WikiDoc Resources for COVID-19-associated hypoxemia
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COVID-19 emerged as a pandemic, after its outbreak in Wuhan, China in December 2019. It is caused by a new type of Coronavirus, which binds to ACE-2 receptors on Type 2 pneumocytes in the lower respiratory tract. The clinical presentation of patients with COVID-19 varies from asymptomatic disease to severe acute respiratory distress syndrome (ARDS). Hypoxemia is present with an increased A-a gradient. Hypoxemia is diagnosed by Pa02<60mmHg in a sample of Arterial Blood Gas. Mechanisms involved in hypoxemia are widely reported to be due to ventilation perfusion mismatch and intrapulmonary shunting. Diffusion impairment can cause hypoxemia during recovery period due to fibrosis in the lungs. Older age, male sex, hypertension and dyspnea have been identified as risk factors for development of hypoxemia in COVID-19. Complications of hypoxemia include acute respiratory failure and multi-organ failure. Treatment is based on oxygen supplementation to keep target Spo2> 90%.
- In December 2019, novel coronavirus outbreak occurred in Wuhan, China
- On 11th March 2020, it was declared as Pandemic by WHO.
- There is no established system for the classification of COVID-19 associated hypoxemia.
- COVID-19 is caused by the novel Coronavirus. It binds to ACE-2 receptors in the lower respiratory tract which causes pulmonary manifestations.
- The virus causes alveolar injury which stimulates an inflammatory response in the host tissue.
- Mononuclear inflammatory cells are recruited at the site of injury which release cytokines e.g Interleukin-6 and activate procoagulants
- As a result of this insult, the alveolar epithelium and capillary endothelium are damaged.
- Alveoli collapse occurs due to fluid accumulation and loss of surfactant
- Simultaneously, the activation of Coagulation cascade by cytokines leads to widespread thrombosis in multiple organs of the body, including lungs.
- It has also been suggested that there is down-regulation of the Hemostatic Oxygen Sensing system of the body (e.g Carotid bodies) through alteration in the expression of mitochondrial proteins by the Coronavirus, occurring at a cellular level.
- The above mechanism support the lack of dyspnea in proportion to the severity of hypoxemia, on clinical presentation, a phenomenon known as "happy hypoxemia".
Mechanisms of Hypoxemia in COVID-19
Ventilation Perfusion Mismatch
- V/Q mismatch is typically seen due to ARDS.
- Initially the lungs have good compliance but there is marked hypoxemia.
- This can be explained by abnormal vasoregulation which disrupts the physiological, hypoxic pulmonary vasoconstriction response to hypoxemia.
- If hypoxemia is not addressed early, the patient increases inspiratory efforts which exerts more pressure on the tissues, causing a rise in the transpulmonary pressure.
- These changes in lung dynamics promote capillary leakage, which further increases alveolar exudates and the lungs become poorly compliant.
- The ventilation-perfusion mismatch, therefore, progresses from a high Va/Q ratio to a low Va/Q ratio.
- Pulmonary vascular thrombi also contributes to Va/Q mismatch.
- Both acute pulmonary embolism and small vessel thrombosis are seen on autopsy.
- This increases the alveolar dead space causing Va/Q mismatch.
- Blood is shunted from the poorly ventilated alveoli to well-aerated lung regions.
- Intra-cardiac shunts can be detected in 20% of the COVID-19 patients treated for ARDS. Patent foramen ovale opens due to positive pressure ventilation.
- Shunt can be differentiated from Va/Q mismatch due to the lack of response to supplemental oxygen.
- A study was conducted in China to measure DLCO of discharged patients. The researchers concluded that the decrease in DLCO correlated with the severity of pneumonia on admission.
|Pulmonary causes||Cardiac causes|
|Non cardiogenic Pulmonary Edema||Myocarditis|
|Pulmonary Hypertension||Heart Failure|
|Pulmonary embolism||Cardiogenic Shock|
|Super imposed bacterial infection||Arrhythmia|
Differentiating COVID-19-associated hypoxemia from other Diseases
- COVID-19-associated hypoxemia should be differentiated from other potential causes of hypoxemia.
- Dyspnea is not a prominent feature of hypoxemia due to COVID-19 in contrast to other diseases causing hypoxemia
- This can be explained by areas of well preserved lung compliance surrounding the damaged tissue.
- It is important to differentiate COVID-19 associated pneumonia from Community acquired pneumonia, as both can present with hypoxemia and pulmonary infection.
|Covid-19 Pneumonia||Community Acquired pneumonia|
|Sars-Cov2||Viral/ Bacterial pathogens e.g Streptococcus Pneumonia, Influenza|
|Pneumonia develops after 6 days of infective symptoms||Rapid development of symptoms of pneumonia|
|Malaise is a prominent feature||Malaise is not a prominent feature|
|Extra Pulmonary symptoms are present ( anosmia, headache, myalgia)||Pulmonary symptoms are more prominent ( Productive cough, fever)|
|Radiology shows Basal atelectasis / Bilateral peripheral Ground Glass opacities||Radiology shows Lobar Consolidation|
Epidemiology and Demographics
- COVID-19 is seen more commonly in men.
- 80% of patients with Coronavirus disease develop a respiratory infection.
- According to a study conducted in Hubei, China, 5%-25% of patients admitted in hospital for COVID-19 needed ICU admission. Of the patients admitted in ICU, 60%-70% developed ARDS.
- There is no geographical association of hypoxemia in COVID-19.
- According to a study conducted in Wuhan, China, the following risk factors were identified in patients presenting with hypoxemia (Spo2< 90%):
Natural History, Complications, and Prognosis
- COVID-19 has a wide range of clinical presentations, varying from asymptomatic to severe disease, requiring ICU admission.
- Acute Respiratory Distress Syndrome (ARDS) (see COVID-19-associated acute respiratory distress syndrome) and pneumonia, which are a common cause of hypoxemia, can develop in 15% of patients.
- Common complications of hypoxemia include acute respiratory failure, (see COVID-19-associated respiratory failure) and multiorgan failure (Acute Kidney injury, Liver dysfunction, Cardiac injury).
- Prognosis is generally poor for patients presenting with hypoxemia. It has been identified as an independent risk factor for mortality due to COVID-19.
- Patients who require mechanical ventilation have a mortality rate of 50%-60%.
- Hypoxemia in COVID-19 patients is associated with the development of the following:
Diagnostic Study of Choice
- The diagnosis of COVID-19 associated hypoxemia can be established by the following investigations:
- Reverse Transcriptase-Polymerase Chain Reaction from nasal or throat swab sample positive for COVID-19
- Chest Tomography images showing peripheral and bilateral ground-glass opacities
- Arterial Blood gas showing Pa02 (Partial Pressure of oxygen) below 60mmHg
- Oxygen Saturation below 90% on Pulse oximeter
History and Symptoms
- Lymphopenia (80% of patients)
- Elevated C- Reactive Protein
- Elevated LDH (40% of patients)
- ELevated D-Dimer
- Elevated level of IL-1, IL-6
- There are no typical electrocardiographic findings for hypoxemia related to COVID-19.
- To view the electrocardiogram findings on COVID-19, click here.
- Chest x-ray demonstrates multi-lobar infiltrates
- To view the x-ray finidings on COVID-19, click here.
Echocardiography or Ultrasound
- There are no typical echocardiographic findings for hypoxemia related to COVID-19.
- To view the echocardiographic findings on COVID-19, click here.
- Computed Tomography shows consolidation and bilateral ground-glass opacities located peripherally.
- To view the CT scan findings on COVID-19, click here.
- There are no specific MRI findings for hypoxemia related to COVID-19.
- To view the MRI findings on COVID-19, click here.
Other Imaging Findings
- To view other imaging findings on COVID-19, click here.
Other Diagnostic Studies
- To view other diagnostic studies for COVID-19, click here.
Treatment of Hypoxemia due to COVID-19
- Hypoxia due to COVID-19 warrants hospital admission.
- Oxygen target should be Spo2>90%.
- Some centres have suggested to restrict oxygen supplementation by High Flow Nasal Cannula (HFNC) and Non-Invasive Ventilation (Bipap, CPAP) as they generate aerosol and pose a threat to the healthcare workers.
- Invasive mechanical Ventilation by early intubation is recommended for hypoxemia not responding to Non-Invasive Ventilation.
- SpO2< 93%-94%
- Respiratory rate > 28-30 breaths per minute.
- Deliver oxygen via 40% Venturi mask.
- If a response is seen in 5-10 minutes, continue treatment for the next 6 hours.
- NIV is recommended if there is no improvement.
High Flow Nasal Oxygenation(HFNO)
- Use is recommended in a negative pressure environment due to aerosol generation.
- Apply if SpO2< 92%.
- No response to Oxygen delivery is observed via nasal cannula, face mask or Venturi mask.
- Use Oxygen flow of 30-50L/min.
- Keep FiO2 between 50%-70%.
Non Invasive Ventilation
- Used when dyspnea/ hypoxemia does not improve within 1 hour of HFNO used at 50L/min and FiO2>70%.
- Recommended to use pressure setting of 8-10cm Hg and FiO2of 60%.
- Monitor with hourly Arterial Blood Gas sampling.
- Use for 4-6 hours, allowing 1-hour break for feeding.
Invasive Mechanical ventilation
- Performed in patients with severe hypoxemia ( Pa02/FiO2 <200) and failure of NIV.
- Rapid Sequence intubation is preferred to avoid aerosolisation by Bag mask ventilation.
- Lung protective Ventilation is used in patients with severe ARDS.
- Tidal Volume at 4-6ml/kg of body weight.
- Plateau Pressure(Pplat) < 30cm H2O.
- High Positive End Expiratory Pressure (PEEP) is recommended to keep driving pressure (Pplat-PEEP)<14cm H2O.
- Recommended in severe ARDS (PaO2/FiO2 <150) along with Invasive Mechanical ventilation.
- Recommended for a total duration of 12-16 hours daily.
- Not recommended for infants less than 6 months of age.
- It decreases Va/Q mismatch by eliminating gravitational forces exerted on lung portion by mediastinal structures, allowing maximum lung recruitment when positive pressure mechanical ventilation is applied.
- Meta analysis have shown that Prone positioning can decrease mortality when used for long duration within intial 48hours in severe ARDS.
- In a pilot study performed in New York emergency, awake proning was associated with improved oxygen saturations in non intubated patients.
- Position should be changed every 2 hours to prevent pressure ulcer formation.
Extra Corporeal Membrane Oxygenation
- Used in refractory hypoxemic respiratory failure.
- PaO2/Fio2 < 50mmHg for more than 1 hour.
- PaO2/FiO2 < 80mmHg for more than 2 hours.
- Arterial Blood Gas indicating pH <7.2 persisting for more than 1 hour, due to uncompensated respiratory acidosis.
- It is has shown improved clinical outcome in severe COVID-19.
- Infection with COVID-19 can be prevented by practicing the following:
- Social distancing
- Frequent hand washing
- Personal Hygiene
- Wearing mask
- Use of Personal Protective equipment by healthcare workers
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