RESUMO
OBJECTIVES AND METHODS: Rapid bronchoscopy in general anaesthesia still has its precise indications, where the high frequency jet ventilation technique offers several advantages. The monitoring of ventilation, however, has been rather unsatisfactory up to date. We therefore studied capnography in 60 bronchoscopies during HFJV (rate: 100/min; I:E = 0.33; driving pressure: 0.08-0.14 MPa) using a rigid bronchoscope with a distally located sampling port. Continuous capnograms were recorded. End-tidal partial pressures of carbon dioxide (petCO2), however, were obtained from 2-3 single breaths by intermittently reducing the jet-frequency to 10-12/min. After 6 min (MP1: whole group; n = 60) and 18 min of HFJV (MP2: n = 34 of this group) petCO2 values were regularly obtained and compared to pCO2 in synchronously drawn capillary blood samples (pcCO2). The jet driving pressure initially adjusted to body weight, however, was only corrected according to petCO2, aiming at 34 mmHg. RESULTS: During HFJV, sinusoidal capnograms permitted the identification of every single jet impulse. With instruments being passed through the bronchoscope, however, these curves were substantially distorted. Mean pcCO2 at MP1 (37.8 +/- 6.7 mmHg) and MP2 (37.2 +/- 6.7 mmHg) demonstrated normal ventilation to light hyperventilation. Differences from mean petCO2 obtained during low frequency breathing were 3.3 mmHg at MP1 and 4.4 mmHg at MP2 (p < 0.05). There were strong correlations between the individual pairs of pcCO2 and petCO2 from MP1 (r = 0.80) and MP2 (r = 0.75) as well as between the pairs of dpcCO2 and dpetCO2 from both MPs (r = 0.77). The accuracy of the ventilator setting according to petCO2 with reference to pcCO2 was 73% for MP1 and 74% for MP2 (sensitivity: 75%/79%; specificity: 72%/67%). CONCLUSIONS: Capnography in rigid bronchoscopy during HFJV proved a clinically applicable addition to monitoring. Its routine use is strongly recommended in interventional bronchoscopy. The true petCO2 values obtained by intermittent single low frequency jet breathing permit estimates of gas exchange sufficiently exact for clinical purposes and for adjustment of the ventilator setting. Wave forms of the continuously recorded capnogram during HFJV are a warning of impeded ventilation or airway obstruction and, thus, of the danger of barotrauma or hypoventilation. Besides contributing to patient safety, this monitoring method might improve the acceptance of HFJV for bronchoscopy. Furthermore, it can also be applied to rigid bronchoscopy with common ventilation.
