Introduction

Acute respiratory failure (ARF) is defined as the failure of gas exchange in the respiratory system, including oxygenation and carbon dioxide elimination. ARF can occur within minutes or hours and can cause either hypoxemia, hypercapnia or both.¹ 

There are two types of ARF: 

• Type 1 – Occurs when the respiratory system cannot provide the body with enough oxygen, which can cause hypoxemia
• Type 2 – Occurs when the respiratory system cannot remove enough carbon dioxide from the body, which can cause hypercapnia 

Several factors affecting the respiratory system can increase the risk of ARF, including abnormalities in the:

• Upper and lower respiratory tracts
• Central and peripheral nervous systems
• Chest wall 
• Muscles used in respiration²

Pathophysiology of acute respiratory failure

Several mechanisms contribute to impaired oxygenation and ventilation in ARF.

Mechanisms affecting oxygenation (Type 1 ARF)

Diffusion defect

When there are structural changes to the alveoli, like decreased surface area or increased thickness, it can cause a diffusion defect. This defect can stop proper diffusion of oxygen across the membrane, and some causes of it are emphysema and interstitial lung disease. 

Ventilation/perfusion mismatch

Ventilation/perfusion (V/Q) mismatch can also cause damaged oxygenation as the ratio depends on ventilation and blood flow. When this ratio is mismatched, it can cause type 1 respiratory failure. Some causes of V/Q mismatch include chronic obstructive pulmonary disease (COPD) and pulmonary embolism. 

Right-to-left shunt

A right-to-left shunt can also disrupt gas exchange, but unlike V/Q mismatch, it doesn’t improve with supplemental oxygen therapy. This can be the result of arteriovenous malformation, severe pneumonia, and severe pulmonary oedema. 

Mechanisms affecting ventilation (Type 2 ARF)

Respiratory pump failure

Respiratory pumps are made up of the chest wall, pulmonary parenchyma, respiratory muscles, and the central and peripheral nervous systems. If any of these components fail, it can cause the pump to become unable to ventilate properly. Some causes of the pump failures are muscle abnormalities, chest wall disorders, and pleural diseases. 

Increased dead space

Dead space in the lungs is the area that doesn't undergo gas exchange. Sometimes these areas of dead space can increase and this can lead to higher carbon dioxide levels. This can be the result of COPD. 

Excess carbon dioxide production

Increased carbon dioxide levels can be the result of fever exercise and sepsis, and this can also cause hypercapnia.²

Clinical presentation of acute respiratory failure

ARF presents with various signs and symptoms, depending on whether it primarily affects oxygenation (hypoxemia) or ventilation (hypercapnia). 

Low oxygen levels (hypoxemia) can cause:

• Shortness of breath (dyspnea) 
• Drowsiness 
• Bluish colour on the fingers, toes, and lips (cyanosis) 
• Difficulty with routine activities such as dressing 

High carbon dioxide levels (hypercapnia) can cause: 

• Blurred vision
• Confusion 
• Headaches 
• Rapid breathing (tachypnoea)³

Diagnosis of acute respiratory failure

To be able to diagnose acute respiratory failure the patient must undergo a physical examination. The criteria of this examination can include:

• A general inspection where the examiner will look at accessory muscle use, altered mental status, dyspnea, diaphoresis, fever, respiratory distress, obesity, and pursed-lip breathing 
• Head examination – The head may have a Cushingoid appearance, central cyanosis, and pale conjunctiva
• Neck examination – There could be jugular venous distention, lymphadenopathy, and tracheal deviation in the neck 
• Thoracic examination – In the thorax, you might be able to hear certain sounds when breathing such as crackles, bronchial breath sounds, and wheezes, along with reduced chest expansion, tachypnea (shortness of breath), and vocal resonance 
• Abdomen – Abnormal growth of the liver in the abdomen
• Extremities:
  • Upper limbs – Digital clubbing, peripheral cyanosis, tremors
  • Lower limbs – Oedema, peripheral cyanosis, unilateral swelling²

Diagnostic tests

There are a range of diagnostic tests that can be performed for acute respiratory failure, the main one being arterial blood gas analysis. This test is used to measure both oxygenation and ventilation so it can be used to diagnose both hypoxemia and hypercapnia. Healthcare professionals can use the information presented to differentiate between acute and chronic respiratory failure by looking at the renal response.² Lung imaging, such as CT scans and chest x-rays, can also be used to create images of the lungs, so any damage can be seen and located. You can also check for conditions such as pneumonia, heart failure, and the causes of any other lung issues.⁴ Bacterial cultures can be used to test for bacterial infections.⁵

Managing acute respiratory failure in ICU settings

Initial stabilisation in ICU

Once admitted to the ICU, patients suffering from acute respiratory failure need to undergo initial stabilisation. This starts with an assessment of the airways, and ensuring the patient can maintain enough oxygen. The physician will also decide if the patient requires intubation or any securing of their airways. High-flow nasal cannula (HFNC) may be required as a form of oxygen therapy. This is where warmed humidified oxygen is administered to the patient via a nasal cannula. Non-invasive positive pressure ventilation (NIV) is the most common type of ventilation. Some examples of non-invasive ventilation include continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BiPAP), and these are standard care for moderate-severe cases of COPD and pulmonary oedema. These both can improve oxygenation, reduce the work of breathing, and reduce the need for intubation.⁶

Mechanical ventilation 

Mechanical ventilation works by using positive airway pressure to push gas into the lungs and is made up of four stages: 

• Trigger Phase – Initiates inhalation, either by the patient or ventilator settings
• Inspiratory Phase – Air intake into the lungs
• Cycling Phase – A temporary pause in inhalation
• Exhalation Phase – The release of air from the lungs

Modes of Mechanical Ventilation

Ventilation can be administered through multiple different modes including either mandatory or assisted. Assisted ventilation is when the patient exhibits inspiratory efforts which triggers mechanical ventilation to deliver a breath. Mandatory ventilation is the cycle of breathing that will continue in the machine, even if the patient doesn’t trigger the initial stage.⁷

Monitoring and Adjustments

While a patient is attached to mechanical ventilation it’s important to keep monitoring them to ensure there are no issues. Key monitoring techniques include:

• Hemodynamic monitoring – Measures blood pressure, central venous pressure, and fluid balance
• ECG – Monitors beating frequency, cardiac rhythm and ischaemic episodes 
• Pulmonary artery catheters – An indicator for adequate fluid resuscitation, and it can be used for both diagnosis and therapy
• Arterial pressure monitoring – The continuous measurement of blood pressure and can be used for sampling blood gases. A strong variation in amplitude can be an indicator of volume deficiency⁸

Physicians may also put patients on oxygenation and ventilation monitoring. 

• Pulse oximetry is a noninvasive continuous monitor that can estimate the oxygen saturation of arterial haemoglobin indicating hypoxemia
• Capnography is the continuous monitor for ventilation and measures the concentration of carbon dioxide in respiration indicating hypoxemia⁹ 

Complications of management

Ventilators may cause complications including ventilator-associated pneumonia. This is defined as when pneumonia develops more than 48 hours after the patient is put on mechanical ventilation. This is normally treated with antibiotics, but with the growing problem of antibiotic resistance other methods of treatment are needed.¹¹ Other complications may include deep vein thrombosis (DVT), stress ulcers, and ICU delirium.

Weaning and extubation

Patients are assessed using spontaneous breathing trials (SBT) to test if they are ready for weaning. The initial stage consists of sedating the patient and conducting daily assessments to see if they are ready for weaning. These assessments are usually done in the ICU and are performed on every stable patient with indications that their need for mechanical ventilation has been resolved. Once the assessments are completed and have positive results, the sedation is either reduced to a minimum or completely removed until the patient is awake, cooperative, and comfortable. 

The next and final stage is performing the actual SBT. The ventilator support is reduced to a minimum via T-piece or pressure support. The SBT is performed for 30-120 minutes and the patient is monitored for respiratory distress. If the patient begins to struggle, they are immediately put back onto the ventilator. Once the SBT is performed and the patient meets the successful criteria, an assessment for airway removal will be done by carrying out a cuff leak test.

The criteria for extubation includes:

• Resolution of the cause of intubation and mechanical ventilation
• Patients must be able to perform and maintain sufficient gas exchange on their own without any external help
• No auto-PEEP (positive end-expiratory pressure)
• The patient must have an adequate cardiovascular reserve
• There must be no secretion of liquids into the endotracheal tube (ET tube) that can cause high airway resistance and obstruction after extubation
• The patient must be able to protect their own airways without any external help¹⁰

Post-ICU care and long-term outcomes

Post-ICU care can consist of physiotherapy, pulmonary rehabilitation, and managing post-intensive care syndrome (PICS). PICS is a long-term health problem, which can consist of physical, cognitive, and mental impairments. This can lead to difficulty with self-care, low quality of life, and difficulty returning to work or engaging in social activities. All these factors can lead to long-term health outcomes.¹²

Summary

Acute respiratory failure is caused by damage to gas exchange, leading to hypoxemia and hypercapnia. This can be the result of various mechanisms including structural changes in the lungs, ventilation/perfusion mismatch, and respiratory pump failure. Diagnosis is based on physical exams, arterial blood gas, and imaging. However, ARF management can be complicated by ventilation-associated risks such as pneumonia, deep vein thrombosis, and ICU delirium.

Weaning from mechanical ventilation is assessed through spontaneous breathing trials to ensure patients can breathe independently. Following extubation, rehabilitation is essential to improve long-term outcomes. Early intervention and appropriate management are crucial for recovery and reducing complications.