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S Andreassen

Publications and source records attributed to S Andreassen.

At least 19 recordsLinked to original sources
PubMed2006

Using physiological models and decision theory for selecting appropriate ventilator settings.

OBJECTIVE: To present a decision support system for optimising mechanical ventilation in patients residing in the intensive care unit. METHODS: Mathematical models of oxygen transport, carbon dioxide transport and lung mechanics are combined with penalty functions describing clinical preference toward the goals and side-effects of mechanical ventilation in a decision theoretic approach. Penalties are quantified for risk of lung barotrauma, acidosis or alkalosis, oxygen toxicity or absorption atelectasis, and hypoxaemia. RESULTS: The system is presented with an example of its use in a post-surgical patient. The mathematical models describe the patient's data, and the system suggests an optimal ventilator strategy in line with clinical practice. CONCLUSIONS: The system illustrates how mathematical models combined with decision theory can aid in the difficult compromises necessary when deciding on ventilator settings.

Carbon Dioxide↗
PubMed2006

Reproduction of MIGET retention and excretion data using a simple mathematical model of gas exchange in lung damage caused by oleic acid infusion.

The multiple inert-gas elimination technique (MIGET) is a complex mathematical model and experimental technique for understanding pulmonary gas exchange. Simpler mathematical models have been proposed that have a limited view compared with MIGET but may be applicable for use in clinical practice. This study examined the use of a simple model of gas exchange to describe MIGET retention and excretion data in seven pigs before and following lung damage caused by oleic acid infusion and subsequently at different levels of positive end-expiratory pressure. The simple model was found to give, on average, a good description of MIGET data, as evaluated by a chi(2) test on the weighted residual sum of squares resulting from the model fit (P > 0.2). Values of the simple model's parameters (dead-space volume, shunt, and the fraction of alveolar ventilation going to compartment 2) compared well with the similar MIGET parameters (dead-space volume, shunt, log of the standard deviation of the perfusion, log of the standard deveation of the ventilation), giving values of bias and standard deviation on the differences between dead-space volume and shunt of 0.002 +/- 0.002 liter and 7.3 +/- 2.1% (% of shunt value), respectively. Values of the fraction of alveolar ventilation going to compartment 2 correlated well with log of the standard deviation of the perfusion (r(2) = 0.86) and log of the standard deviation of the ventilation (r(2) = 0.92). These results indicate that this simple model provides a good description of lung pathology following oleic acid infusion. It remains to be seen whether physiologically valid values of the simple model parameters can be obtained from clinical experiments varying inspired oxygen fraction. If so, this may indicate a role for simple models in the clinical interpretation of gas exchange.

Animals↗
PubMed2005

Model predictive glycaemic regulation in critical illness using insulin and nutrition input: a pilot study.

Stress-induced hyperglycaemia is prevalent in intensive care, impairing the immune response. Nutritional support regimes with high glucose content further exacerbate the problem. Tight glucose control has been shown to reduce mortality by up to 43% if levels are kept below 6.1 mmol/L. This research develops a control algorithm with insulin and nutritional inputs for targeted glucose control in the critically ill. Ethics approval for this research was granted by the Canterbury Ethics Committee. Proof-of-concept clinical pilot trials were conducted on intubated, insulin-dependent Christchurch ICU patients (n=7) on constant nutritional support. A target 10-15% reduction in glucose level per hour for a desired glucose level of 4-6 mmol/L was set. 43% and 91% of glucose targets were achieved within +/-5 and +/-20%, respectively. The mean error was 8.9% (0.5 mmol/L), with an absolute range [0, 2.9] mmol/L. End glucose levels were 40% lower compared to initial values. All large target errors are attributable to sudden changes in patient physiology at low glucose values, rather than systemic deficiencies. Target errors are consistent with and explainable by published sensor error distributions. The results show that intensive model-based glucose management with nutrition control reduced absolute glucose levels progressively while reducing the severity of glycaemic fluctuation even with significant inter-patient variability and time-varying physiological condition. Trials spanning longer periods of time are in development to verify the short-term pilot studies performed and to test the adaptability of the controller. Clinically, these results indicate potential in clinical use to reduce ICU mortality as well as reduce risk of severe complications.

Aged↗
PubMed2005

A method for calculation of arterial acid-base and blood gas status from measurements in the peripheral venous blood.

In non-emergency medical departments such as internal medicine sampling of arterial blood and analysis for acid-base status is not routinely performed. Peripheral venous blood is routinely taken but interpretation of its acid-base status is difficult. This paper presents a method for calculation of arterial acid-base and blood gas status from measurements in peripheral venous blood combined with a pulse oximeter measurement of arterial saturation. The use of the method has been illustrated using the data of three patients with different acid-base, haemodynamic, and metabolic conditions. The sensitivity of the method has been tested for measurement errors including venous blood acid-base and blood gas status and pulse oximetry; errors due to physiological assumptions including the values of RQ and strong acid production at the tissues; and errors due to air bubbles in the blood. Errors due to these effects are relatively insignificant except for errors in calculated arterial PO(2), particularly when SpO(2) is greater than 97%; and errors when the change in base excess across the sampling site due to strong acid production is greater that 1.3 mmol/l.

Acid-Base Equilibrium↗
PubMed2005

Family members' experiences, information needs and information seeking in relation to living with a patient with oesophageal cancer.

A qualitative study was conducted with the aim to describe family members' experiences, information needs and information seeking in relation to living with a patient suffering from oesophageal cancer. Data were collected by means of semi-structured interviews with nine family members. A content analysis was used in order to organize data. It was found that family members were not aware of the severe diagnosis and the illness caused intrusions on the family. The time following diagnosis family members' information seeking was low. They used interpersonal as well as mass media sources to obtain knowledge about and handle the uncertainty related to the illness. Some family members did not actively seek information. In conclusion, healthcare professionals are expected to improve quality of cancer care. These improvements are unlikely to occur without an understanding of family members' needs. This study shows that the family members were unprepared of receiving a diagnosis of oesophageal cancer. They emphasized the importance of including the children in the care given. Moreover, the whole family was faced with uncertainty, which led to intrusion on everyday life. Therefore, family members used different strategies for managing the uncertainty, whereof one was searching for information. The primary source of information was the physician.

Adult↗
PubMed2005

Mathematical models of oxygen and carbon dioxide storage and transport: the acid-base chemistry of blood.

This article describes a mathematical model of the acid-base chemistry of blood. The model is formulated from first principles by considering the "components" of blood and the reaction equations in the plasma and erythrocyte fractions. Equations are formulated to describe the total concentration of blood components, the physicochemical properties, and the equilibrium position of reactions. The model includes 28 equations and 12 parameters. All equations can be solved from six variables included in the model. The model uses simple mathematics, without introducing intermediate concepts or linear coefficients necessary for algebraic solution. Model equations are solved simultaneously using numerical methods. Model parameters are estimated and the model verified for plasma, fully oxygenated blood, and deoxygenated blood. Published data are used to estimate model parameters and normal conditions and to verify model simulations. The model reproduces experimental results, including addition or removal of CO2, or strong acid to plasma; CO2, strong acid or haemoglobin to blood; and the effects of deoxygenating blood. The model can also be used as the basis for models of whole body CO2 transport as illustrated in the accompanying article. As such, it is possible to simulate the effects on blood of physiological changes in ventilation or metabolism.

Acid-Base Equilibrium↗
PubMed2005

Mathematical models of oxygen and carbon dioxide storage and transport: interstitial fluid and tissue stores and whole-body transport.

This article describes a mathematical model of whole-body O2 and CO2 transport. The model includes representation of the acid-base chemistry of the blood, interstitial fluid, and tissues, plus transport of O2 and CO2 between compartments representing tissues, interstitial fluid, arterial and venous blood, and lungs. The model includes equations for calculation of all concentrations in the compartments, including equations describing the physicochemical properties and reaction equations of interstitial fluid and tissues. In addition, the model includes equations that describe the flow of substrate between the compartments and differential equations allowing calculation of the changes in state variables caused by the flow of substrates between the compartments. This model is designed to calculate the effects of metabolic and respiratory perturbations, such as variation in breathing pattern or production of strong acid at the tissues. The model reproduces the results of published experiments when used to simulate (1) normal conditions in the lungs, arterial and venous blood, interstitial fluid, and tissues during normal ventilation; (2) the characteristic two-exponential response to changes in minute ventilation; and (3) the relationship between arterial blood values of PCO2 and HCO3,p during inspiration of different fractions of CO2.

Acid-Base Equilibrium↗
PubMed2004

Hypoxaemia after cardiac surgery: clinical application of a model of pulmonary gas exchange.

BACKGROUND AND OBJECTIVE: To investigate the clinical application of a mathematical model of pulmonary gas exchange, which ascribes hypoxaemia to shunt and ventilation/perfusion mismatch. Ventilation/perfusion mismatch is quantified by deltaPO2, which is the drop in oxygen pressure from alveoli to lung capillaries. Shunt and deltaPO2 were used to describe changes in oxygenation after coronary artery bypass grafting. METHODS: Fourteen patients were studied 2-4 h after surgery and on postoperative days 2, 3 and 7. On each occasion inspired oxygen fraction was changed in four to six steps to obtain arterial oxygen saturation (SaO2) in the range of 90-100%, enabling construction of FEO2/SaO2 curves. Measurements of ventilation, circulation and oxygenation were entered in a previously described mathematical model of pulmonary gas exchange. RESULTS: We found that oxygenation was most impaired 3 days after surgery. By fitting the mathematical model to the FEO2/SaO2 curve, we found that shunt remained constant throughout the study period. However, deltaPO2 increased from 0.5 kPa (median, range 0-3.8) 2-4 h after surgery, to 3.2 kPa (range 1.2-6.4, P < 0.05) on day 2, and to 4.0 kPa (range 1.2-8.3) on day 3. On day 7, deltaPO2 decreased to 2.2 kPa (range 0-3.5, P < 0.05). CONCLUSIONS: Ventilation/perfusion mismatch (deltaPO2), rather than shunt, explains the changes in postoperative oxygenation. The model of pulmonary gas exchange may serve as a useful and potentially non-invasive clinical tool for monitoring patients at risk of postoperative hypoxaemia.

Aged↗
PubMed2001

Modelling of hypoxaemia after gynaecological laparotomy.

BACKGROUND: Late postoperative arterial hypoxaemia is common after major surgery, and may contribute to cardiovascular, cerebral or wound complications. This study investigates the time course of hypoxaemia following gynaecological laparotomy, and estimates parameters of mathematical models of pulmonary gas exchange to describe hypoxaemia. METHODS: Twelve patients were studied on four occasions; preoperatively, 2, 8 and 48 h after surgery. On each occasion inspired oxygen fraction (FIO2) was varied, changing end-expired oxygen fraction (FEO2) to achieve arterial oxygen saturations (SaO2) ranging from 90% to 100%. Measurements of ventilation and blood gases were taken. Oxygenation was characterized plotting FEO2 against SaO2. The shape and position of the FEO2/SaO2 curve was described using two mathematical models including parameters describing gas exchange: either shunt and resistance to oxygen diffusion (Rdiff); or shunt and asymmetry of ventilation-perfusion (fA2). RESULTS: Two hours after surgery SaO2 was reduced from 97.5%+/-1.2% (mean+/-SD) to 93.8%+/-2.7% (mean+/-SD) (P<0.001). Values of shunt, Rdiff and fA2 were significantly changed at 2 and 8 h postoperatively. Forty-eight hours postoperatively Rdiff and fA2 were still significantly changed. CONCLUSION: Oxygenation in 12 patients preoperatively, 2, 8 and 48 h after gynaecological laparotomy is described. Two patients were hypoxaemic (SaO2 <92%) 48 h postoperatively. When two different models of oxygen transport are fitted to patient data, high values of Rdiff or low values of fA2 describe the right shift in the FEO2/SaO2 curve seen in patients with oxygenation problems. These models fit patient data identically, and may be useful in quantifying postoperative hypoxaemia.

Adult↗
PubMed2001

A method for diagnosing multiple diseases in MUNIN.

A new method for diagnosing multiple diseases in large medical decision support systems based on causal probabilistic networks is proposed. The method is based on characteristics of the diagnostic process that we believe to be present in many diagnostic tasks, both inside and outside medicine. The diagnosis must often be made under uncertainty, choosing between diagnoses that each have small prior probabilities, but not so small that the possibility of two or more simultaneous diseases can be ignored. Often a symptom can be caused by several diseases and the presence of several diseases tend to aggravate the symptoms. For diagnostic problems that share these characteristic, we have proposed a method that operates in a number of phases: in the first phase only single diseases are considered and this helps to focus the attention on a smaller number of plausible diseases. In the second phase, pairs of diseases are considered, which make it possible to narrow down the field of plausible diagnoses further. In the following phases, larger subsets of diseases are considered. The method was applied to the diagnosis of neuromuscular disorders, using previous experience with the so-called MUNIN system as a starting point. The results showed that the method gave large reductions in computation time without compromising the computational accuracy in any substantial way. It is concluded that the method enables practical inference in large medical expert systems based on causal probabilistic networks.

Decision Trees↗
PubMed2000

A model of the endogenous glucose balance incorporating the characteristics of glucose transporters.

This paper describes the development and preliminary test of a model of the endogenous glucose balance that incorporates the characteristics of the glucose transporters GLUT1, GLUT3 and GLUT4. In the modeling process the model is parameterized with nine parameters that are subsequently estimated from data in the literature on the hepatic- and endogenous- balances at various combinations of blood glucose and insulin levels. The ability of the resulting endogenous balance to fit blood glucose measured from patients was tested on 20 patients. The fit obtained with this model compared favorably with the fit obtained with the endogenous balance currently incorporated in the DIAS system.

Biological Transport, Active↗
PubMed1999

Using probabilistic and decision-theoretic methods in treatment and prognosis modeling.

Causal probabilistic networks, also called Bayesian networks, allow both qualitative knowledge about the structure of a problem and quantitative knowledge, derived from case databases, expert opinion and literature to be exploited in the construction of decision support systems for diagnosis, treatment and prognosis. This mixing of qualitative and quantitative knowledge will be illustrated, using the selection of antibiotics for a subset of patients with severe infections. The subset consists of patients where bacteria or fungi have been found in the blood. A simple pathophysiological model of infection is used to calculate a prognosis, dependent on the choice of antibiotics. A decision-theoretic approach is used to balance the therapeutic benefit of antibiotic treatment against the cost of antibiotics in the form of direct monetary cost, side effects and ecological cost. A retrospective trial on patients with bacteria or fungi in the blood stemming from the urinary tract indicates that with this approach, it may be possible to suggest balanced choices of antibiotics that not only achieve greater therapeutic benefit, but also reduce the cost of therapy.

Anti-Bacterial Agents↗
PubMed1999

Hypoxemia after coronary bypass surgery modeled by resistance to oxygen diffusion.

OBJECTIVE: To evaluate a model describing postoperative hypoxemia after cardiac surgery by using two variables, i.e., shunt and resistance to oxygen diffusion (Rdff). DESIGN: Estimation of these two variables in normal subjects and postoperative cardiac patients. SETTING: The pulmonary function laboratory for the normal subjects and the intensive care unit for the cardiac patients. PATIENTS/SUBJECTS: Nine postoperative cardiac patients and six healthy subjects. INTERVENTIONS: Inspired oxygen fraction was varied in normal subjects and in cardiac patients 3-6 hrs after surgery. This variation occurred in four to seven steps to achieve arterial oxygen saturations in the range 0.90-1.00. MEASUREMENTS AND MAIN RESULTS: Measurements were taken of arterial oxygen saturation, cardiac output, ventilation, and end-tidal gases at each inspired oxygen fraction. These measurements gave the following estimates for the normal subjects: shunt = 3.9+/-5.4% (mean +/- SD) and Rdiff = -5+/-16 torr/(L/min) [-0.7+/-2.2 kPa/(L/min)]; for the cardiac patients: shunt = 7.7+/-1.8% and Rdiff = 212+/-230 torr/(L/min) [28.2+/-30.6 kPa/(L/min)]. The increase in Rdiff (P = .01) was sufficient to explain the observed hypoxemia in these patients. The value for shunt was not significantly increased in the patients (p = .09). The two-variable model (shunt and Rdff) gave a better prediction of arterial oxygen saturation than a model with shunt as the only variable (p = .02). CONCLUSIONS: In cardiac patients requiring supplementary oxygen, the respiratory abnormality could, in our model, be best described by an increased Rdiff, not by an increased shunt value.

Adult↗
PubMed1999

Dynamic updating in DIAS-NIDDM and DIAS causal probabilistic networks.

Diabetes advisory system (DIAS) is a decision support system, which has been developed to provide advice on the amount of insulin injected by subjects with insulin-dependent diabetes mellitus (IDDM). DIAS employs a temporal causal probabilistic network (CPN) to implement a stochastic model of carbohydrate metabolism. The CPN network has recently been extended to provide also advice to subjects with noninsulin-dependent diabetes mellitus (NIDDM). However, due to increased complexity and size of the extended CPN the calculations became unfeasible. The CPN network was, therefore, simplified and a novel approach employed to generate conditional probability tables. The principles of dynamic CPN's were adopted and, in combination with the method of conditioning, learning, and forecasting, were implemented in a time- and memory-efficient way. An evaluation using experimental data was carried out to compare the original and revised DIAS implementations employing data collected by patients with IDDM, and to assess the a posteriori identifiability of model parameters in patients with NIDDM.

Adult↗
PubMed1999

Empirical treatment of bacteraemic urinary tract infection. Evaluation of a decision support system.

INTRODUCTION: In a Danish county with a low prevalence of antibiotic resistance to most antibiotics, we have constructed and evaluated a decision support system (DSS) for guidance of empirical antibiotic therapy in patients with bacteraemia originating from the urinary tract. METHODOLOGY: The DSS was based upon a causal probabilistic network, and a decision theoretic approach was used to balance the costs of antibiotic therapy against the therapeutic benefit. The costs included direct cost of purchasing antibiotics, side effects, and the risk of development of antibiotic resistance. The therapeutic benefit was defined as the increase in life-expectancy caused by antibiotic therapy. Life-years were chosen as the common currency unit. Four hundred and ninety-one bacteraemias seen during 1992-1994 were used to construct the DSS (derivation set), and 426 bacteraemias during 1995-1996 were used for evaluation (validation set). The cases were identified in a regional register of bacteraemias. The study was non-interventional. RESULTS: The DSS suggested antibiotics which would provide coverage in 377 of the 426 episodes (88.5%) compared to 259 episodes (60.8%) for which empirical therapy actually provided coverage (p < 0.01, McNemar-test). The regimens suggested by the DSS included mecillinam as monotherapy in 240 episodes (56.3%), gentamicin as monotherapy in 81 (19.0%), and a combination of gentamicin and ampicillin in 80 (18.8%). CONCLUSION: A decision theoretic approach shows promise of improving empirical antibiotic treatment, and may be a measure to support an antibiotic policy.

Aged↗