Implications of a biphasic two-compartment model of constant flow ventilation for the clinical setting

Alveolar dynamics during mechanical ventilation in the healthy and injured lung

Mechanical ventilation is a life-saving therapy in patients with acute respiratory distress syndrome (ARDS). However, mechanical ventilation itself causes severe co-morbidities in that it can trigger ventilator-associated lung injury (VALI) in humans or ventilator-induced lung injury (VILI) in experimental animal models. Therefore, optimization of ventilation strategies is paramount for the effective therapy of critical care patients. A major problem in the stratification of critical care patients for personalized ventilation settings, but even more so for our overall understanding of VILI, lies in our limited insight into the effects of mechanical ventilation at the actual site of injury, i.e., the alveolar unit. Unfortunately, global lung mechanics provide for a poor surrogate of alveolar dynamics and methods for the in-depth analysis of alveolar dynamics on the level of individual alveoli are sparse and afflicted by important limitations. With alveolar dynamics in the intact lung remaining largely a "black box," our insight into the mechanisms of VALI and VILI and the effectiveness of optimized ventilation strategies is confined to indirect parameters and endpoints of lung injury and mortality.In the present review, we discuss emerging concepts of alveolar dynamics including alveolar expansion/contraction, stability/instability, and opening/collapse. Many of these concepts remain still controversial, in part due to limitations of the different methodologies applied. We therefore preface our review with an overview of existing technologies and approaches for the analysis of alveolar dynamics, highlighting their individual strengths and limitations which may provide for a better appreciation of the sometimes diverging findings and interpretations. Joint efforts combining key technologies in identical models to overcome the limitations inherent to individual methodologies are needed not only to provide conclusive insights into lung physiology and alveolar dynamics, but ultimately to guide critical care patient therapy.

Optimal determination of respiratory airflow patterns using a nonlinear multicompartment model for a lung mechanics system

We develop optimal respiratory airflow patterns using a nonlinear multicompartment model for a lung mechanics system. Specifically, we use classical calculus of variations minimization techniques to derive an optimal airflow pattern for inspiratory and expiratory breathing cycles. The physiological interpretation of the optimality criteria used involves the minimization of work of breathing and lung volume acceleration for the inspiratory phase, and the minimization of the elastic potential energy and rapid airflow rate changes for the expiratory phase. Finally, we numerically integrate the resulting nonlinear two-point boundary value problems to determine the optimal airflow patterns over the inspiratory and expiratory breathing cycles.

Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years

Auto-positive end-expiratory pressure (auto-PEEP; AP) and dynamic hyperinflation (DH) may affect hemodynamics, predispose to barotrauma, increase work of breathing, cause dyspnea, disrupt patient-ventilator synchrony, confuse monitoring of hemodynamics and respiratory system mechanics, and interfere with the effectiveness of pressure-regulated ventilation. Although basic knowledge regarding the clinical physiology and management of AP during mechanical ventilation has evolved impressively over the 30 years since DH and AP were first brought to clinical attention, novel and clinically relevant characteristics of this complex phenomenon continue to be described. This discussion reviews some of the more important aspects of AP that bear on the care of the ventilated patient with critical illness.

The Language of Caring: Quantitating Medical Practice Patterns using Symbolic Dynamics

Real-world medical decisions rarely involve binary sole condition present or absent-patterns of patient pathophysiology. Similarly, provider interventions are rarely unitary in nature: the clinician often undertakes multiple interventions simultaneously. Conventional approaches towards complex physiologic derangements and their associated management focus on the frequencies of joint appearances, treating the individual derangements of physiology or elements of intervention as conceptually isolated. This framework is ill suited to capture either the integrated patterns of derangement displayed by a particular patient or the integrated patterns of provider intervention. Here we illustrate the application of a different approach-that of symbolic dynamics-in which the integrated pattern of each patients derangement, and the associated provider response, are captured by defining words based on the elements of the pattern of failure. We will use as an example provider practices in the context of mechanical ventilation- a common, potentially harmful, and complex life support technology. We also delineate other domains in which symbolic dynamics approaches might aid in quantitating practice patterns, assessing quality of care, and identifying best practices.

Closed-loop control for intensive care unit sedation

The potential clinical applications of active control for pharmacology in general, and anesthesia and critical care unit medicine in particular, are clearly apparent. Specifically, monitoring and controlling the depth of anesthesia in surgery and the intensive care unit is of particular importance. Nonnegative and compartmental models provide a broad framework for biological and physiological systems, including clinical pharmacology, and are well suited for developing models for closed-loop control for drug administration. These models are derived from mass and energy balance considerations that involve dynamic states whose values are nonnegative and are characterized by conservation laws (e.g., mass, energy, fluid, etc.) capturing the exchange of material between kinetically homogenous entities called compartments. Compartmental models have been particularly important for understanding pharmacokinetics and pharmacodynamics. One of the basic motivations for pharmacokinetic/pharmacodynamic research is to improve drug delivery. In critical care medicine it is current clinical practice to administer potent drugs that profoundly influence levels of consciousness, respiratory, and cardiovascular function by manual control based on the clinician's experience and intuition. Open-loop control (manual control) by clinical personnel can be tedious, imprecise, time-consuming, and sometimes of poor quality, depending on the skills and judgement of the clinician. Closed-loop control based on appropriate dynamical systems models merits investigation as a means of improving drug delivery in the intensive care unit. In this article, we discuss the challenges and opportunities of feedback control using nonnegative and compartmental system theory for the specific problem of closed-loop control of intensive care unit sedation. Several closed-loop control paradigms are investigated including adaptive control, neural network adaptive control, optimal control, and hybrid adaptive control algorithms for intensive care unit sedation.

Theoretical study of inspiratory flow waveforms during mechanical ventilation on pulmonary blood flow and gas exchange

A lumped two-compartment mathematical model of respiratory mechanics incorporating gas exchange and pulmonary circulation is utilized to analyze the effects of square, descending and ascending inspiratory flow waveforms during mechanical ventilation. The effects on alveolar volume variation, alveolar pressure, airway pressure, gas exchange rate, and expired gas species concentration are evaluated. Advantages in ventilation employing a certain inspiratory flow profile are offset by corresponding reduction in perfusion rates, leading to marginal effects on net gas exchange rates. The descending profile provides better CO2 exchange, whereas the ascending profile is more advantageous for O2 exchange. Regional disparities in airway/lung properties create maldistribution of ventilation and a concomitant inequality in regional alveolar gas composition and gas exchange rates. When minute ventilation is maintained constant, for identical time constant disparities, inequalities in compliance yield pronounced effects on net gas exchange rates at low frequencies, whereas the adverse effects of inequalities in resistance are more pronounced at higher frequencies. Reduction in expiratory air flow (via increased airway resistance) reduces the magnitude of upstroke slope of capnogram and oxigram time courses without significantly affecting end-tidal expired gas compositions, whereas alterations in mechanical factors that result in increased gas exchanges rates yield increases in CO2 and decreases in O2 end-tidal composition values. The model provides a template for assessing the dynamics of cardiopulmonary interactions during mechanical ventilation by combining concurrent descriptions of ventilation, capillary perfusion, and gas exchange.