Sarah Buehler

The dynamics of carbon dioxide equilibration after alterations in the respiratory rate

Manual or automated control of mechanical ventilation can be realized as an open or closed-loop system for which the regulation of the ventilation parameters ideally is tuned to the dynamics and equilibration time of the biological system. We investigated the dynamic, transient state and equilibration time (teq) of the CO2 partial pressure (PCO2) after changes in the respiratory rate (RR). In 17 anaesthetized patients without known history of lung disease, respiratory rate was alternately increased and decreased and end-tidal CO2 partial pressures (PetCO2) were measured. Linear relations were found between ΔRR and PetCO2 changes (ΔPetCO2 = 0.3 − 1.1 ΔRR) and between ΔRR and teq for increasing and decreasing RR (teq(hypervent) = 0.5 |ΔRR|, teq(hypovent) = 0.7 |ΔRR|). Extrapolation of the transition between two PCO2 steady-states allowed for the prediction of the new PCO2 steady-state as early as 0.5  teq with an error <4 mmHg. At bedside or in automated ventilation systems, the linear dependencies between ΔRR and ΔPCO2 and between ΔRR and teq as well as early steady-state prediction of PCO2 could be used as a guidance towards a timing and step size regulation of RR that is well adapted to the biological system.

Breathing-phase selective filtering of respiratory data improves analysis of dynamic respiratory mechanics

BACKGROUND The analysis of non-linear respiratory system mechanics under the dynamic conditions of controlled mechanical ventilation is affected by systemic disturbances of the respiratory signals. Cardio-pulmonary coupling induces cardiogenic oscillations to the respiratory signals, which appear prominently in the second half of expiration. OBJECTIVE We hypothesized that breathing phase-selective filtering of expiratory data improves the analysis of respiratory system mechanics. METHODS We retrospectively analyzed data from a multicenter-study (28 patients with injured lungs, under volume-controlled ventilation) and from two additional studies (3 lung healthy patients and 3 with injured lungs, under pressure-controlled ventilation). Data streams were recorded at different levels of positive end-expiratory pressure. Using the gliding-SLICE method, intratidal dynamic respiratory mechanics were analyzed with and without low-pass filtering of expiratory or inspiratory data separately. The quality of data analysis was derived from the coefficient of determination R^2. RESULTS Without filtering, R^2 lay below 0.995 for 87 of 280 investigated data streams. In 68 cases expiration-selective low-pass filtering improved the quality of analysis to R^2 ⩾ 0.995. In contrast, inspiration-selective filtering did not improve R^2. CONCLUSIONS The selective filtering of expiration data eliminates negative side-effects of cardiogenic oscillations thus leading to a significant improvement of the analysis of dynamic respiratory system mechanics.

Detection of thoracic vascular structures by electrical impedance tomography: a systematic assessment of prominence peak analysis of impedance changes

OBJECTIVE Electrical impedance tomography (EIT) is a non-invasive and radiation-free bedside monitoring technology, primarily used to monitor lung function. First experimental data shows that the descending aorta can be detected at different thoracic heights and might allow the assessment of central hemodynamics, i.e. stroke volume and pulse transit time. APPROACH First, the feasibility of localizing small non-conductive objects within a saline phantom model was evaluated. Second, this result was utilized for the detection of the aorta by EIT in ten anesthetized pigs with comparison to thoracic computer tomography (CT). Two EIT belts were placed at different thoracic positions and a bolus of hypertonic saline (10 ml, 20%) was administered into the ascending aorta while EIT data were recorded. EIT images were reconstructed using the GREIT model, based on the individuals thoracic contours. The resulting EIT images were analyzed pixel by pixel to identify the aortic pixel, in which the bolus caused the highest transient impedance peak in time. MAIN RESULTS In the phantom, small objects could be located at each position with a maximal deviation of 0.71 cm. In vivo, no significant differences between the aorta position measured by EIT and the anatomical aorta location were obtained for both measurement planes if the search was restricted to the dorsal thoracic region of interest (ROIs). SIGNIFICANCE It is possible to detect the descending aorta at different thoracic levels by EIT using an intra-aortic bolus of hypertonic saline. No significant differences in the position of the descending aorta on EIT images compared to CT images were obtained for both EIT belts.

The Equilibration of PCO2 in Pigs Is Independent of Lung Injury and Hemodynamics.

Objectives: In mechanical ventilation, normoventilation in terms of PCO2 can be achieved by titration of the respiratory rate and/or tidal volume. Although a linear relationship has been found between changes in respiratory rate and resulting changes in end-tidal cO2 (△PetCO2) as well as between changes in respiratory rate and equilibration time (t eq) for mechanically ventilated patients without lung injury, it is unclear whether a similar relationship holds for acute lung injury or altered hemodynamics. Design: We performed a prospective randomized controlled animal study of the change in PetCO2 with changes in respiratory rate in a lung-healthy, lung-injury, lung-healthy + altered hemodynamics, and lung-injury + altered hemodynamics pig model. Setting: University research laboratory. Subjects: Twenty mechanically ventilated pigs. Interventions: Moderate lung injury was induced by injection of oleic acid in 10 randomly assigned pigs, and after the first round of measurements, cardiac output was increased by approximately 30% by constant administration of noradrenalin in both groups. Measurements and Main Results: We systematically increased and decreased changes in respiratory rate according to a set protocol: +2, -4, +6, -8, +10, -12, +14 breaths/min and awaited equilibration of Petco2. We found a linear relationship between changes in respiratory rate and △PetCO2 as well as between changes in respiratory rate and t eq. A two-sample t test resulted in no significant differences between the lung injury and healthy control group before or after hemodynamic intervention. Furthermore, exponential extrapolation allowed prediction of the new PetCO2 equilibrium and t eq after 5.7 ± 5.6 min. Conclusions: The transition between PetCO2 equilibria after changes in respiratory rate might not be dependent on moderate lung injury or cardiac output but on the metabolic production or capacity of cO2 stores. Linear relationships previously found for lung-healthy patients and early prediction of PetCO2 equilibration could therefore also be used for the titration of respiratory rate on the PetCO2 for a wider range of pathologies by the physician or an automated ventilation system.

Simultaneous monitoring of intratidal compliance and resistance in mechanically ventilated piglets: A feasibility study in two different study groups.

Compliance measures the force counteracting parenchymal lung distension. In mechanical ventilation, intratidal compliance-volume (C(V))-profiles therefore change depending on PEEP, tidal volume (VT), and underlying mechanical lung properties. Resistance counteracts gas flow through the airways. Due to anatomical linking between parenchyma and airways, intratidal resistance-volume (R(V))-profiles are hypothesised to change in a non-linear way as well. We analysed respiratory system mechanics in fifteen piglets with lavage-induced lung injury and nine healthy piglets ventilated at different PEEP/VT-settings. In healthy lungs, R(V)-profiles remained mostly constant and linear at all PEEP-settings whereas the shape of the C(V)-profiles showed an increase toward a maximum followed by a decrease (small PEEP) or volume-dependent decrease (large PEEP). In the lavage group, a large drop in resistance at small volumes and slow decrease toward larger volumes was found for small PEEP/VT-settings where C(V)-profiles revealed a volume-dependent increase (small PEEP) or a decrease (large PEEP and large VT). R(V)-profiles depend characteristically on PEEP, VT, and possibly whether lungs are healthy or not. Curved R(V)-profiles might indicate pathological changes in the underlying mechanical lung properties and/or might be a sign of derecruitment.

Dynamic Hysteresis Behaviour of Respiratory System Mechanics

The static pressure-volume (PV) curve of the respiratory system is characterized by hysteresis behaviour. Determination of separate inspiratory and expiratory compliance is required to analyse this phenomenon during the dynamic situation of mechanical ventilation. In five piglets expiratory flow was linearized (flow-controlled expiration, FLEX) to allow for compliance estimation separately for inspiration and expiration. Expiratory compliance was higher than inspiratory compliance along the entire intratidal course, converging at higher volumes. At higher PEEP levels expiratory and inspiratory compliance tended to run more in parallel. We conclude that the analysis of the separate inspiratory and expiratory compliance profiles allows for indicating unfavourable mechanical ventilation settings.

Quality index control system for shape identification of intratidal compliance-volume curve with fuzzy logic

One of the goals of mechanical ventilation is to sustain alveolar recruitment while avoiding excessive lung overinflation as well as underinflation [1]. To achieve this goal the patient should be ventilated within that range of the PV-loop, where the compliance of the respiratory system is maximal. In the compliance-volume curve six shape categories can be identified [2]. Our quality index control system with fuzzy logic evaluates the quality and reliability of the compliance shape identification. If the quality index is evaluated as “o.k.”, “good” or “very good” the identified shape will be considered, otherwise the identified compliance shape will be neglected. The quality index control system helps to obtain maximal compliance avoiding errors by the compliance shape identification.

The shape of intratidal resistance-volume and compliance-volume curves in mechanical ventilation - an animal study.

In respiratory system mechanics, the shape of the intratidal pulmonary compliance-volume curve can be used to detect atelectasis or overdistension in the mechanically ventilated lung and thus to op ...

Analysis of Dynamic Respiratory Mechanics Profits from Breathing-Phase Selective Filtering.

Cardio-pulmonary coupling induces cardiogenic oscillations to the respiratory signals, which appear most prominently in the expiration. We hypothesized that the analysis of respiratory system mechanics profits from the breathing phase-selective filtering of expiratory data. Using the gliding-SLICE method, intratidal dynamic respiratory system mechanics were analyzed without and with low-pass filtering (cut-off-frequency f=4 Hz) of expiratory or inspiratory data separately. The quality of data analysis was derived quantitatively from the coefficient of determination (R). The selective filtering of expiration data eliminates negative side-effects of cardiogenic oscillations thus leading to a significant improvement of the analysis of dynamic respiratory system mechanics.