Influenza respiratory failure is a distressing secondary complication of the flu. The World Health Organization has elevated the pandemic alert for the swine flu H1N1 from 4 to 5, meaning the risk of a global pandemic is imminent. A rating of 6 means a full-fledged pandemic is underway, with potentially millions of people being infected.
As the currently circulating virus is new, there is no natural immunity to it. The fatality rate for the circulating strains is estimated to be between 1% and 4%, which is not particularly lethal for an influenza virus.
However, even that relatively low virulence rate has huge ramifications for the health systems of even developed countries. A 2% fatality rate for 10 million infections is still 200,000 deaths.
Of huge concern to intensive care physicians that will be tasked with taking care of the sickest patients is the resources required to care for patients that become severely ill, but who ultimately recover. The number could be in the millions.
One reassuring aspect in this care scenario of influenza respiratory failure is that all ICU physicians have taken care of multiple severely ill influenza patients. It is expected that the care of a swine flu patient who is severely ill will not differ significantly from a seasonal flu patient.
What happens in respiratory failure from swine flu and how is it treated?
Background of Influenza Respiratory Failure
Influenza viruses are categorized into three types (A, B, and C) and are differentiated by their proteins. The genetic material is RNA which is enveloped by a two layer structure made up of lipids.
All type A and B virus particles have two different types of glycoproteins on their surfaces – hemagglutinin (H or HA) and neuraminidase (N or NA). Type C influenza has less genetic material and only one type of glycoprotein on the surface.
Type B and C influenza infects mainly humans. Type A influenza primarily infects birds and other animals such as pigs, horses, cats, and dogs.
Type A influenza viruses are further subdivided based on the differences in the H and N proteins. Sixteen H and 9 N variants have been described.
Influenza is associated with the classic abrupt onset of systemic symptoms in about 60% – 70% of cases. These include fever (38 to 40 degrees Celsius) , chills, rigors (shakes), headache, myalgias (muscle pains), and malaise. The fever is usually continuous and lasts most typically 2 – 4 days.
During the early stage of systemic symptoms in influenza respiratory failure, the patient usually looks ill with a flushed face and watery eyes.
Other common symptoms include sore throat, dry cough, arthralgias (joint pain) and eye symptoms such as pain on moving the eyes, photophobia (pain when the eye is exposed to light), and burning and tearing of the eye.
Some influenza type A strains have more pronounced gastrointestinal symptoms such as abdominal pain, nausea, and diarrhea.
After several days the systemic symptoms diminish and respiratory complaints become noticeable. These include cough, chest pain, nasal discharge, hypoxia (shortness of breath), and dyspnea (increased work of breathing.)
Bilateral patchy infiltrates in moderate ARDS.
Type A and B flu infections can cause several different types of respiratory complications including tracheobronchitis, asthma attackes, exacerbations of cystic fibrosis, croup, and bronchiolitis.
Pneumonia can also result from flu infections and are described as primary influenza viral pneumonia, secondary bacterial pneumonia, and a mixed viral and bacterial pneumonia. Somewhat more unusual is a syndrome that mimics pulmonary embolism as seen with abnormal imaging studies.
In type A flu pandemics, about 20% of patients that develop an pneumonia will have the primary viral type. It occurs particularly in patients who are immunocompromised or who have underlying cardiac or pulmonary conditions. It also occurs with an increased frequency in pregnancy.
Primary viral influenza is marked by high fever, cough, dypsnea, and cyanosis. Patients can become rapidly hypoxic (low blood oxygen) and cyanotic (blue colored skin.) The sputum may contain blood and sputum cultures show many polymorphonuclear leukocytes but few bacteria.
Radiographic examination (chest X-ray) shows most typically patchy bilateral infiltrates.
Secondary bacterial pneumonia is associated with the elderly or those with chronic medical conditions, although many young, healthy patients suffer from this condition.
Most typically the clinical course with secondary bacterial pneumonia is marked by an initial phase with influenza type symptoms that is followed by a period (1 – 4 days) of seeming improvement. Recrudescence (reappearance) of fever occurs with sputum production, cough, pleuritic chest pain (pain on inspiration).
Radiographic examination (chest X-ray) shows a localized area of consolidation (pneumonia.)
Sputum cultures show most typically show presence of Streptococcus pneumoniae, Staphyloccocus aureus (including methicillin resistant Staph aureus), or Haemophilus influenzae.
Treatment is with antibiotics that have efficacy against the specific organism identified. The most worrisome infection is with Staphylococcus as these infections can be virulent and cause significant lung destruction.
An ominous development in the respiratory status of an influenza patient is the onset of acute respiratory distress syndrome or ARDS.
This syndrome is not unique to patients with pulmonary viral infections and can occur in toxic inhalations, near drownings, cardiac failure, and multiple blood transfusions among other causes.
Rather, it is considered the final common pathway of a host physiological derangements that result in acute lung injury.
A formal set of five diagnostic criteria have been developed for ARDS. They are:
Absence of cardiac factors as a cause of pulmonary edema – that is, the fluid filling the lungs can not be a result of a heart attack or other acute heart problem.
Identification of associated or causative condition– that is, that patient must have suffered a known injury or insult.
Dyspnea (usually severe) – that is, severe shortness of breath.
Bilateral radiographic infiltrates – that is, patchy white areas on chest x-ray.
Hypoxemia with PaO2 to FiO2 < 200 – that is, low blood oxygen even in the setting of administered supplemental oxygen.