The Physics of RSV
If you have been paying attention to recent news, you are aware of the extremely early and intense surge of respiratory viruses across the United States, including respiratory syncytial virus (RSV), rhinovirus, influenza, and SARS-CoV-2. This increased activity—along with the strain placed on healthcare facilities—prompted the issuance of a CDC Health Advisory this week.1 Of note is the rise in RSV-associated hospitalizations to levels normally seen in December and January.
Infection with RSV can lead to significant airway changes including loss of cilia, sloughing of epithelial cells, collections of cellular debris and mucus, and edema around the airway. Whenever I teach about RSV, I channel my inner nerd and revisit my college physics. If nothing else, RSV is about mucus and debris—a lot of mucus and debris. This is where I invoke Poiseuille's Law, describing the laminar flow of fluids (including air) through a tube. Resistance to flow is proportional to the inverse of the radius of the tube, raised to the 4th power. In other words, as the radius is decreased, the resistance to flow increases, in a big way, especially when one accounts for differences in airway diameters.
If we consider a coating of mucus 1 mm thick within the trachea, the proportional reduction is far greater in an infant (6 mm diameter reduced to 4 mm) than in an adult (20 mm reduced to 18 mm). Doing the math, we find that resistance to airflow in the infant increases by more than 5 times as compared to about 1.5x in an adult (note: this is over-simplified… but does make the point!).
Adding to this dilemma in infants are the following:
- dry autumnal and winter air leads to thickened and harder to clear mucus
- reluctance to suspend breathing to eat and drink results in less intake
- dehydration produces even more hard to clear mucus
- energy stores, needed to maintain the significant work of breathing, decrease
- running out of fuel can lead to respiratory collapse.
Hydration, feeding, and humidification of air are all important for initial care of infants with RSV. Close observation is essential, with early evaluation should respiration become labored. Hospital care is largely supportive, but necessary in severe cases. For eligible, high-risk children palivizumab can lower the risk of hospitalization. Finally, RSV vaccines—though not available now—are on the horizon.
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