patm minus PH2O) minus PaCO2/R, where R is the respiratory quotient of approximately 0.8) is used to calculate the A-a gradient (PAO2 minus PaO2). The normal A-a gradient is approximately 2.5 plus (age divided by 4). An elevated A-a gradient indicates V/Q mismatch, shunt, or diffusion impairment, while a normal A-a gradient points to hypoventilation or low FiO2. The P/F ratio (PaO2 divided by FiO2) provides rapid assessment of oxygenation severity: normal is above 400, mild ARDS is 200 to 300, moderate ARDS is 100 to 200, and severe ARDS is below 100. Shunt, in which blood bypasses ventilated alveoli, is distinguished from V/Q mismatch by its failure to correct with supplemental oxygen.

Supplemental Oxygen Delivery Systems

Low-Flow Systems

Nasal cannula delivers 1 to 6 L/min, providing approximately 24 to 44 percent FiO2, with each liter per minute adding roughly 4 percent FiO2. A simple face mask operates at 5 to 10 L/min and delivers 35 to 55 percent FiO2. A non-rebreather mask uses 10 to 15 L/min with a reservoir bag and one-way valves to achieve 60 to 90 percent FiO2.

High-Flow Systems

The Venturi mask provides precise FiO2 delivery ranging from 24 to 60 percent, making it particularly useful in COPD where avoiding oxygen-induced hypercapnia is important. High-flow nasal cannula (HFNC) can deliver up to 60 L/min at a controlled FiO2 and temperature. Its benefits include dead space washout, generation of low-level PEEP (approximately 3 to 5 cmH2O), reduced work of breathing, and improved mucociliary clearance. The FLORALI trial demonstrated that HFNC reduced 90-day mortality compared to standard oxygen and NIV in acute hypoxemic respiratory failure, particularly in the subgroup with P/F ratio below 200.

Non-Invasive Ventilation (NIV)

CPAP (Continuous Positive Airway Pressure)

CPAP provides constant positive pressure throughout the respiratory cycle. It recruits alveoli, improves functional residual capacity, and reduces both preload and afterload. There is strong evidence supporting its use in cardiogenic pulmonary edema (3CPO trial) and obstructive sleep apnea.

BiPAP (Bilevel Positive Airway Pressure)

BiPAP delivers separate inspiratory (IPAP) and expiratory (EPAP) pressures, with the difference between them (pressure support) assisting ventilation and reducing work of breathing. Strong evidence supports its use in COPD exacerbation with respiratory acidosis, where it reduces both intubation rates and mortality. Moderate evidence supports its use in immunocompromised patients with respiratory failure and for post-extubation support in high-risk patients.

Contraindications to NIV

NIV is contraindicated in cardiac or respiratory arrest, inability to protect the airway (decreased consciousness or excessive secretions), upper GI surgery or GI bleeding, facial trauma or surgery, and hemodynamic instability (with the exception of cardiogenic pulmonary edema).

Monitoring NIV Success

Reassessment should occur within 1 to 2 hours of initiating NIV. Signs of success include improved respiratory rate, reduced accessory muscle use, improved gas exchange, and patient comfort. Signs of failure that should prompt consideration of intubation include worsening respiratory rate, persistent hypoxemia, encephalopathy, and hemodynamic instability.

Indications for Intubation and Mechanical Ventilation

Intubation is indicated for failure to protect the airway (GCS of 8 or less, loss of gag or cough reflexes, excessive secretions), refractory hypoxemia despite maximal non-invasive support, refractory hypercapnia with respiratory acidosis and altered mental status, anticipated clinical deterioration (such as massive hemoptysis or expanding angioedema), and patient fatigue with impending respiratory arrest (tachypnea above 35, accessory muscle use, paradoxical abdominal movement).

Mechanical Ventilation Modes

Volume-Controlled Ventilation (VCV)

VCV delivers a set tidal volume at a set rate, guaranteeing minute ventilation. Its disadvantage is that airway pressure is variable, creating a risk of barotrauma if compliance worsens. Peak inspiratory pressure and plateau pressure should be monitored closely.

Pressure-Controlled Ventilation (PCV)

PCV delivers a set inspiratory pressure for a set time at a set rate. It limits peak airway pressure and uses a decelerating flow pattern that may improve gas distribution. Its disadvantage is that tidal volume varies with changes in compliance and resistance.

Pressure Support Ventilation (PSV)

PSV is patient-triggered, pressure-targeted, and flow-cycled, meaning it supports only spontaneous breaths. It is used for weaning and spontaneous breathing trials. The clinician sets the pressure support level and PEEP, while the patient controls rate, tidal volume, and inspiratory time.

SIMV (Synchronized Intermittent Mandatory Ventilation)

SIMV combines mandatory breaths with spontaneous breaths. It was historically used for weaning but has been largely replaced by PSV, which has been shown to wean patients faster.

Initial Ventilator Settings

General Starting Parameters

The typical starting mode is volume-controlled assist/control or pressure control. Tidal volume should be set at 6 to 8 mL/kg of ideal body weight (IBW), calculated as 50 plus 2.3 times (height in inches minus 60) for males and 45.5 plus 2.3 times (height in inches minus 60) for females. Respiratory rate is generally started at 14 to 18 breaths per minute and adjusted for target pH and PaCO2. FiO2 begins at 100 percent and is weaned rapidly to keep SpO2 at 92 to 96 percent. PEEP starts at 5 cmH2O and is increased as needed. Inspiratory flow rate is typically 60 L/min for VCV, and the I:E ratio is set at 1:2 to 1:3 to allow adequate expiratory time, especially in obstructive disease.

Disease-Specific Settings

In ARDS, tidal volume is lowered to 6 mL/kg IBW with higher PEEP and a target plateau pressure of 30 cmH2O or less. In obstructive disease (COPD, asthma), a lower respiratory rate with longer expiratory time (I:E ratio 1:3 to 1:4) is used, with monitoring for auto-PEEP. In metabolic acidosis, minute ventilation must be set high enough to match the patient's pre-intubation compensatory effort, as failure to do so can precipitate peri-intubation cardiac arrest.

Ventilator Waveforms and Troubleshooting

Key Waveforms

On the pressure-time waveform in VCV, peak pressure reflects both airway resistance and lung compliance, while plateau pressure (measured by an end-inspiratory hold) reflects compliance alone. On the flow-time waveform, expiratory flow should return to zero before the next breath; failure to do so indicates air trapping and auto-PEEP. The volume-time waveform is useful for identifying air leaks when exhaled volume is less than inhaled volume.

Critical Pressure Measurements

Peak inspiratory pressure (PIP) represents the total pressure including airway resistance. Plateau pressure (Pplat) reflects alveolar pressure and should be kept at or below 30 cmH2O. Driving pressure, calculated as Pplat minus PEEP, is ideally kept at or below 15 cmH2O and is the strongest independent predictor of mortality in ARDS. When PIP is elevated but Pplat is normal, the problem is increased airway resistance (bronchospasm, mucus plug, kinked tube). When both PIP and Pplat are elevated, the problem is decreased compliance (ARDS, pneumothorax, pleural effusion, abdominal distension).

Auto-PEEP (Intrinsic PEEP)

Auto-PEEP results from air trapping due to incomplete exhalation. It is detected by an end-expiratory hold maneuver or by observing that flow does not return to zero on the flow-time waveform. Management involves reducing the respiratory rate, reducing tidal volume, increasing expiratory time, and treating bronchospasm. Applying extrinsic PEEP at up to 80 percent of the measured auto-PEEP can reduce the triggering threshold and improve patient-ventilator synchrony.

ARDS: Acute Respiratory Distress Syndrome

Berlin Definition (2012)

ARDS is defined by timing within 1 week of a known insult or new/worsening respiratory symptoms, bilateral opacities on imaging not fully explained by effusions, atelectasis, or nodules, respiratory failure not fully explained by heart failure or fluid overload, and oxygenation impairment on PEEP of at least 5 cmH2O.

Lung-Protective Ventilation (ARDSNet Protocol)

Low tidal volume ventilation at 6 mL/kg IBW (range 4 to 8 mL/kg) is the cornerstone of ARDS management. The landmark ARDSNet ARMA trial demonstrated that this approach reduced mortality from 40 to 31 percent compared to 12 mL/kg IBW. Plateau pressure should be targeted at or below 30 cmH2O. PEEP is titrated using low or high PEEP/FiO2 tables; the ALVEOLI, LOV, and EXPRESS trials did not show a clear overall mortality benefit of higher PEEP but did suggest benefit in moderate-to-severe ARDS. Permissive hypercapnia, accepting elevated PaCO2 as long as pH remains above 7.20, is an expected consequence of low tidal volume ventilation. The FACTT trial demonstrated that a conservative fluid management strategy improved oxygenation and shortened time on the ventilator, though without a mortality difference.

Adjunctive Therapies in ARDS

Prone positioning for 16 or more hours per day in moderate-to-severe ARDS (P/F below 150) reduced mortality from 33 to 16 percent in the PROSEVA trial, with a number needed to treat of 6. For neuromuscular blockade, the ACURASYS trial showed benefit with cisatracurium in severe ARDS, but the larger ROSE trial showed no difference with early continuous neuromuscular blockade, so current practice favors selective use. The ART trial demonstrated harm with aggressive recruitment maneuvers combined with high PEEP, so these should be used cautiously. For ECMO, the EOLIA trial in refractory severe ARDS did not show definitive mortality benefit, though Bayesian analysis suggests probable benefit. Growing evidence supports early moderate-dose dexamethasone in moderate-to-severe ARDS, though optimal timing and patient selection remain debated.

Weaning and Liberation from Mechanical Ventilation

Readiness Assessment

Before attempting liberation, the underlying cause should be resolving or improved. Oxygenation should be adequate (FiO2 40 percent or less, PEEP 8 cmH2O or less, P/F above 200). The patient should be hemodynamically stable on no or low-dose vasopressors, have adequate mental status to protect the airway, and have no planned procedures requiring sedation.

Spontaneous Breathing Trial (SBT)

An SBT can be performed using a T-piece trial or low-level pressure support (5 to 8 cmH2O) for 30 to 120 minutes. Success is defined by respiratory rate below 35, stable heart rate, SpO2 of 90 percent or greater, absence of distress, and a rapid shallow breathing index (RSBI) below 105. The RSBI, calculated as respiratory rate divided by tidal volume in liters, below 105 predicts successful extubation. The ABC trial protocol, combining daily spontaneous awakening trials with daily SBTs, has been shown to reduce ventilator days and ICU length of stay.

Post-Extubation

High-risk patients benefit from prophylactic NIV or HFNC after extubation to reduce reintubation rates. Post-extubation stridor should be monitored for, and a cuff leak test can help identify patients at risk. Dexamethasone may be administered prophylactically in high-risk cases.

<image> A comprehensive diagram comparing the four types of respiratory failure. Each type is displayed in a quadrant with the mechanism of gas exchange impairment illustrated at the alveolar-capillary level. Type 1 shows V/Q mismatch and shunt with collapsed or fluid-filled alveoli. Type 2 shows decreased alveolar ventilation with normal alveoli but reduced airflow. Include corresponding ABG patterns (PaO2, PaCO2, A-a gradient) and common clinical causes for each type. </image>

<image> A ventilator waveform interpretation guide showing three panels of pressure-time, flow-time, and volume-time waveforms for volume-controlled ventilation. The first set shows normal waveforms with labeled PIP, plateau pressure, and PEEP. The second set shows elevated PIP with normal plateau (airway resistance problem). The third set shows elevated PIP with elevated plateau (compliance problem). The fourth set shows auto-PEEP with incomplete expiratory flow. Use color coding to highlight abnormalities. </image>

<image> An infographic summarizing the key ARDS management evidence. Display the ARDSNet lung-protective ventilation protocol centrally (6 mL/kg IBW, Pplat <= 30, PEEP/FiO2 ladder). Surround it with satellite boxes for each adjunctive therapy: prone positioning (PROSEVA trial), neuromuscular blockade (ACURASYS/ROSE), conservative fluids (FACTT), corticosteroids, and ECMO (EOLIA). Each box includes the trial name, key finding, and a green checkmark or yellow caution symbol indicating strength of evidence. </image>

Clinical Pearls

Always calculate ideal body weight for tidal volume settings, because using actual body weight in an obese patient leads to volutrauma and lung injury. Driving pressure (Pplat minus PEEP) is the best single ventilator parameter associated with mortality in ARDS and should be targeted at or below 15 cmH2O. In ARDS, prone positioning for 16 or more hours per day is one of the few interventions with a proven mortality benefit and should be implemented early in moderate-to-severe cases. When a ventilated patient suddenly desaturates, the DOPE mnemonic provides a rapid differential: Displacement of the tube, Obstruction (mucus plug), Pneumothorax, and Equipment failure. Peri-intubation cardiac arrest is a real risk in patients with severe metabolic acidosis who are compensating with high minute ventilation; their respiratory rate must be matched immediately post-intubation. Auto-PEEP is the hidden killer in obstructive lung disease on the ventilator -- if the patient is hypotensive, disconnecting the circuit briefly to allow full exhalation can be lifesaving. Daily paired spontaneous awakening trials and spontaneous breathing trials represent the evidence-based standard for ventilator liberation.

References

• ARDS Definition Task Force. Acute Respiratory Distress Syndrome: The Berlin Definition. JAMA. 2012.
• ARDSNet. Ventilation with Lower Tidal Volumes for ALI and ARDS. NEJM. 2000.
• Guerin C, et al. PROSEVA Trial: Prone Positioning in Severe ARDS. NEJM. 2013.
• National Heart, Lung, and Blood Institute. FACTT Trial: Fluid Management in ALI. NEJM. 2006.
• Papazian L, et al. ACURASYS Trial: Neuromuscular Blockade in Early ARDS. NEJM. 2010.
• Moss M, et al. ROSE Trial: Early Neuromuscular Blockade in ARDS. NEJM. 2019.
• Combes A, et al. EOLIA Trial: ECMO for Severe ARDS. NEJM. 2018.
• Frat JP, et al. FLORALI Trial: High-Flow Oxygen in Acute Hypoxemic Respiratory Failure. NEJM. 2015.
• Girard TD, et al. Efficacy and Safety of a Paired Sedation and Ventilator Weaning Protocol (ABC Trial). Lancet. 2008.

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