Lung Mechanics and Gas Exchange during Pressure-Control Ventilation in Dogs: Augmentation of CO₂Elimination by an Intratracheal Catheter

Abstract

Increased awareness of pressure-related injury to the alveolar-capillary interface has renewed interest in modes of ventilation that limit alveolar distention such as pressure-controlled ventilation (PCV). We examined respiratory system mechanics and gas exchange during PCV in six dogs. Our data conformed to the predictions of our single-compartment mathematical model of respiratory dynamics during PCV (J Appl Physiol 1989; 67:1081-92). For a fixed pressure (Pset) and inspiratory time fraction (Tl/Ttot) (15 cm H2O and 0.3, respectively), minute ventilation (VE) reached a well-defined plateau as frequency (f) increased from 10 to 50 breaths/min and tidal volume (VT) fell progressively. Concomitantly, the physiologic dead-space fraction (VD/VT) increased from 0.50 +/- 0.04 to 0.85 +/- 0.04, and arterial PCO2 (PaCO2) rose from 39 +/- 4 to 76 +/- 12 mm Hg. At a fixed combination of frequency, applied pressure, and Tl/Ttot (40 breaths/min, 15 cm H2O, and 0.3), VE did not change when we introduced fresh gas continuously from an intratracheal catheter. However, PaCO2 and VD/VT fell progressively as catheter flow increased from zero to 14 L/min (60 +/- 12 to 40 +/- 12 mm Hg and 0.83 +/- 0.03 to 0.25 +/- 0.14 mm Hg, respectively). We conclude that during PCV at a fixed Pset and Tl/Ttot increasing frequency caused VT to fall and VE to reach a plateau. Declining VT was associated with a rise in PaCO2 because of a subsequent fall in alveolar ventilation. Insufflating fresh gas by an intratracheal catheter increased alveolar ventilation and improved CO2 elimination by washing out the anatomic dead space.(ABSTRACT TRUNCATED AT 250 WORDS)

Tracheal gas insufflation

“…The technique of tracheal gas insufflation was described as long ago as 1969 [28], and has P a co 2 reduction with tracheal gas insufflation as an adjunct to pressure or volume controlled ventilation [8,9,11,12,15,16,29]. Expiratory washout is potentially attractive as it does not augment inspiratory tidal volume and may cause less dynamic hyperinflation than continuous tracheal gas insufflation.…”

“…In addition, all such volumes were corrected for the compliance of the breathing circuit. This was done using the formula 2:3 Â peak airway pressure 2 PEEP ; where 2.3 is the compliance of the breathing system, expressed as ml.cmH 2 O 21 [11]. This breathing system compliance was obtained from a previously constructed pressure/volume curve for the breathing system.…”

“…Tracheal gas insufflation is most effective when the tracheal end-expiratory carbon dioxide concentration is elevated as the mass of carbon dioxide flushed from the airway dead space is maximised. Tracheal gas insufflation can produce a degree of autopositive end expiratory pressure (auto-PEEP); however, in theory this can be reduced by using a low-resistance expiratory valve and tracheal tube, a small diameter insufflation catheter and low tracheal gas insufflation flow [11]. Efficiency of tracheal gas insufflation is reduced in conditions such as acute lung injury as the central airway dead space is relatively small compared with the physiological dead space [8,9].…”

Mechanical Ventilation

“…Nahum et al developed an animal model combining conventional mechanical ventilation with tracheal gas insufflation via a catheter positioned above the carina and inserted in parallel with the ETT. These authors showed that the technique decreased PaCO 2 (Nahum et al 1992a). Efficacy depended on the flow rate used (up to 10 L/min) and was little dependent on catheter positioning.…”

Influência da insuflação de gás traqueal sobre a capnografia de pacientes anestesiados

“…Minimal changes in residual volume cause huge changes in PaCO 2 . This implies in the use of TGI when the VD/VT ratio is elevated (permissive hypercapnia) 6,10 . During the use of TGI, one should ideally monitor PEEP, peak inspiratory pressure, airway resistance, pulmonary complacency, and tidal volume to prevent possible complications 11 .…”