Respiratory Failure and Ventilation

Acute respiratory failure is a common reason for admission to the intensive care unit (ICU). Patients may arrive requiring support to oxygenate arterial blood (as reflected by low partial pressure of arterial oxygen [PaO₂]) or to achieve adequate ventilation (as reflected by high partial pressure of arterial carbon dioxide [PaCO₂]). In this section, we cover the following approaches to management of respiratory failure:

• Mechanical (Invasive) Ventilation

• Noninvasive Ventilation

Mechanical (Invasive) Ventilation

A patient may need endotracheal intubation for many reasons. The broad categories include:

• hypoxemic respiratory failure

  • low PaO₂ (as measured by arterial blood gas [ABG] or by proxy low arterial oxygen saturation [SaO₂]), which may be due to partial pressure of inspired oxygen, hypoventilation, intracardiac or intrapulmonary shunting, ventilation-perfusion (V/Q) mismatch, or oxygen diffusion limitation (very rare at sea level)

  • V/Q mismatch is often associated with a high alveolar-arterial oxygen gradient (A-a gradient); for sea-level breathing ambient air and a body temperature of 37°C, mean alveolar oxygen tension can be calculated as PAO₂ = 150 mm Hg − [PaCO₂ / 0.8]); A-a gradient is PAO₂ − PaO₂. Calculating this gradient can be helpful for determining the etiology of hypoxemia (a normal A-a gradient varies with age and can be estimated as the age in years divided by 4 plus 4)

  • common causes: pulmonary embolism, acute respiratory distress syndrome (ARDS), pneumonia

• hypercarbic (hypercapnic) respiratory failure

  • high PaCO₂ (as measured by ABG or by proxy venous blood gas) due to reduced alveolar ventilation from increased dead space (i.e., ventilated areas of the lung without perfusion) or decreased minute ventilation (tidal volume × respiratory rate)

  • comparing a patient’s current PaCO₂ to a prior value can be helpful in someone with chronic hypercarbia

  • possible causes: severe asthma, chronic obstructive pulmonary disease (COPD), obstructive sleep apnea (OSA), neuromuscular disease, decreased respiratory drive (e.g., overdose or stroke)

• mixed hypercarbic and hypoxemic respiratory failure

  • examples: neuromuscular disease with pneumonia, overdose with pulmonary aspiration

• airway protection

  • in patients with preserved respiratory drive and mechanics but at risk of aspiration due to altered mental status or upper-airway compromise

  • possible causes: upper-airway problem (e.g., tonsillitis, angioedema, or epiglottitis), intoxication or overdose, neuromuscular disease impacting the bulbar muscles, encephalopathy

Ventilator Settings

Mechanical ventilators and their modes have become increasingly complicated in recent years and can vary among brands. The following are the basic modes used in most intubated patients:

• assist-control ventilation (AC)/continuous mandatory ventilation ventilator sets the fraction of inspired oxygen (FiO₂) and a minimum respiratory rate (patient can trigger more); ventilator supports all breaths with either volume or pressure cycling as follows:

  • volume-controlled (VC): set tidal volume; ventilator delivers breath until set tidal volume is reached (end-inspiratory pressure varies with lung compliance)

    • benefits: more control over ventilation, best for ARDSNet ventilation (see ARDS in this rotation guide)

    • drawbacks: ventilator-patient dyssynchrony (i.e., patient does not synchronize their breathing with the ventilator cycle), risk of lung injury from high airway pressures due to dyssynchrony

    • as a safety feature in the VC mode, most ventilators either stop the inspiratory gas flow when the “cutoff pressure” is reached or change the delivery system such that gas is delivered for the remainder of the ventilatory cycle at the set pressure

  • pressure-controlled (PC): set airway pressure; ventilator delivers breath until set pressure is reached (tidal volume varies with lung compliance)

    • benefits: variable flow during inspiration, less dyssynchrony

    • drawbacks: no guaranteed minute ventilation, unable to guarantee low tidal volume ventilation (see ARDS in this rotation guide)

• pressure support ventilation (PSV): for spontaneously breathing patients; used to wean from mechanical ventilation; delivers a set level of airway pressure (usually 5-15 cm H₂O above positive end-expiratory pressure [PEEP] during inspiration) to augment spontaneous breath; patient controls duration, respiratory rate, and tidal volume

General Principles

• Titrate the ventilator settings to acceptable physiological parameters (i.e., some degree of hypoxemia and hypercapnia is acceptable).

• Use low tidal volumes to avoid ventilator-induced lung injury.

• Use the lowest fraction of inspired oxygen (FiO₂) to maintain oxygenation at 90%-92%. Positive end-expiratory pressure (PEEP) can recruit collapsed lung units and improve oxygenation.

• Note: If the patient’s respiratory status acutely decompensates or the ventilator is sounding an alarm or malfunctioning, you can collaborate with the respiratory therapist to disconnect the patient from the ventilator and manually ventilate with a bag-valve mask while you troubleshoot.

The following algorithm can help you choose a ventilation strategy:

(Source: Ventilator-Induced Lung Injury. N Engl J Med 2013.)

Weaning Patients from the Ventilator

When the underlying cause for mechanical ventilation starts to improve, it’s time to think about weaning. Being on a ventilator can cause serious harm. The following are some evidence-based strategies that can reduce the duration of mechanical ventilation:

• low tidal volumes (6 mL/kg of ideal body weight), especially in ARDS

• daily interruption of sedation (i.e., a spontaneous awakening trial)

• daily assessment of readiness for spontaneous breathing trial

• early physical and occupational therapy

• minimum or no sedation

• conservative fluid management, especially in acute lung injury

• strategies to reduce ventilator-associated pneumonia

Weaning strategy: All mechanically ventilated patients should, when appropriate, have a daily spontaneous-awakening trial (SAT; i.e., interruption of sedatives) with a paired spontaneous-breathing trial (SBT). In the Awakening and Breathing Controlled trial, this SAT/SBT strategy resulted in more days breathing without assistance, shorter ICU length of stay, shorter hospital length of stay, and lower mortality than standard of care. However, there is no one-size-fits-all strategy for liberating patients from the ventilator. Much variation exists for determining when a patient can be weaned and extubated, balancing a shorter duration on ventilation with a higher likelihood of needing reintubation.

The following is one possible algorithm to determine when to extubate. An SBT may involve placing a patient on continuous positive airway pressure (CPAP) at 5 cm H₂O or ventilation through a T-piece (no CPAP). Some clinicians calculate a rapid shallow breathing index (RSBI; respiratory rate divided by tidal volume in liters) at the end of an SBT to help determine readiness; the lower the RSBI, the more likely a patient is to succeed. An RSBI <105 is a typical cut-off.

(Source: Weaning Patients from the Ventilator. N Engl J Med 2012.)

Noninvasive Ventilation

When a patient has imminent respiratory failure, but before he/she progresses to requiring endotracheal intubation, other methods of providing respiratory support can temporize or even prevent the need for conventional mechanical ventilation.

• low-flow oxygen delivery

  • includes nasal cannula, oxygen mask, reservoir mask, nonrebreather mask that all deliver FiO₂ greater than ambient air as oxygen flow levels (up to 15 liters/min) often improve a patient’s oxygenation; flows above 15 liters/min (up to flows of 60 liters/min) are usually considered “high-flow”

• high-flow nasal cannula

  • Unlike standard nasal cannula, high-flow nasal cannula can deliver humidified oxygen at a precisely set FiO₂ up to rates of 60 liters per minute.

  • High-flow nasal cannula can generate low levels of PEEP in the upper airway and flush out CO₂ to decrease dead space.

  • The FLORALI trial showed that high-flow nasal cannula can reduce 90-day mortality in patients with nonhypercapnic acute hypoxemic respiratory failure.

  • Several trials have shown that high-flow nasal cannula can reduce reintubation rate when utilized immediately upon extubation in certain patient groups.

• noninvasive positive pressure ventilation (NIPPV; see 2022 NEJM review)

  • NIPPV includes BiPAP (bilevel positive airway pressure) and CPAP (continuous positive airway pressure). The machine provides, via a tightly fitted face mask, nasal pillow device, or helmet, a baseline continuous positive pressure, and during BiPAP an additional higher level of positive pressure during inspiration.

  • NIPPV has been shown to reduce the need for intubation in acute hypoxemic respiratory failure from cardiogenic pulmonary edema and in other types of acute hypercapnic respiratory failure (e.g., chronic obstructive pulmonary disease [COPD] exacerbation).

  • NIPPV use upon extubation has reduced the need for reintubation and has helped to facilitate earlier ventilator weaning in patients with COPD.

  • Note: NIPPV is contraindicated in patients with altered mental status, aspiration risk, and ARDS.