Catheter position and blood gases during constant-flow ventilation

Abstract

We studied the effect of catheter position and flow rate on gas exchange during constant-flow ventilation (CFV) in eight anesthetized, paralyzed dogs. The distal tips of the insufflation catheters were positioned 0.5, 2.0, 3.5, and 5.0 cm from the tracheal carina. Flow rates were varied between 10 and 55 l/min and steady-state arterial blood gases were measured. At a given flow rate, arterial CO2 pressure (PaCO2) decreased as CFV was administered further into the lung up to a distance of 3.5 cm from the carina; there were no significant differences in PaCO2 at 3.5 and 5.0 cm. For a given catheter position, PaCO2 decreased with increasing flow rate up to a flow rate of 40 l/min. Further increases in flow rate had no significant effect on PaCO2. Arterial O2 pressure (PaO2) was relatively constant at all flow rates and catheter positions. We conclude that, up to a point, CO2 elimination can be improved by positioning the catheters further into the lung; advancing the catheters further than 3.5 cm from the carina may cause over-ventilation of specific lung regions resulting in a relative plateau in CO2 elimination and relatively constant PaO2's. Positioning the catheters further into the lung permits the use of lower flow rates, thus potentially minimizing the risk of barotrauma.

Tracheal gas insufflation

“…Indeed, movement of the chest wall appeared to be almost absent. These gains were achieved with a relatively low insufflation gas flow of 108 ml.kg 21 .min 21 . Greater efficiency might have been achieved with a higher flow.…”

“…Anaesthesia and muscle relaxation were maintained by an i.v. infusion of alphaxalone/alphadolone 15 mg.kg 21 .h 21 supplemented with morphine. A relative overdose of anaesthetic agents was used to obtund any muscle tone that may have altered thoracic wall compliance.…”

“…The time taken for the expiratory washout device to disable itself and the maximum pressure reached in the distal trachea were noted. The ventilator settings were: rate 12 breath.min 21 , I : E ratio 1 : 2, pressure controlled ventilation with a peak airway pressure of 20 cmH 2 O and a PEEP of 2 cmH 2 O.…”

Mechanical Ventilation

“…Intratracheal Pressure Rise and on Upstream Pressure The increase in distal pressure is due to a moment transfer mechanism (Slutsky and Menon 1987) created at the continuous-flow exit in the endotracheal tube lumen. Increasing CTGI flow rates (ranging from 0.5 to 2 L/min) while using various endotracheal tubes (Fig.…”

Gas exchange by intratracheal insufflation in a ventilatory failure dog model.

“…These studies showed that low flow tracheal insufflation of oxygen, when used without external vibration, can sustain oxygenation during apnea, but cannot sufficiently remove CO2 from the lungs ( 10-1 1 ). Addition of external chest wall vibration at frequencies of [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Hz and an amplitude of 0.2 cm to ITI improved alveolar ventilation, so that normal or low Paco2 levels were reached (8)(9). Recently, ITI was used by Bergofsky and Hurewitz ( 12) in five patients and by Couser and Make (13) in five patients suffering from hypoventilation and CO2 retention.…”