COVID-19: Epidemiology and pathophysiology

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  • Coronaviruses are common human pathogens
    • Cause the common cold
    • In epidemics, cause up to one-third of community-acquired upper respiratory tract infections in adults; and may cause diarrhea in infants and children
  • SARS-CoV-2 is a novel coronavirus (a new strain not previously identified in humans)
    • Likely primary source = bats
    • It is a betacoronavirus in the same subgenus as the severe acute respiratory syndrome (SARS) virus. Middle East respiratory syndrome (MERS) virus is another, more distantly related, betacoronavirus. Like the SARS coronavirus, SARS-CoV-2 uses angiotensin-converting enzyme 2 [ACE2] for cell entry


COVID Lung Phenotypes and Their Management

Hypoxemic patients can be divided into two general phenotypes[2]

COVID L Lung Phenotype

  • Characterized by Low elastance (i.e., high compliance), Low ventilation to perfusion ratio, Low lung weight and Low recruitability
  • Often referred to as the “happy hypoxemic”
  • Normal lung volumes and low lung recruitability.
  • Hypoxemia may be due to loss of regulation of perfusion and loss of hypoxic vasoconstriction.
  • These patients can be damaged iatrogenically if you respond to their pulse ox with standard vent modes
  • Do poorly with low tidal volume (TV) and high PEEPs
  • Best managed with high FiO2 which allows you to limit the PEEP
  • Recommended initial vent settings:
    • 8 ml/kg TV, 100% FiO2
    • Increase the PEEP only if the patient is desaturating on a high FiO2.
    • Can turn into COVID H patients on the vent.

COVID H Lung Phenotype

  • Characterized by High elastance, High right-to-left shunt, High lung weight and High recruitability.
  • Increased permeability of the lung leads to edema, atelectasis, decreased gas volume, and decreased TV for a given inspiratory pressure.
  • High degree of lung recruitability.
  • 20 – 30% of patients fit ARDS criteria:
    • Hypoxemia
    • Bilateral infiltrates
    • Decreased the respiratory system compliance
    • Increased lung weight and potential for recruitment
  • The ARDS ladder applies only to this subset of COVID patients.


  • Disease Severity
    • 80% have mild symptoms
    • 15% have severe disease requiring hospitalisation
    • 5% require mechanical ventilation
  • Risk Factors
    • Older age
    • Underlying conditions (lung disease, Renal Failure, Malignancy, heart disease, diabetes)
Ro Example
  • Reproduction Number (R0) SARS-CoV-2:
    • R0 = 2.2 - 4.2
    • Where R0 = expected number of secondary cases produced by a single typical infection in a susceptible population (basic reproductive rate)
    • R0 for seasonal flu ~ 1.3
    • R0 for pandemic flu ~ 1.5-1.8
  • Incubation Period:
    • Incubation = 5 days (median); range of 2-14 days
    • Serial interval duration = 7.5 days
      • Serial interval refers to the time from illness onset in successive cases in a transmission chain
  • Surface survival time of SARS-CoV-2:
    • stainless steel: persists for 3 hours (or longer)
    • underscores the importance of environmental cleaning / disinfection
    • cleaning gets rid of the proteins that would interfere with a disinfectants effectiveness
  • How long to shut a patient room down after a COVID patient is in there?
    • It’s not about the risk of contracting the infection but about the ability to clean room safely without respiratory protection precautions by the cleaner
    • 30-40 minutes usually sufficient (for most modern facilities) as long as no aerosol-generating procedure performed (longer, time not clearly stated at this time)
      • Most modern rooms designed to have 12 air exchanges per hour
      • Ventilation symptoms vary. So, older / fewer exchanges per hour => more time.
      • Note: studied in a simulated lab environment. Lab virions not covered in protein and mucus and other things that would mimic real life and that could prolong survival

See Also


  1. Xu Z, Shi L, Wang Y et al. Pathological findings of COVID-19 associated with acute respiratory distress syndrome. The Lancet Respiratory Medicine. 2020;8(4):420-422. doi:10.1016/s2213-2600(20)30076-x
  2. Gattinoni L et al. Covid-19 pneumonia: different respiratory treatment for different phenotypes. Intensive Care Medicine. 2020.