Acute respiratory failure refers to acute compromise in the alveolar gas exchange resulting in an inability of the pulmonary functions to meet the body's metabolic demand for oxygen and elimination of CO2.
Acute respiratory failure refers to the inability of pulmonary system to meet oxygen demand for blood oxygenation and /or CO2 elimination. The causes of acute respiratory failure are grouped into four types on the basis of the etiopathogenesis.
Type 1 (hypoxemic type) is the commonest acute respiratory failure. Type 2 is marked by hypercapnia and is caused by diseases which impair minute ventilation either by inhibiting respiratory drive or impairing respiratory muscles or neuromuscular activity. Type 1 respiratory failure is generally caused by diseases which damage the alveoli such as pneumoconiosis, pulmonary embolism, and acute respiratory distress syndrome.
Types 3 and 4 occur preoperatively and as a result of shock, respectively. Respiratory infections especially bronchiolitis and pneumonia are the most common indications for mechanical ventilation in children . Inadequate production or abnormal functioning of surfactant is implicated in the etiology of acute respiratory failure in children due to secondary acute lung injury, acute respiratory distress syndrome , and bronchiolitis  .
The signs and symptoms of acute respiratory failure are those of the underlying disease and the features of hypoxemia and hypercapnia.
Diagnosis of acute respiratory failure depends on a high index of suspicion which is confirmed by arterial blood gas analysis with pulse oximetry. This is followed by laboratory and imaging studies to determine the underlying disease cause.
Management of acute respiratory failure involves urgent administration of supplemental oxygen which is followed by diagnosis and management of the underlying cause.
Acute respiratory failure is classified into four main groups largely based on the etiopathogenesis. These include:
Type 1 (Hypoxemic type). This type is characterized by hypoxemia (PO2 <50 mmHg) and is generally the result of diseases which impair alveolar exchange by damaging the alveolar structures. These diseases include acute respiratory distress syndrome (ARDS), pulmonary embolism, bronchiectasis, pneumoconiosis, pulmonary edema, asthma, idiopathic pulmonary fibrosis, and chronic obstructive pulmonary disease (COPD).
Extrapulmonary diseases which cause hypoxemic respiratory failure include kyhoscoliosis, cyanotic congenital heart disease, and obesity. Hypoxemic respiratory failure is the commonest type of acute respiratory failure.
Type 2 (Hypercapnic) respiratory failure is also called ventilatory respiratory failure and is caused by ventilatory problems. These diseases can be grouped into four types for descriptive purposes.
Hypercapnic respiratory failure is marked by a PCO2 of over 50mmHg and may be associated with hypoxemia.
Type 3 respiratory failure occurs preoperatively while type 4 occurs secondary to shock.
The number of inpatient cases diagnosed with acute respiratory failure between 2001 to 2009 increased from 1,007,549 to 1,917,910 according to the Nationwide inpatient sample based on the international classification of diseases (ICD). The total hospital costs rose from $30.1 billion in 2001 to $54.3 billion in 2009; the change in the mean hospital cost per case was insignificant. The mortality from respiratory failure in the inpatients during this period dropped from 27.6% to 20.6% .
It's not fully settled whether there is a racial predisposition to acute respiratory failure. More so, as reported by Khan et al in a study, there were no racial differences in mortality rates of acute respiratory failure among Asians and the native patients . However, some studies have pointed significant differences in mortality rates between African Americans and whites .
Severe acute respiratory syndrome (SARS), an infectious pulmonary disease, causes acute respiratory failure. SARS is most prevalent in Asia, Europe and North America. It was recorded that at least 8,000 people contracted SARS between November, 2002 and August, 2003 with countries worst hit were Singapore, Taiwan, Vietnam, Canada, and China .
SARS is associated with significant morbidity and mortality; 32% of cases of SARS are associated with significant mortality while mortality is present in up to 80% of severe cases   .
In the physiologic state of the body, the ventilatory capacity always exceeds the ventilatory demand. In acute respiratory failure, however, the reverse happens such that either the ventilatory capacity becomes inadequate to meet the body's ventilatory demands or there is increased ventilatory demand.
The functional unit of the respiratory system is the alveolus, in which gas exchange takes place. During alveolar capillary gas exchange, oxygen diffuses into the blood while carbon dioxide diffuses out. Oxygen in the blood reversibly binds to hemoglobin such that 1.36 ml of oxygen is bound to 1g of hemoglobin. However, what amount of oxygen binds to hemoglobin depends on Pa O2, the partial pressure of oxygen in the blood. This relationship is represented by oxygen-hemoglobin dissociation curve.
In physiologic respiration, there are occurrences of ventilation-perfusion mismatch whereby ventilation doesn't match the perfusion provided in terms of the oxygen tension (PO2) thereby creating a gradient. Therefore, this creates a physiologic scenario in which some alveoli are over-ventilated while others are underventilated for a given alveolar perfusion and creating a spectrum of hypoperfused and hyper-perfused alveoli for a given ventilation. The over-ventilated alveoli with poor or no perfusion are termed high Ventilation/Perfusion (V/Q) units while the alveoli which are poorly ventilated but highly perfused are termed low V/Q units. The former category of alveoli is regarded as dead space because of the poor perfusion for gas exchange while the later group of alveoli is described as shunt.
Carbon dioxide is transported in the blood in three main forms: as a simple solution, as bicarbonate, and bound to hemoglobin. Normally, the rate of tissue production of carbon dioxide is equal to its rate of pulmonary elimination. This is represented by the equation below:
VA = K x VCO2/ Pa CO2.
(K is a constant with a value of 0.863; VA, alveolar ventilation; VCO2, carbon dioxide ventilation).
V/Q mismatch and shunting are the main processes which cause hypoxemic respiratory failure by causing a large alveolar-arterial PO2 gradient. Although, these two processes may occur simultaneously, V/Q mismatch is the commonest cause of hypoxaemia. The low V/Q units are particularly responsible for causing hypoxemia while the high V/Q units do not cause significant respiratory complication unless the underlying disease becomes severe. The low V/Q match may occur as a result of either under-ventilation or hyperperfusion. Under-ventilation may be caused by the diseases listed above which inhibit respiratory drive or impair chest wall movement while over-perfusion may be seen in pulmonary embolism.
Shunt is referred to as an unresolved hypoxaemia despite administration of 100% oxygen. This is a result of several hyperperfused alveolar units with minimal ventilation. This causes a reduced arterial blood content. Shunt is seen in pneumonia, severe pulmonary edema, and atelectasis. Hypoxaemia secondary to shunt is relatively more difficult to treat by oxygen supplementation. Hypercapnia may result if shunt is severe.
Hypercapnic respiratory failure basically results from reduction in alveolar ventilation. In these settings, both hypoxemia and hypercapnia may occur, however, alveolar-arterial PO2 gradient remains normal. Hyperventilation rarely causes respiratory failure and it commonly results from drug toxicity.
Mortality from acute respiratory failure varies depending on the etiology. ARDS is associated with a mortality rate of 40-45% which has remained high for a long period of time  . COPD contributes to a rate of 10% mortality, whereas, an acute exacerbation of COPD causes death in 30% of patients. Generally, mortality rates are highest among the elderly and over two-thirds of survivors may develop pulmonary sequela within a year of treatment.
Hypercapnic respiratory failure is associated with a relatively higher mortality rate, possibly attributable to the chronicity of the underlying disease, notable systemic comorbidities, and poor nutritional status.
As noted in a study by Noveanu et al, administration of beta-blockers before and during admission is associated with significant reduction in mortality rates within a year of treatment .
Acute respiratory failure causes various pulmonary and extrapulmonary complications including the underlisted:
Typically, the presentation of acute respiratory failure is a reflection of the underlying etiology. Depending upon the underlying pathology and the associated finding of hypoxemia or hypercapnia, the clinical presentation can vary. Hypercapnia causes asterixis and tachycardia, while hypoxemia causes cyanosis and neurologic impairment including restlessness, confusion, convulsions, and coma. Cyanosis is the bluish discoloration of the skin and mucous membranes caused by hypoxemia. Cyanosis becomes obvious if the concentration of deoxygenation hemoglobin in the tissues is or above 5g/dl.
Once acute respiratory failure is suspected, arterial blood gas analysis should be done to confirm the diagnosis. Complete blood count may be necessary to reveal common findings such as anemia.
Further investigations are necessary to detect the underlying cause of the acute respiratory failure including chest radiographs or CT scans and pulmonary function tests to check for pulmonary pathology. If cardiac pathology is suspected as the cause of the disease, an echocardiogram may be recommended. Blood electrolytes are also analysed to assess renal function.
The management of acute respiratory failure begins with emergency resuscitation which aims to restore ventilation and oxygenation. After the acute management has been achieved, further treatment involves determining the diagnosis and controlling the underlying disease.
Oxygenation is achieved with administration of supplemental oxygen and all patients should have their oxygen saturation monitored with a pulse oximeter. Oxygen saturation should generally be maintained at over 90%. Adequate oxygenation is usually successful with supplemental oxygen because the rate of oxygen diffusion into the alveolar capillaries is directly proportional to the PaO2.
There are various oxygen-delivery devices which differ in their rate of oxygen delivery and therefore, their effectiveness also varies. These devices may be open or closed systems. These oxygen-delivery devices include:
Patients with acute respiratory failure are treated with supportive care which includes mechanical ventilation and prophylactic treatment of thromboembolism, stress ulcers, and pressure sores. Oxygenation has been shown to be better in both positions, prone and upright, with the prone position being preferred in patients with ARDS. Robak et al in a report noted short-term benefits of combined prone and upright positions . However, in ARDS patients, prone position is preferred with the upright position being regarded as an effective alternative  . It has also been reported that both positions cause a reduction in pneumonia associated with ventilator   .
Primary prevention of acute respiratory failure includes strategies to prevent the underlying causes of acute respiratory failure. It has been suggested that influenza and pneumococcal vaccinations may reduce the risk of acute respiratory failure in high risk patients such as those with COPD. Quitting smoking also reduces the incidence of acute respiratory failure in patients with lung disease.
Secondary prevention of acute respiratory failure involves constant management of chronic pulmonary diseases to prevent progression to respiratory failure.
Acute respiratory failure is a medical emergency in which oxygen and carbon dioxide can no longer be adequately exchanged in the lungs. Normally, the oxygen we inhale gets exchanged with carbon dioxide in small structures within the lungs called alveoli; the carbon dioxide is then expired while oxygen diffuses into the blood for use by every cell in the body. In acute respiratory failure, this exchange is compromised, therefore, the body is short of oxygen to provide for the body.
Several diseases can cause acute respiratory failure. Some of these diseases destroy the alveoli while others impair breathing. Diseases which destroy the alveoli include acute respiratory distress syndrome, pneumonia, pulmonary fibrosis, and asthma. Diseases which reduce deep breathing include brain damage, drug toxicity, disorders of the chest wall muscles, spinal cord injury, brain tumors and airway tumors. All of these conditions significantly reduce the amount of oxygen made available to the body either by destroying the alveoli or by reducing the amount of oxygen which is made available for diffusion.
Although, death from acute respiratory failure depends on the cause, the respiratory failure is fatal, if it is not treated urgently. Survivors may still be left with some damage done to the lungs. Furthermore, in the absence of early treatment, acute respiratory failure may cause significant complications in many tissues of the body, such as heart attack, heart failure, low blood pressure, stomach ulcers, diarrhea, kidney failure, and lung infections.
Symptoms of acute respiratory failure are difficulty in breathing, fast heart rate, confusion, restlessness, convulsions and even coma. A very typical finding in this condition is the bluish discoloration of the lips, skin, and other body linings. This is called cyanosis and is due to very low oxygen content of the blood.
First, once this condition is suspected, doctors would perform a test called arterial blood gas analysis to see if the oxygen is low or the carbon dioxide is high. If this is confirmed, doctors would perform several tests to determine the cause of the respiratory failure. These tests would include chest x-ray, chest CT scan, and electrocardiogram (ECG).
The mainstay of treatment of acute respiratory failure is providing an artificial source of oxygen via mechanical ventilation. Here, oxygen is delivered through the patient's nose via face mask, tubes passed down the throats, or via small tubes passed into the nostrils. Once the delivery of adequate oxygen is achieved, the doctors would proceed to treating the underlying cause and prevent the development of complications.