Volume and pressure during transient added resistance

Effectiveness of mask and helmet interfaces to deliver noninvasive ventilation in a human model of resistive breathing

The helmet, a transparent latex-free polyvinyl chloride cylinder linked by a metallic ring to a soft collar that seals the helmet around the neck, has been recently proposed as an effective alternative to conventional face mask to deliver pressure support ventilation (PSV) during noninvasive ventilation in patients with acute respiratory failure. We tested the hypothesis that mechanical characteristics of the helmet (large internal volume and high compliance) might impair patient-ventilator interactions compared with standard face mask. Breathing pattern, CO2 clearance, indexes of inspiratory muscle effort and patient-ventilator asynchrony, and dyspnea were measured at different levels of PSV delivered by face mask and helmet in six healthy volunteers before (load-off) and after (load-on) application of a linear resistor. During load-off, no differences in breathing pattern and inspiratory muscle effort were found. During load-on, the use of helmet to deliver pressure support increased inspiratory muscle effort and patient-ventilator asynchrony, worsened CO2 clearance, and increased dyspnea compared with standard face mask. Autocycled breaths accounted for 12 and 25% of the total minute ventilation and for 10 and 23% of the total inspiratory muscle effort during mask and helmet PSV, respectively. We conclude that PSV delivered by helmet interface is less effective in unloading inspiratory muscles compared with PSV delivered by standard face mask. Other ventilatory assist modes should be tested to exploit to the most the potential benefits offered by the helmet.

Approach to the estimation of alveolar pressure from noninvasive measurement of upper airway resistance

An alternative to whole body plethysmography is proposed for estimating upper airway resistance. The feasibility study indicates that the method should be suitable for diagnostic and clinical purposes, following a proper survey and the testing of its reproducibility. 11 subjects are asked to breath normally through 12 small resistances while flow and pressure are monitored at the mouth. Chest movements are recorded by sampling a chest cross-section, and relative variations of the cross-section are compared to the expired and inspired volumes. No viscoelastic effects are significantly documented so that in a first approximation a model consisting only of resistors is applied to the airways. In each subject the patterns of the flow and mouth-pressure are heuristically estimated as a polynomial function of the added resistance. From the fit of the resistor model, the internal resistance of the airways is estimated as that value able to link the flow-resistance function to the pressure-resistance function, according to the classic Kirchhoff laws. We obtain resistances of (mean resistance +/- SD) 2.9 +/- 1.1, range 1.1 to 5.2, hPasdm-3 during expiration and 2.5 +/- 0.8, range 1.3 to 3.7, hPasdm-3 during inspiration.

Improved computation of respiratory resistance as measured by transiently increased resistance

The authors have previously described an automated system for measuring total respiratory resistance. The technique used by this system involves a transient, externally applied increase in resistance to breathing. The utility of this technique is limited by a number of assumptions and to free it from the constraints imposed by one of these assumptions a new method for analysing the data was developed. The paper describes the new method of analysis and presents data comparing the resulting Rrss with those computed using the old method.