Static versus dynamic respiratory mechanics for setting the ventilator

The lower inflection point (LIP) of the inspiratory limb ofa static pressure-volume (PV) loop is assumed to indicate thepressure at which most lung units are recruited. The LIP isdetermined by a static manoeuvre with a PV-history that is differentfrom the PV-history of the actual ventilation. In nine surfactant-deficientpiglets, information to allow setting PEEP and V_T was obtained,both from the PV-curve and also during ongoing ventilation fromthe dynamic compliance relationship. According to LIP, PEEPwas set at 20 (95% confidence interval 17–22) cm H₂O.Volume-dependent dynamic compliance suggested a PEEP reduction(to 15 (13–18) cm H₂O). Pulmonary gas exchange remainedsatisfactory and this change resulted in reduced mechanicalstress on the respiratory system, indirectly indicated by volume-dependentcompliance being consistently great during the entire inspiration. Br J Anaesth 2000; 85: 577–86 ^* Corresponding author: Klinik für Anaesthesiologie und OperativeIntensivmedizin am Zentralklinikum Augsburg, Stenglinstr. 2,D-86156 Augsburg, Germany     

Humidification during high-frequency oscillation ventilation is affected by ventilator circuit and ventilatory setting

Background:  High-frequency oscillation ventilation (HFOV) is an accepted ventilatory mode for acute respiratory failure in neonates. As conventional mechanical ventilation, inspiratory gas humidification is essential. However, humidification during HFOV has not been clarified. In this bench study, we evaluated humidification during HFOV in the open circumstance of ICU. Our hypothesis is that humidification during HFOV is affected by circuit design and ventilatory settings.
Methods/Materials:  We connected a ventilator with HFOV mode to a neonatal lung model that was placed in an infant incubator set at 37°C. We set a heated humidifier (Fisher & Paykel) to obtain 37°C at the chamber outlet and 40°C at the distal temperature probe. We measured absolute humidity and temperature at the Y-piece using a rapid-response hygrometer. We evaluated two types of ventilator circuit: a circuit with inner heating wire and another with embedded heating element. In addition, we evaluated three lengths of the inspiratory limb, three stroke volumes, three frequencies, and three mean airway pressures.
Results:  The circuit with embedded heating element provided significantly higher absolute humidity and temperature than one with inner heating wire. As an extended tube lacking a heating wire was shorter, absolute humidity and temperature became higher. In the circuit with inner heating wire, absolute humidity and temperature increased as stroke volume increased.
Conclusion:  Humidification during HFOV is affected by circuit design and ventilatory settings.