Agneta Markström

Ventilation with constant versus decelerating inspiratory flow in experimentally induced acute respiratory failure.

Background Recognition of the potential for ventilator-associated lung injury has renewed the debate on the importance of the inspiratory flow pattern. The aim of this study was to determine whether a ventilatory pattern with decelerating inspiratory flow, with the major part of the tidal volume delivered early, would increase functional residual capacity at unchanged (or even reduced) inspiratory airway pressures and improve gas exchange at different positive end-expiratory pressure levels. Methods Surfactant depletion was induced by repeated bronchoalveolar lavage in 13 anesthetized piglets. Decelerating and constant inspiratory flow ventilation was applied at positive end-expiratory pressure levels of 22, 17, 13, 9, and 4 cm H2 O. Tidal volume, inspiration-to-expiration ratio, and ventilatory frequency were kept constant. Airway pressures, gas exchange, functional residual capacity (using a wash-in/washout method with sulfurhexafluoride), central hemodynamics, and extravascular lung water (using the thermo-dye-indicator dilution technique) were measured. Results Decelerating inspiratory flow yielded a lower arterial carbon dioxide tension compared to constant flow, that is, it improved alveolar ventilation. There were no differences between the flow patterns regarding end-inspiratory occlusion airway pressure, end-inspiratory lung volume, static compliance, or arterial oxygen tension. No differences were seen in hemodynamics and oxygen delivery. Conclusions The decelerating inspiratory flow pattern increased carbon dioxide elimination, without any reduction of inspiratory airway pressure or apparent improvement in arterial oxygen tension. It remains to be established whether these differences are sufficiently pronounced to justify therapeutic consideration.

Impact of Different Inspiratory Flow Patterns on Arterial C02- tension

Abstract Ventilation with decelerating inspiratory flow is known to reduce the dead space fraction and to decrease PaCO2. Constant inspiratory flow with an end-inspiratory pause (EIP) is also known to increase the removal of CO2. The aim of the study was to elucidate the effect of the pause/no-flow period while both the pattern and rate of inspiratory flow was unchanged, and when the lung was ventilated with sufficient PEEP to prevent end-expiratory collapse. Surfactant depleted piglets were assigned to decelerating or constant inspiratory flow with 24 breaths per minute (bpm) or 12 bpm, or to constant flow, without and with an end-inspiratory pause of 25%. By adding an EIP the total time without active inspiratory flow of the respiratory cycle was kept unchanged. Gas exchange, airway pressures, functional residual capacity (using sulfurhexafluoride) and haemodynamics (thermo-dye indicator dilution technique) were measured. Irrespective of ventilatory frequency, PaCO2 was lower and serial dead space reduced with decelerating flow, compared with constant inspiratory flow. With an end-inspiratory pause added to constant inspiratory flow, serial dead space was reduced but did not decrease PaCO2. The results of this study corroborate the assumption that total time without active inspiratory flow is important for arterial CO2-tension.

Under Open Lung Conditions Inverse Ratio Ventilation Causes Intrinsic Peep and Hemodynamic Impairment

Inverse ratio ventilation (IRV) is commonly used in clinical practice. Several studies have used IRV in order to recruit collapsed alveoli. In a randomised trial in twelve surfactant depleted piglets, the lungs were ventilated with sufficient positive end-expiratory pressure (PEEP) to prevent end-expiratory collapse, and the effects of increased inspiration-to-expiration (I:E ratio) were evaluated. Pressure regulated ventilation (with I:E of 1:1, constant tidal volume and decelerating inspiratory flow) was used at 30 breaths per minute (bpm). I:E ratios of 1.5:1, 2.3:1 and 4:1 were applied sequentially. When the I:E ratio was increased, external PEEP had to be reduced in order to keep total PEEP constant. Functional residual capacity, airway pressures, gas exchange, extrathermal volume and hemodynamics were measured. With I:E ratios above 2:1 intrinsic PEEP was generated and with concomitant decrease in cardiac index. PaO2 was not affected, but oxygen delivery was reduced. It is concluded that I:E ratios of 2:1, or above, generate increased intrinsic PEEP with compromised hemodynamics.

Effects of sustained pressure application on compliance and blood gases in healthy porcine lungs

Background: Short periods of sustained increase in airway pressures (Pressup) are believed to re‐open lung areas that collapsed upon induction of anaesthesia. Recruitment of alveolar surface is usually assessed in terms of changes in the pressure–volume (PV) curve. The purpose of this study was to analyse PV‐curves before and after a Pressup and to ascertain whether such changes are compatible with the concept of recruitment of lung volume.

Experimental studies on lung mechanics, gas exchange and oxygen delivery under open lung conditions. Mechanical ventilation with decelerating versus constant inspiratory flow.

Experimental studies on lung mechanics, gas exchange and oxygen delivery under open lung conditions : mechanical ventilation with decelerating versus constant inspiratory flow