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 (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 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)
  • 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, PaO₂ = 150 mmHg – [PaCO₂ / 0.8]) can be helpful for determining the etiology (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 respiratory failure
  • high PaCO₂ (as measured by ABG) due to reduced alveolar ventilation from increased dead space (i.e. ventilated areas of the lung without perfusion) or decreased minute ventilation
  • 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, anaphylaxis, 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, but few new modes have shown a definitive benefit over the basic modes below used for most patients.

• assist–control ventilation (AC) / continuous mandatory ventilation (CMV): ventilator sets 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 reached (end inspiratory pressure varies with lung compliance, but there is a pressure limit beyond which the ventilator will not deliver more gas)
  • benefits: more control over ventilation, best for ARDSNet ventilation (see section on ARDS)
  • drawbacks: ventilator–patient dyssynchrony (i.e., patient does not synchronize their breathing with the ventilator cycle), risk of lung injury from airway high pressures due to dyssynchrony 
• pressure-controlled (PC): set airway pressure; ventilator delivers breath until set pressure reached (tidal volume varies with lung compliance)
  • benefits: variable flow during inspiration, less dyssynchrony
  • drawbacks: no guaranteed minute ventilation
• 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 okay.
• Use low tidal volumes to avoid ventilator-induced lung injury.
• Use the lowest fraction of inspired oxygen (F_IO₂) 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 always 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:

Figure 3. Ventilatory Strategies. Shown are strategies for the use of a ventilator in a patient with ARDS (Panel A), a heart-beating organ donor (Panel B), and a patient with normal lungs (Panel C). A protective ventilation strategy is defined as one in which the goal is to minimize the injury that can be caused by mechanical ventilation; components of this strategy include minimization of end-inspiratory stretching and minimization of injury caused by ventilation at low lung volumes. A protective lung strategy includes a protective ventilation strategy plus approaches to minimize derecruitment of the lung (e.g., the use of continuous positive airway pressure during apnea tests and the use of closed circuits during suctioning). There is currently no evidence showing that any mode of ventilation is better than any other in delivering the tidal volume of 6 ml per kilogram of predicted body weight (PBW) or limiting the plateau pressure. There is less evidence for the strategies for heart-beating organ donors and patients with normal lungs in the intensive care unit (ICU) than for the strategies for patients with ARDS7 and for anesthetized patients undergoing major abdominal surgery. Rescue therapy refers to treatments that may improve oxygenation in life-threatening situations but for which there are insufficient data clearly showing improved clinical outcomes. Some of these treatments have been shown to be ineffective in terms of clinical outcomes (e.g., the use of nitric oxide and high-frequency ventilation), whereas others have not been adequately evaluated (e.g., extracorporeal support). Their use should be carefully considered before they are implemented. PEEP denotes positive end-expiratory pressure, and P/F ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen.
(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
• interruption of sedation before 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 pressure support). 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.

Transition from Mechanical Ventilation to Spontaneous Breathing

(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.

• standard oxygen delivery
• Includes nasal cannula, oxygen mask, reservoir mask, nonrebreather mask that provide escalating amounts of F_IO₂.
• high-flow nasal cannula
  • Unlike standard nasal cannula, can deliver humidified oxygen at a precisely set F_IO₂ up to rates of 50 liters per minute.
  • 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.
• noninvasive intermittent positive pressure ventilation (NIPPV)
  • NIPPV includes BiPAP (bilevel positive airway pressure) and CPAP (continuous positive airway pressure). The machine provides, via a tightly fitted facemask, a baseline continuous positive pressure and a higher level of positive pressure during inspiration.
  • BiPAP has been shown to reduce the need for intubation in acute hypoxemic respiratory failure from cardiogenic pulmonary edema and in acute hypercapnic respiratory failure (e.g., chronic obstructive pulmonary disease [COPD] exacerbation).
  • Note: NIPPV is contraindicated in patients with altered mental status, aspiration risk, and ARDS.