High‐Frequency Positive‐Pressure Ventilation (HFPPV) Applied in Bronchoscopy under General Anaesthesia An Experimental Study

Abstract

Techniques for ventilation in bronchoscopy under general anaesthesia using intermittent positive‐pressure ventilation directly via the fixed side‐arm of the open bronchoscope have recently been described by several authors. Our present technique utilizes a high pressure oxygen/nitrous oxide gas mixture which is obtained from a gas‐conditioning system. This gas is intermittently supplied at a high insufflation frequency to the side‐arm of the open bronchoscope.In lung model experiments and experiments in dogs the use of Storz bronchoscopes 4–9 mm in diameter was studied. With the volumes of delivered gas kept constant, the lung model studies showed that the “intrapulmonary pressure” increases with decreasing insufflation frequencies; in parallel the “pulmonary ventilation” decreases due to increasing gas loss through the outer opening of the open bronchoscope. In both the dog and the lung model experiments increasing frequencies resulted in reduced pulmonary ventilation. Both of these studies showed that when the relative insufflation time was shortened from 32 to 22 % of the period time ventilation increased. As demonstrated previously, the central venous pressure and the pulmonary arterial pressure were not affected by this type of high‐frequency positive‐pressure ventilation.It is concluded that this study favours an insufflation frequency (f) of 60 per min and a relative insufflation time (t%) of 22% to be used in the clinical situation.If f/t% is kept at 60/22 in bronchoscopic HFPPV the driving gas pressure is the principal determinant of the total gas input and pneumatic valve function, which are the main factors in the generation of tidal volume and alveolar ventilation with this “open” ventilator system. Consequently, and as may be demonstrated clinically (Eriksson & Sjöstrand 1977a), the ventilation of the patient is regulated by the driving gas pressure. As there is no air entrainment with bronchoscopic HFPPV the oxygenation is regulated by means of the oxygen concentration in the oxygen/ nitrous oxide mixture delivered intermittently to the side‐arm of the bronchoscope.