Respiratory failure overview

# Overview
Respiratory failure is the result of inadequate gas exchange, that in turn results in hypoxemia, hypercapnia or both. There is always an underlying disease that results in respiratory failure. The cause must be identified in order two classify, diagnose and treat respiratory failure. In some instances it could result in persistent lung injury. Mortality rate increases with age. The incidence is approximately 137.1 per 100,000 individuals in the united states. Common risk factors include smoking, alcohol abuse, COPD exacerbation and obesity. If left untreated patients with respiratory failure may develop sepsis and multiple organ failure which could result in death. It is diagnosed based on clinical presentation, the hallmark being shortness of breath which could be correlated with arterial blood gases to classify and diagnose the patient. Correction of ABG's by mechanical ventilation is the mainstay of treatment.

# Historical Perspective
The diagnostic and treatment strategies for respiratory failure and it's associated conditions have come a long way. Since the discovery of the stethoscope by Rene Laennec in 1816, to the work of Fenn and his team in 1946 on pulmonary gas exchange, the use of cuffed endotracheal tubes by Ibsen in 1954 to administer positive pressure ventilation to patients in respiratory failure who were admitted to the intensive care units, that became common in the United States in 1960.

# Classification
Respiratory failure may be classified into several subtypes as follows; Type I, Type II, Type III, Type IV.

# Pathophysiology
Respiratory failure is the result of inadequate gas exchange in the pulmonary circulation. This could result from lung failure or pump failure. Lung failure which is failure of gas exchange results in hypoxemia (PaCO2). Type I respiratory failure has a V/Q mismatch that causes hypoxemia. Type II respiratory failure is caused by alveolar hypoventilation which results in hypercapnia. Type III respiratory failure in the peri-operative period clinically progresses to Type I or Type II respiratory failure. Type IV respiratory failure results from an underlying circulatory collapse.

# Causes
Common causes of respiratory failure include pneumonia, pulmonary edema, pulmonary embolism, acute respiratory distress syndrome, atelectasis, asthma, COPD, neuromuscular and chest wall disorders, inadequate post-operative analgesia, smoking, obesity and shock. Life-threatening causes of respiratory failure include chronic obstructive pulmonary disease, acute on chronic respiratory failure, pulmonary infection, pulmonary embolism, heart failure, cardiac arrhythmia and lung cancer.

# Differentiating Respiratory Failure from other Diseases
As respiratory failure manifests in a variety of clinical forms, differentiation must be established in accordance with the particular type of respiratory failure. Type I respiratory failure must be differentiated from other disease that cause hypoxia, such as acute decompensated heart failure, adult respiratory distress syndrome, high altitude pulmonary edema, neurogenic pulmonary edema, pulmonary embolism, pneumonia and idiopathic chronic lung fibrosis. In contrast Type II respiratory failure must be differentiated from other diseases that cause hypercapnia, such as COPD, status asthmaticus, opioid toxicity, myasthenia crisis, Guillain-Barre syndrome. As well as Type III preoperative respiratory failure and Type IV respiratory failure.

# Epidemiology and Demographics
The incidence of respiratory failure is approximately 137.1 per 100,000 individuals in the United States and the mortality rate is approximately 29%-42%. The incidence is higher among patients  65 years of age and the mortality rate is higher among African-Americans and Hispanics compared to Caucasians. Men and women are equally affected.

# Risk Factors
Common risk factors in the development of respiratory failure include smoking, alcohol abuse, COPD exacerbation, obesity.

# Screening
There is insufficient evidence to recommend routine screening for respiratory failure.

# Natural History, Complications, and Prognosis
Common complications of respiratory failure include, pulmonary emboli, barotrauma, fibrosis and pneumonia. If left untreated patients with respiratory failure may progress to develop sepsis and multiple organ failure which increases mortality. The mortality rate of respiratory failure is approximately 29%-42%.

# Diagnosis

## Diagnostic Study of Choice
Respiratory failure is mainly diagnosed based on clinical presentation. There is no single diagnostic study of choice for the diagnosis of respiratory failure, but respiratory failure can be diagnosed based on history, examination and arterial blood gases.

## History and Symptoms
A positive history of sudden onset of shortness of breath, trauma to the neck or thorax and change in mental status is suggestive of respiratory failure. Common symptoms of respiratory failure include tachypnea , stridor and dyspnea. Less common symptoms of respiratory failure include anxiety, headache, and asterixis.

## Physical Examination
Patients with respiratory failure usually appear distressed with altered mental status. Physical examination of patients with respiratory failure is usually remarkable for dyspnea, stridor, and tachypnea.

## Laboratory Findings
Laboratory findings consistent with the diagnosis of respiratory failure include abnormal bicarbonate, oxygen, phosphate, and magnesium levels.

## Chest X-ray
X-ray may be helpful in establishing the etiology of respiratory failure as it may detect underlying disease. Findings on x-ray suggestive of pre-existing COPD include hyperinflation and a flattened diaphragm. X-ray findings suggestive of interstitial lung disease include reticular nodular shadows. Findings on x-ray suggestive of acute respiratory distress include cardiomegaly, redistribution of vessels, peribronchial cuffing, pleural effusion, lines within the septum, and bat-wing distribution of perihilar infiltrates. Finally, thoracic cage abnormalities may be detected such as kyphosis, scoliosis, pectus excavatum, fractured ribs and ankylosing spondylitis, as well as diaphragmatic paralysis.

## Electrocardiogram
An ECG may be helpful in detecting underlying cardiovascular disease and to diagnose arrhythmia arising as a complication of severe hypoxemia and acidosis.

## CT scan
A CT scan may be helpful in establishing the etiology of respiratory failure as it may detect underlying disease. CT can also predict the incidence of respiratory failure after trauma to the spine. Findings on CT tend to be similar to those found on plain x-ray. Findings on CT scan suggestive of pre-existing COPD include hyperinflation and a flattened diaphragm. CT findings suggestive of interstitial lung disease include reticular nodular shadows. Findings on CT scan suggestive of acute respiratory distress include cardiomegaly, redistribution of vessels, peribronchial cuffing, pleural effusion, lines within the septum, and bat-wing distribution of perihilar infiltrates. CT may also reveal pathology of the neck, brainstem and peripheral nervous system, such as stroke, tumor and transection of the spinal cord.

## MRI
MRI may be helpful in establishing the etiology of respiratory failure, in particular if the cause is due to a pathology of the neck, central or peripheral nervous system. MRI can predict the occurence of respiratory failure in a patient with cervical spine injury. MRI may suggest that stroke, tumor, spinal cord injury and/or complete spinal transection are the cause of respiratory failure. Findings on MRI may include embolism, thrombosis, and haemorrhage.

## Echocardiography and Ultrasound
Echocardiography may be helpful in the diagnosis of a cardiac cause of respiratory failure. Findings on an echocardiography suggestive of cardiac cause of respiratory failure include dilatation of the left ventricle, focal or global wall motion irregularities, severe mitral regurgitation. If patients show a normal size of their heart and a normal blood pressure then this suggests an etiology of acute respiratory distress. Echocardiography is also useful in patients with chronic hypercapnic respiratory failure as the function of the right ventricle and the pulmonary artery pressure may be monitored. Thoracic ultrasound is a part of critical care ultrasonography and may be helpful in the diagnosis of acute cardiopulmonary respiratory failure. Findings on an ultrasound suggestive of respiratory failure include the presence of pneumothorax, alveolar and interstitial aeration abnormalities, and pleural effusion.

## Other Imaging Findings
There are no other imaging findings associated with respiratory failure.

## Other Diagnostic Studies
Despite most patients being unable to perform a pulmonary function test during acute respiratory failure, pulmonary function testing may be useful with chronic respiratory failure. Chronic respiratory failure is usually due to an underlying restrictive disease, pulmonary function test findings include a decrease in FEV1 and a significant decrease in FVC with an overall increase in the FEV1/FVC ratio. It is unlikely that an obstructive lung disease would cause respiratory failure. In respiratory failure with significant cardiac function compromise, a right side cardiac catheter may be used. Catheterization is controversially used to assess those patients with acute hypoxemic respiratory failure where cardiac function is uncertain.

# Treatment

## Medical Therapy
There is no treatment for respiratory failure; however, medication may be used to allow for easier intubation and to ease anxiety in the patient. Recently, studies have demonstrated a strong recommendation against the use of sedatives or analgesics. The use of these agents has been implicated in decreasing the success rates of ventilation mechanisms.

## Oxygen Therapy
A trial of non-invasive ventilation (NIV) may be carried out in order to achieve hypoxemic correction. NIV is advantageous in carrying less infection and mortality rates than traditional mechanical ventilation. ECMO is a cardiopulmonary support machine that is useful in cases of acute severe respiratory failure.

## Mechanical Ventilation
Mechanical ventilation aims to correct abnormalities in oxygenation of the blood and tissues, reduce the respiratory effort and prevent dynamic hyperinflation. Different modes of ventilation are available to suit each patient's individual needs, such as assisted-control ventilation.

## Primary Prevention
Effective measures for the primary prevention of respiratory failure include control of primary illness that may lead to respiratory failure. In addition, the administration of influenza and pneumococcal vaccinations to patients at risk such as COPD and asthma patients is very important in preventing respiratory failure. Finally, smoking cessation will help to minimize the risk for respiratory failure.

## Secondary Prevention
Effective measures for the secondary prevention of respiratory failure include monitoring and medical compliance of patients with chronic lung disease such as asthma.

## Future or Investigational Therapies
Noninvasive pressure support ventilation (NPSV) using a new special helmet has undergone a pilot trial, where it successfully treated hypoxemic ARF, with a higher tolerance and less complications than a conventional face mask.