12th January 2024

Respiratory Failure and NIV

Background

Respiratory failure is defined as abnormalities of oxygenation or ventilation (CO2 elimination) leading to respiratory dysfunction severe enough to threaten the function of vital organs. 

Respiratory failure is one of the most common reasons for ICU admission, resulting in up to ≥ 75% of ICU patients requiring mechanical ventilation during their stay. 

Types and Pathophysiology

Respiratory failure is generally categorised into two types depending on the underlying pathophysiology and subsequent arterial blood gas picture.

Type 1 Respiratory Failure

When blood flows through a diseased part of the lung (e.g. asthma, pneumonia) with impaired ventilation, it results in hypoxic and CO2-rich blood entering pulmonary veins. Increased ventilation of neighbouring regions of normal lung can increase CO2 excretion but cannot augment oxygen uptake as the haemoglobin flowing through these normal regions is already fully saturated. 

Hence, blood coming from normal and under-ventilated areas results in hypoxia with normocapnia, leading to Type 1 respiratory failure.

Type 2 Respiratory Failure

This happens when there is not sufficient normal lung left to correct PaCO2, leading to severe ventilation – perfusion mismatch. Seen in conditions that cause generalised reduction in total ventilation, including diseases of the lung as well as neuromuscular disorders. 

Interpreting blood gas abnormalities in Respiratory Failure 

Table 1: Blood gas interpretation in respiratory failure

Ralston, S. H., Penman, I. D., Strachan, M. W. J., & Hobson, R. (Eds.). (2018). Davidson’s Principles and Practice of Medicine (23rd ed.). Elsevier Health Sciences

Aetiology

Acute Respiratory Failure may occur in a variety of pulmonary and non-pulmonary disorders. A few selected causes are mentioned here: 

Table 2: Various causes of respiratory failure

Chesnutt A.N., & Chesnutt M.S., & Prendergast N.T., & Prendergast T.J. (2021). Acute respiratory failure. Papadakis M.A., & McPhee S.J., & Rabow M.W.(Eds.), Current Medical Diagnosis & Treatment 2021. McGraw Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2957&sectionid=249370465

References

• Chesnutt A.N., & Chesnutt M.S., & Prendergast N.T., & Prendergast T.J. (2021). Acute respiratory failure. Papadakis M.A., & McPhee S.J., & Rabow M.W.(Eds.), Current Medical Diagnosis & Treatment 2021. McGraw Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2957&sectionid=249370465 
• Kress J.P., & Hall J.B. (2018). Approach to the patient with critical illness. Jameson J, & Fauci A.S., & Kasper D.L., & Hauser S.L., & Longo D.L., & Loscalzo J(Eds.), Harrison’s Principles of Internal Medicine, 20e. McGraw Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2129&sectionid=188731384
• Ralston, S. H., Penman, I. D., Strachan, M. W. J., & Hobson, R. (Eds.). (2018). Davidson’s Principles and Practice of Medicine (23rd ed.). Elsevier Health Sciences.
• Fitzpatrick M.E., & Prendergast N.T., & Rivera-Lebron B (2022). Acute respiratory failure. Papadakis M.A., & McPhee S.J., & Rabow M.W., & McQuaid K.R.(Eds.), Current Medical Diagnosis & Treatment 2022. McGraw Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=3081&sectionid=25896008

Presentation

Symptoms and Signs of Acute Respiratory Failure are those of the underlying disease combined with those of hypoxaemia and hypercapnia. Clinical features are often non-specific; hence, attention must be paid to serial ABGs. 

Table 3: Signs and symptoms of hypoxaemia vs hypercapnia

Rupa Bessant, & Jonathan Birns, & Charlotte Ford (2020). Non-invasive ventilation. Richard Lee., & Brintha Selvarajah, & William L. G. Oldfield, Medicine For MRCP 2020. OXFORD University Press

Differential Diagnosis

Due to varied clinical presentations of respiratory failure, a few differentials must be ruled out. 

Type 1 respiratory failure (T1RF)

T1RF must be differentiated from other diseases that cause hypoxia, such as acute decompensated heart failure, adult respiratory distress syndrome (ARDS), pulmonary oedema, pneumonia and lung fibrosis.

Type 2 respiratory failure (T2RF)

Different pathologies leading to hypercapnia should be ruled out, including COPD, opioid toxicity, myasthenia gravis, status asthmaticus, Guillain barre syndrome, etc.

Management

Treatment of Acute Respiratory Failure

Patients identified to be in acute respiratory failure need to be managed in a stepwise ABCDE approach.

Acute management of Type 1 respiratory failure (TR1F)

In type 1 respiratory failure (oxygenation deficit), high concentration oxygen will usually relieve hypoxia by increasing alveolar PO2. In some cases, however, mechanical ventilation may be needed where there is severe alveolar damage.

One of the treatment options for T1RF, which has not responded to medical management, is to give Continuous Positive Airways Pressure (CPAP). It involves delivering a continuous positive pressure during both inspiration and expiration, improving oxygenation and reducing work of breathing. It is usually indicated in worsening T1RF, acute cardiogenic pulmonary oedema and sometimes in obstructive sleep apnoea (domiciliary setting).

Acute management of Type 2 respiratory failure (T2RF)

Type 2 respiratory failure in an acute setting is an emergency needing immediate management, including ventilatory support.

It is very important to identify the underlying condition precipitating T2RF and to treat it with pharmacological interventions first. For instance, in patients with COPD, initial management will involve rigorous use of back-to-back nebulisers, intravenous antibiotics, steroids and serial ABGs for monitoring. If the pH, Pco2 and PO2 do not show a convincing improvement after 1 hour of active medical management, non-invasive ventilation should be considered.

Non-Invasive Ventilation (NIV)

It uses a tightly fitted mask covering nose and mouth and delivers higher inspiratory pressures, which increase tidal volume and lower expiratory pressures at the end of expiration.

NIV Settings

Initial inspiratory positive airway pressure (IPAP) is generally set at 12cmH2O, and the expiratory positive airway pressure (EPAP) at 4cmH2O.

There can be an increment of 2-4 cmH2O in IPAP every 15 minutes to achieve an adequate tidal volume. 

Indications for NIV 

1. Acute exacerbation of COPD (T2RF not responding to optimal medical management)
2. Decompensated obstructive sleep apnoea/obesity hypoventilation syndrome
3. T2RF secondary to neuromuscular weakness or chest wall deformity (Myasthenia Gravis, Guillain-Barre syndrome, amyotrophic lateral sclerosis) 

Contraindications for NIV 

1. Impaired consciousness or confusion (GCS <8)
2. Life-threatening hypoxemia 
3. Facial surgery or burns
4. Undrained pneumothorax
5. Vomiting

Invasive Ventilation

Involves endotracheal intubation in cases where medical and non-invasive methods to improve ventilation have failed.

Treatment of chronic and acute-on-chronic Respiratory Failure 

COPD has been identified as the most common cause leading to chronic type 2 respiratory failure. Body compensates for persistently raised PaCO2 by retaining bicarbonate and hence correcting acidaemia. In acute-on-chronic respiratory failure, this balance is disturbed, resulting in an acidic pH followed by drowsiness and coma. These patients lose their chemosensitivity to raised PaCO2 and, hence, paradoxically depend on hypoxia for respiratory drive and are at risk of respiratory depression if given high concentrations of oxygen. Such patients may not appear distressed despite being critically ill. Attention must, therefore, be paid to serial ABGs and signs of CO2 narcosis (flapping tremors, bounding pulse, confusion).

The key to treatment is to identify and treat the precipitating cause while trying to achieve a safe PaO2 (>7.0 kPa) without increasing PaCO2 and acidosis. Patients who are conscious and have adequate respiratory drive may benefit from non-invasive ventilation. Patients on the other side of the spectrum who are drowsy with low respiratory effort will need an urgent decision regarding intubation and ventilation. The decision is sometimes challenging and factors like patient’s and family wishes, functional status and quality of life must be considered.

Domiciliary care for Chronic Respiratory Failure 

Advanced lung conditions like COPD, cystic fibrosis or certain neuromuscular deformities / central alveolar hypoventilation can lead to chronic respiratory failure. NIV can be of great value in such patients. In initial stages, patients may only require ventilatory support during the night when there is a physiological decline in ventilatory drive. However, over time, CO2 retention becomes chronic with renal compensation of acidosis, and in advanced cases, daytime NIV can also be helpful.

Single/double lung transplantation is reserved for patients with advanced chronic disease after exhaustion of all available medical treatment. 

Palliative Care in Respiratory Failure

Early palliative care can relieve suffering and provide support for people with end-stage respiratory failure. Interventions to provide symptomatic management of dyspnoea, cough, haemoptysis, excessive respiratory secretions, fatigue and anxiety should be offered, especially as the end of life nears.

Regular doses of short-acting morphine 2-4 hourly can help relieve both the pain and sense of breathlessness. Low dose of an anxiolytic-like Lorazepam may help alleviate anxiety/agitation. Certain anticholinergics like Glycopyrronium can help get rid of excessively troublesome respiratory secretions. 

Respiratory stimulant drugs like Doxapram can be tried in patients not suitable for ventilation.

References

• Rupa Bessant, & Jonathan Birns, & Charlotte Ford (2020). Non-invasive ventilation. Richard Lee., & Brintha Selvarajah, & William L. G. Oldfield, Medicine For MRCP 2020. OXFORD University Press 
• Robertson HT. Dead space: the physiology of wasted ventilation. Eur Respir J. 2015 Jun;45(6):1704–16. Epub 2014 Nov 13. Erratum in: Eur Respir J. 2015 Oct;46(4):1226.
• Ralston, S. H., Penman, I. D., Strachan, M. W. J., & Hobson, R. (Eds.). (2018). Davidson’s Principles and Practice of Medicine (23rd ed.). Elsevier Health Sciences.