Decision analysis for large capital purchases: how to buy a ventilator.

We describe a formal decision-making procedure for purchasing intensive care ventilators. We adapted a general decision-making tool known as an additive, compensatory, multiattribute utility model. The model incorporates input from the various stakeholders in the decision. It identifies the factors that are important in the decision and the alternative decision options, weights the factors, ranks the alternative decisions on how well they serve the factors, and finally provides an overall score that identifies the best option. This model provides a more objective and analytical approach than is often used in purchasing decisions. The benefits include simplifying discussion among stakeholders and assisting administrators in justifying major purchase proposals.

A new system for understanding modes of mechanical ventilation.

Numerous ventilation modes and ventilation options have become available as new mechanical ventilators have reached the market. Ventilator manufacturers have no standardized terminology for ventilator modes and ventilation options, and ventilator operator's manuals do not help the clinician compare the modes of ventilators from different manufacturers. This article proposes a standardized system for classifying ventilation modes, based on general engineering principles and a small set of explicit definitions. Though there may be resistance by ventilator manufacturers to a standardized system of ventilation terminology, clinicians and health care equipment purchasers should adopt such a system in the interest of clear communication--the lack of which prevents clinicians from fully understanding the therapies they administer and could compromise the quality of patient care.

An outbreak of Salmonella enteritidis food poisoning from a commercially produced cheese.

Nine suspected cases of food poisoning were reported from three hospitals to the epidemiology and prevention service (Servizio di Epidemiologia e Prevenzione - SEP) of the local health authority in Naples district (Azienda Sanitaria Locale, ASL NA 4) betw

A three-dimensional virtual environment for modeling mechanical cardiopulmonary interactions.

We have developed a real-time computer system for modeling mechanical physiological behavior in an interactive, 3-D virtual environment. Such an environment can be used to facilitate exploration of cardiopulmonary physiology, particularly in situations that are difficult to reproduce clinically. We integrate 3-D deformable body dynamics with new, formal models of (scalar) cardiorespiratory physiology, associating the scalar physiological variables and parameters with the corresponding 3-D anatomy. Our framework enables us to drive a high-dimensional system (the 3-D anatomical models) from one with fewer parameters (the scalar physiological models) because of the nature of the domain and our intended application. Our approach is amenable to modeling patient-specific circumstances in two ways. First, using CT scan data, we apply semi-automatic methods for extracting and reconstructing the anatomy to use in our simulations. Second, our scalar physiological models are defined in terms of clinically measurable, patient-specific parameters. This paper describes our approach, problems we have encountered and a sample of results showing normal breathing and acute effects of pneumothoraces.

Anatomical and physiological simulation for respiratory mechanics.

Injuries in trauma affect anatomical structures, indirectly affecting physiological systems through mechanical behavior and physical proximity. This paper describes the theory for and preliminary results from our approach to couple a three-dimensional (3-D) anatomical model of the chest with a physiological model of respiratory mechanics. In particular, we investigated behavior in quiet, normal breathing and in an open, sucking chest wound. We envision that our integrated simulation of respiratory anatomy and respiratory mechanics could assist students in visualizing and predicting relationships between structural-anatomical and functional-physiological changes in an interactive, 3-D environment.

Modeling respiratory anatomy and physiology in VR.

In trauma, many injuries impact anatomical structures, which may in turn affect physiological processes--not only those processes within the structures, but ones occurring in physical proximity to them as well. Our goal is to endow a 3D anatomical model with physiological mechanisms to demonstrate such effects. Our approach couples deformable object simulation for organs with physiological modeling, in a way that supports three-dimensional animated simulation. We demonstrate our approach through our current model of respiratory mechanics in a virtual 3D environment. Anatomical models that can capture physiological and pathophysiological changes can serve as an infrastructure for more detailed modeling, as well as benefiting surgical planning, surgical training, and general medical education.

Water vapour and temperature dynamics in the upper airways of normal and CF subjects.

Water vapour partial pressure (PH2O) and temperature (T) were measured together, continuously, at the airway opening (either lips or nares) and at the oropharynx of human subjects with normal lungs or with cystic fibrosis (CF). No apparent differences in PH2O or T were found between normal and CF groups breathing ambient air (22 +/- 2 degrees C). During inspiration the relative humidity at the pharynx for nose breathing (95%) was higher than for mouth breathing (75%). For hot air breathing (48 +/- 2 degrees C), the PH2O and relative humidity of inspired gas at the pharynx was lower for the CF group than for the normal group. Also, the CF group had a higher airway surface temperature at the airway openings on inspiration. These data suggest that when the rate of evaporation is sufficiently high, the rate-limiting step may be water transport through the mucosal tissue and/or secretions. At least for the upper airways, this rate limitation is more evident for CF patients than for normal subjects.

Parameter estimation and sensitivity analysis of a nonlinearly elastic static lung model.

A model for the static pressure-volume behavior of the lung parenchyma based on a pseudo-elastic strain energy function was tested. Values of the model parameters and their variances were estimated by an optimal least-squares fit of the model-predicted pressures to the corresponding data from excised, saline-filled dog lungs. Although the model fit data from twelve lungs very well, the coefficients of variation for parameter values differed greatly. To analyze the sensitivity of the model output to its parameters, we examined an approximate Hessian, H, of the least-squares objective function. Based on the determinant and condition number of H, we were able to set formal criteria for choosing the most reliable estimates of parameter values and their variances. This in turn allowed us to specify a normal range of parameter values for these dog lungs. Thus the model not only describes static pressure-volume data, but also uses the data to estimate parameters from a fundamental constitutive equation. The optimal parameter estimation and sensitivity analysis developed here can be widely applied to other physiologic systems.

Mechanics and gas distribution in normal and obstructed lungs during tidal breathing.

Quantitative characteristics of the dynamic mechanical and gas distribution behavior in 6 normal subjects and 5 subjects with COPD were compared during tidal breathing. Transpulmonary pressure, total lung volume, flow, and N2 fraction at the mouth were measured while N2 was washed out from the lung. The washouts were performed at several frequencies and lung volumes. As an index of nonuniform mechanical behavior, we calculated the frequency variation in dynamic pulmonary compliance (CLdyn). Based on a moment analysis of the multibreath N2 washout, we calculated a mean dilution number (MDN) as an index of the inhomogeneity of alveolar gas distribution and mixing. At FRC or above FRC the CLdyn decreased much more with frequency for the COPD subjects than for the normal subjects. The MDN was also much greater in the presence of COPD. However, the frequency dependence of the MDN was small for both the normal and COPD subjects and uncorrelated with the frequency dependence of CLdyn. Because the multibreath N2 washout and the frequency dependence of CLdyn reflect different aspects of ventilation inhomogeneity, these two responses are unique.

Conditioning of inspired air by a hygroscopic condenser humidifier.

The heat and water content of inspired air is critical to the pulmonary viability of patients with artificial airways. By continuously measuring gas conditions in the ventilator circuits of 6 adult ICU patients, we studied the heat and water reclaimed from expired air by a hygroscopic condenser humidifier (HCH) in the circuit. Temperature, partial pressure of water vapor (PH2O) and relative humidity (RH) were determined at the tracheal outlet of the endotracheal tube. The HCH was 63% efficient; the end-inspiratory gas delivered to the patients averaged 30.9 degrees C with a PH2O of 32.5 mm Hg and an RH of 97.3% or, equivalently, an RH of 69.2% referenced to 37 degrees C. These values are lower than those reported in the literature for gas in the trachea during nose breathing of ambient air, but greater than the values reported for mouth breathing of ambient air.

Measurement system for respiratory water vapor and temperature dynamics.

An instrumentation system has been developed to simultaneously measure water vapor and temperature at the same point within respiratory airways during breathing. A mass spectrometer was used to analyze gas continuously sampled through a modified inlet catheter. At the tip of the catheter, gas temperature is sensed by a microbead thermistor. Adequate water vapor dynamics is achieved by a two-step procedure. First, the tip of the sampling catheter is constricted to reduce the catheter's internal pressure and thereby prevent condensation and evaporation. Second, the water vapor signal from the mass spectrometer is compensated electronically to improve its transient response. As part of the evaluation of the system, water vapor and gas temperature were measured in the oropharynx of human subjects.

Dynamic water vapor and temperature calibration system.

The objective evaluation of thermal and humidification processes in the pulmonary system requires accurate dynamic measurements of temperature and water vapor concentration of a flowing gas mixture. The adequacy of instruments used for such measurements can only be determined by dynamic calibration techniques. We have developed a method of producing step changes in temperature and water vapor content of a gas mixture undergoing controlled steady flow. The system consists of two reservoirs and a slide valve that switches a test section between them. The inlet (usually a probe or catheter tip) of the device to be calibrated is positioned in the test section. The flow rate through the test section is minimally changed during the transition between gas from one reservoir to that of the other. The system has been used to analyze the response of a thermistor and a respiratory mass spectrometer to changes in gas temperature and water vapor.

Static mechanics of excised whole lung: theoretical framework and experimental studies.

A theoretical framework is presented in which to view models of static pulmonary mechanics. To test common simplifying assumptions of these models, we performed a set of experiments using normal lungs excised from dogs. Transpulmonary pressure (Ptp) and lung volume (VL) were measured for air-filled lungs in air and saline-filled lungs in saline during stepwise-static deflations at different vascular volumes and temperatures. Simultaneously, we measured displacements between points on the lung surface. Changes in vascular volume shift the location but not the shape of the Ptp-VL relationship. As long as the vascular pressure is in the normal range, changes in the volume (and weight) of the perfusate do not significantly stiffen the parenchyma. Furthermore, Ptp-VL data obtained between 16 degrees C and 40 degrees C were superimposable, indicating that parenchymal mechanical properties evaluated at room temperature are valid at body temperature. Finally, the common assumptions of uniform deflation, homogeneity, and isotropy of bulk lung tissue appear consistent with the relationship between surface displacement and volume changes.

Static mechanics of excised whole lung: pleural mechanics.

Continuum analyses of lung mechanics require that the boundary condition of stress transmitted to the outermost alveoli be known. Depending upon the exact geometry of the pleural-parenchymal coupling, this stress could possibly be influenced by the pleural mechanical properties. The relation between pleural tension and extension ratio was obtained from tissue specimens from mongrel dog lungs. Using the worst-case geometry, this relationship was compared with the equivalent relation between pressure and volume ratio for the whole lung of the same mongrel dogs. The results of this comparison and a suitable mathematical analysis indicate that the pleura transmits applied pressure differences to the underlying alveolar walls essentially without modification.

Computation of respiratory impedance from forced sinusoidal oscillations during breathing.

Computation of impedances from forced oscillation data during breathing can yield results which reflect not only changes in respiratory mechanics, but artifacts related to the signal analysis. A method has been developed, employing sinusoidal forcing, to determine intra-breath variations of respiratory impedance. The measured pressure and flow waveforms are each the sum of a slowly varying constituent associated with breathing, and a high-frequency oscillatory constituent, whose amplitude and phase vary with time. The signal constituents were separated with a moving-average filter. Characteristic amplitudes and phases of the oscillatory constituents over a short time interval (window) were determined by correlating the constituents with sine waves of the same frequency. Continuous estimates of the time-varying impedance were obtained by moving the window over the data. Two procedures were developed to examine the accuracy of impedances computed with this technique: (1) the analysis of fixed-amplitude sinusoids superimposed on a breathing pattern; and (2) the analysis of a known, time-varying impedance. The effects of forcing frequency, window size, breathing frequency, and the position within the respiratory cycle on the computed impedances were examined. For quiet breathing, the technique can yield impedances which are accurate to within 5% in magnitude, and 5 degrees in phase angle at all instants within the breath. An efficient algorithm was developed for implementing the technique on a computer.

Early echocardiographic and pulmonary function findings in idiopathic scoliosis.

Thirty-six children and adolescents with early stages of idiopathic scoliosis underwent evaluation by echocardiography and pulmonary function testing. Mildly increased pulmonary vascular resistance was inferred from an elevated ratio of right preejection period to right ventricular ejection time, an increased right ventricular dimension, and a decreased left ventricular dimension. Since neither decreased arterial oxygen saturation nor increased end-tidal expired carbon dioxide partial pressure was seen, desaturation and hypoventilation should not account for these abnormalities. Pulmonary function parameters showed no distinct patterns of abnormality. Even though the patients were divided into two groups by severity of spinal curvature, the cardiopulmonary measures did not correlate with thoracic deformity. Billowing of the mitral leaflets, termed mitral valve prolapse, was demonstrated in 25% of the subjects. Our findings suggest that cardiopulmonary and thoracic changes in idiopathic scoliosis may develop in parallel and may be expressions of a common collagen defect. However, study of sleep and exercise arterial saturation may be required to rule out intermittent hypoxemia as a precipitating factor of cor pulmonale in scoliosis.

Model simulation of heat and water transport dynamics in an airway.

Heat and water transport processes in the respiratory tract depend on environmental conditions, breathing patterns, and the physiological state of the respiratory system. To study these processes, we have developed a mathematical model of the dynamics of temperature and water vapor in the radial and axial directions of an idealized trachea. The model is expressed as two implicit finite-difference equations and solved using an alternating-direction algorithm. Using these equations, we simulated the effects of inspired gas temperature and humidity, velocity profile, and flow rate on heat and water transport between the gas and airway wall. Under inspired gas conditions of low temperature or high relative humidity, supersaturation occurs. Increasing either the velocity gradient at the wall or the flow rate increases the heat and water transport rates. However, these rates change by only 10 percent when the velocity gradient is doubled, and by about 35 percent when flow rate undergoes a two-fold change. The model can be used with in-vivo data from the trachea to test hypotheses concerning normal and abnormal heat and water transport.

Ventilation inhomogeneity: alveolar mechanics and gas distribution.

The effects of regional lung differences in alveolar mechanics on the transpulmonary pressure-volume (Ptp-V) relationship and the single-breath washout (SBW) of nitrogen were investigated by mathematical modeling and postmorten human lung experiments. Regional nonuniformity in alveolar collapse and re-opening were associated with differences in gravitational stress or elasticity. Model simulations predict that neither type of regional nonuniformity qualitatively affects the shape of the Ptp-V curve, but does affect the terminal (or small-volume) portion of the SBW. Comparisons of characteristics of the Ptp-V and SBW curves indicate that regional nonuniformity in alveolar collapse is an important mechanism associated with ventilation inhomogeneity.