PubMed · 13125785
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P CROCE. 1953-11-18. [Bronchospirometry].. https://pubmed.ncbi.nlm.nih.gov/13125785/
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There is an urgent need for diagnostic procedures that can detect aspiration of oral and gastrointestinal (GI) secretions into the respiratory tract. Current approaches are limited by poor sensitivity and specificity. These techniques include (1) adding indicators to feedings; (2) recovery of lipid-filled macrophages in respiratory secretions; (3) measurement of changes in the pH of the upper GI and respiratory tracts; (4) endoscopic visualization of reflux events; and (5) measurement of increased glucose concentrations in respiratory secretions. Ideally, specific markers from various sites in the oral and GI tracts might be discovered in respiratory secretions, but conventional bronchoalveolar lavage for sampling respiratory secretions is not practical and involves some risk. Noninvasive measurements of indicators in the exhaled breath condensates could be used to detect aspiration, but a number of theoretical and practical aspects of such studies must be considered before this approach can be applied to the problem of aspiration.
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The ventilatory response to carbon dioxide (CO2) measured by the steady-state method is lower than that measured by Read's rebreathing method. A change in end-tidal P CO2 (PET CO2) results in a lower increment change in brain tissue P CO2 (Pt CO2) in the steady-state than with rebreathing: since Pt(CO2) determines the ventilatory response to CO2, the response is lower in the steady-state. If cerebral blood flow (CBF) responds to Pt CO2, the CBF-CO2 response should be lower in the steady-state than with rebreathing. Six subjects undertook two protocols, (a) steady-state: PET CO2 was held at 1.5 mmHg above normal (isocapnia) for 10 min, then raised to three levels of hypercapnia, (8 min each; 6.5, 11.5 and 16.5 mmHg above normal, separated by 4 min isocapnia). End-tidal P O2 was held at 300 mmHg; (b) rebreathing: subjects rebreathed via a 6 L bag filled with 6.5% CO2 in O2. Transcranial Doppler-derived CBF yielded a higher CBF-CO2 sensitivity in the steady-state than with rebreathing, suggesting that CBF does not respond to Pt CO2.