COVID-19: Epidemiology and pathophysiology

Virology
  • 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

Pathophysiology

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.

Epidemiology
  • 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

Special:Prefixindex/COVID-19

References
  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. https://www.esicm.org/wp-content/uploads/2020/04/684_author-proof.pdf
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