Direct tracheal airway pressure measurements are essential for safe and accurate dynamic monitoring of respiratory mechanics. A laboratory study.
BACKGROUND
All monitoring of respiratory mechanics should depend on tracheal pressures (Trach-P) as endotracheal tube resistance (ETT-Res) will otherwise distort them. The aim of this study was to investigate factors that may vary ETT-Res, causing difficulties in ETT-Res estimation clinically, and to evaluate a method for direct Trach-P measurements to obviate these problems.


METHODS
In a model we studied: 1) The influence on ETT-Res caused by different connectors and secretions; 2) Direct Trach-P measurements with a catheter (o.d. 2 mm, i.d. 0.9 mm) with either end or side hole, filled with either air or liquid, introduced through the ETT lumen and evaluated regarding response time and position.


RESULTS
The pressure drop between trachea and Y-piece increased by 15% when respectively a swivel connector and a humidification device were connected to the ETT. When injecting 1 ml and 2 ml gel into the ETT lumen the inspiratory resistance increased 100% and 600% respectively. The response time of all catheters was < or = 12 ms. During constant flow in inspiratory and expiratory directions the pressure difference between an end hole catheter positioned from 2 cm above the ETT tip to 4 cm below and a reference pressure in the artificial trachea was less than 1.5 cmH2O.


CONCLUSIONS
ETT connections and secretions cause a variance in resistance. Tracheal pressure can be measured with high precision with an air- or liquid-filled catheter. An end hole catheter placed within 2 cm above or below the ETT tip will give sufficiently precise measurements for clinical purposes.
Kárason
S
S
Department of Anaesthesia and Intensive Care, Sahlgrenska University Hospital, Göteborg, Sweden. sigge.karason@molndal.mail.telia.com
Søndergaard
S
S
Lundin
S
S
Wiklund
J
J
Stenqvist
O
O
D000399:Air Pressure; D002404:Catheterization; D006801:Humans; D007442:Intubation, Intratracheal; D008168:Lung / Q000502:physiology; D008953:Models, Anatomic; D016343:Monitoring, Intraoperative; D012129:Respiratory Function Tests / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D015656:Respiratory Mechanics / Q000502:physiology*; D013147:Spirometry / Q000379:methods; D014132:Trachea / Q000502:physiology*