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Changes in respiratory rate affect tidal expiratory flow indices in infants with airway obstruction
F Rusconi1, L Gagliardi, H Aston
1Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London, United Kingdom.
Insights
The ratio of time to peak expiratory flow to total expiratory time (t(pef)/t(e)) may mask airway obstruction in infants. Increased respiratory rate (RR) can alter this ratio, making it an unreliable measure of obstruction in infants with recurrent wheeze.
Area of Science:
- Pediatric Pulmonology
- Respiratory Physiology
- Clinical Diagnostics
Background:
- Airway obstruction is a significant concern in infants with recurrent wheeze.
- Tidal expiratory flow measurements are explored as non-invasive tests for airway obstruction.
- Time to peak expiratory flow (t(pef)) relative to total expiratory time (t(e)), expressed as t(pef)/t(e), is a commonly used index.
Purpose of the Study:
- To investigate if changes in respiratory rate (RR) during airway challenge tests can mask the true severity of airway obstruction in infants.
- To determine the reliability of the t(pef)/t(e) ratio as an indicator of airway obstruction in the presence of varying respiratory rates.
Main Methods:
- Thirty-three infants (6.5-23 months) with recurrent wheeze underwent methacholine or histamine challenge during sedated sleep.
- Tidal breathing and maximal expiratory flow at functional residual capacity (V(maxFRC)) were measured before and after challenges.
- Respiratory rate (RR), t(pef), t(e), and the t(pef)/t(e) ratio were analyzed in relation to changes in airway obstruction.
Main Results:
- All infants showed signs of airway obstruction post-challenge (decreased P(tCO2) and V(maxFRC)).
- A significant increase in RR was observed, along with a significant decrease in both t(pef) and t(e).
- The t(pef)/t(e) ratio did not significantly decrease, and its change correlated positively with RR changes. Infants with higher RR increases showed masked changes in t(pef)/t(e).
Conclusions:
- The t(pef)/t(e) ratio is not a reliable surrogate marker for assessing airway obstruction in infants when respiratory rate changes occur concurrently.
- Increased respiratory rate can artificially normalize or mask the expected decrease in t(pef)/t(e) during airway challenges.
- Clinical interpretation of t(pef)/t(e) in infants requires careful consideration of concurrent respiratory rate variations.
Abstract:
Among the tidal expiratory flow measurements that have been suggested as surrogate tests for airway obstruction, a short time to reach peak tidal expiratory flow (t(pef)) is the most widely used. Time to peak expiratory flow is most often expressed as the ratio between t(pef) and total expiratory time (t(e)). As te strictly depends inversely on respiratory rate (RR), we studied the hypothesis that an increase in RR (and a fall in t(e)) with the development of airway obstruction during methacholine or histamine challenge in infants could mask a decrease of t(pef) when expressed as t(pef)/t(e). Thirty-three infants (ages 6.5-23 months) with recurrent wheeze were studied during sedated sleep. Runs of tidal breathing and maximal expiratory flow at FRC (V(maxFRC)) measured by the squeeze technique were obtained before and after the challenge. All infants responded to the challenge: the median fall in P(tCO2) was 25%, and it was 43% in V(maxFRC). RR increased from a median value of 31.1 to a median of 35.1 breaths/min. Both t(pef) and t(e) were significantly shorter after the challenge (P < 0.001 and 0.004, respectively); however, the decrease in t(pef)/t(e) was not significant (P=0.081). The change in t(pef)/t(e) was positively correlated with the change in RR (r = 0.51, P = 0.003). To analyze better the effect of changes in RR on various indices, we divided the patients into two groups: in 17 subjects with a small increase in RR (< 10%), t(e) did not change significantly, while t(pef) and t(pef)/t(e) did; in 16 subjects with a more marked increase in RR (> or = 10%), the shortening of te masked the simultaneous shortening of t(pef), so that t(pef)/t(e) did not change. These data demonstrate that t(pef)/t(e) cannot be reliably used to evaluate changes in airway obstruction when concomitant changes in RR occur.