In their recent paper Nickol et al 1 studied the possible mechanisms by which non-invasive ventilation (NIV) improves ventilatory failure in patients with a restrictive defect due to either neuromuscular disease or kyphoscoliosis. They investigated three possible hypotheses for reduction in daytime hypercapnia—namely, increased ventilatory sensitivity to CO2, improved respiratory muscle function, and increased respiratory system compliance. They showed that the reduction in diurnal Paco2 after treatment was accompanied by an increase in hypercapnic ventilatory response (HCVR), with no changes in non-volitional tests of respiratory muscle strength or respiratory mechanics. They conclude that an increased ventilatory response to CO2 is the principal mechanism underlying the long term improvement in gas exchange associated with NIV.
Interpretation of HCVR in patients with lung disease is often difficult and, as the authors point out, the measurement is highly variable. In attempting to minimise this variability they report the mean of two tests, but the finding of no significant difference between the first and second test is insufficient evidence to …
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