S. Kjærgaard

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

ObjectiveTo present a decision support system for optimising mechanical ventilation in patients residing in the intensive care unit.MethodsMathematical 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.ResultsThe 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.ConclusionsThe system illustrates how mathematical models combined with decision theory can aid in the difficult compromises necessary when deciding on ventilator settings.

The automatic lung parameter estimator (ALPE) system: non-invasive estimation of pulmonary gas exchange parameters in 10-15 minutes.

Objective.Clinical measurements of pulmonary gas exchangeabnormalities might help prevent hypoxaemia and be useful in monitoringthe effects of therapy. In clinical practice single parameters are oftenused to describe the abnormality e.g., the “effectiveshunt.” A single parameter description is often insufficient,lumping the effects of several abnormalities. A more detailed picturecan be obtained from experiments where FIO2 is varied and twoparameters estimated. These experiments have previously taken30–40 minutes to complete, making them inappropriate for routineclinical use. However with automation of data collection and parameterestimation, the experimental time can be reduced to 10–15 minutes.Methods.A system has been built for non-invasive, Automatic,Lung Parameter Estimation (ALPE). This system consists of a ventilator,a gas analyser with pulse oximeter, and a computer. Computer programscontrol the experimental procedure, collect data from the ventilator andgas analyser, and estimate pulmonary gas exchange parameters. Use of theALPE system, i.e. in estimating gas exchange parameters and reducingexperimental time, has been tested on five normal subjects, two patientsbefore and during diuretic therapy, and on 50 occasions in patientsbefore and after surgical intervention. Results.The ALPE systemprovides estimation of pulmonary gas exchange parameters from a simple,clinical, non-invasive procedure, automatically and quickly. For normalsubjects and in patients receiving diuretic therapy, data collection byclinicians familiar with ALPE took (mean ± SD) 13 min 40 sec± 1 min 23 sec. For studies on patients before and after surgery,data collection by an intensive care nurse took (mean ± SD) 10min 47 sec ± 2 min 14 sec. Parameter estimates were: for normalsubjects, shunt = 4.95% ± 2.64% and fA2 = 0.89± 0.01; for patients with heart failure prior to diuretictherapy, patient 1, shunt = 11.50% fA2 = 0.41, patient 2 shunt =11.61% fA2 = 0.55; and during therapy: patient 1, shunt =11.51% fA2 = 0.71, patient 2, shunt = 11.22% fA2 = 0.49.Conclusions.The ALPE system provides quick, non-invasiveestimation of pulmonary gas exchange parameters and may have severalclinical applications. These include, monitoring pulmonary gas exchangeabnormalities in the ICU, assessing post-operative gas exchangeabnormalities, and titrating diuretic therapy in patients with heartfailure.

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.

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 ΔPO2, which is the drop in oxygen pressure from alveoli to lung capillaries. Shunt and ΔPO2 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, ΔPO2 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, ΔPO2 decreased to 2.2 kPa (range 0-3.5, P < 0.05). Conclusions: Ventilation/perfusion mismatch (ΔPO2), 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.

In-hospital outcome for diabetic patients with acute myocardial infarction in the thrombolytic era.

A 3-year retrospective study was carried out at the Department of Cardiology, Aalborg Hospital, Denmark. The aim of the study was to investigate the in-hospital mortality and complications resulting from acute myocardial infarction in diabetic patients compared with non-diabetic patients in the thrombolytic era and to investigate the correlation between mortality and blood glucose levels in diabetic patients. All patients admitted to the study suffered acute myocardial infarctions. One hundred and twenty-three patients with diabetes and 856 patients without diabetes were included. Mortality was 13% (110 patients) in non-diabetic patients compared with 28% (34 patients) in diabetic patients (p = 0.00002). Eighty-nine patients with diabetes (72%) experienced heart failure or a worsening of heart failure compared with 424 patients without diabetes (50%), p = 0.00001. Twenty-eight diabetic patients (23%) had high-degree atrioventricular block, compared with only 99 non-diabetic patients (12%), p = 0.001. Atrial fibrillation developed in 35 patients with diabetes (28%) and in only 141 patients without diabetes (16%), p = 0.002. No difference was seen in occurrence of ventricular tachyarrhythmias. Diabetic patients with a fatal outcome had significantly higher blood glucose values at admission compared with diabetic patients who survived (17.1 +/- 8.3 vs 13.5 +/- 6.3 mmol/l; p = 0.034), and during hospitalization (85.7 +/- 26.0% of blood glucose values exceeding 10 mmol/l vs 64.5 +/- 33.1; p = 0.00065). In the thrombolytic era diabetic patients with acute myocardial infarction had a higher mortality and experienced more complications during hospitalization compared with non-diabetic patients, and diabetic patients with a fatal outcome had higher blood glucose levels compared with surviving diabetic patients.

Oxygenation within the first 120 h following coronary artery bypass grafting. Influence of systemic hypothermia (32 ⁰C) or normothermia (36 ⁰C) during the cardiopulmonary bypass: a randomized clinical trial

Background:  Lung function is often impaired after cardiac surgery performed under cardiopulmonary bypass (CPB). Normothermic CPB has become more common, but it remains unknown whether it reduces post‐operative lung function compared with hypothermic CPB. The aim of this study was to investigate oxygenation within the first 120 h after systemic hypothermia and normothermia under CPB.

Thrombolytic Therapy in Diabetic Patients with Acute Myocardial Infarction

OBJECTIVE To compare the frequency of thrombolytic therapy in diabetic and nondiabetic patients with acute myocardial infarction (MI) and to examine why some diabetic patients do not receive thrombolytic therapy. RESEARCH DESIGN AND METHODS Retrospective study of all diabetic patients with acute MI admitted to the coronary care unit of Aalborg Hospital within a 3-year period. RESULTS Only 35% (43 of 123) of patients with diabetes compared with 47% (404 of 856) of patients without diabetes received thrombolytic therapy (P < 0.002). There was no difference in the percentage of patients thrombolyzed among patients admitted to the hospital within 12 h after onset of symptoms. Of diabetic patients who were not thrombolyzed, 60% (48 of 80) arrived at the hospital later than 12 h after onset of symptoms. Among patients who arrived late, 63% (35 of 56) had Q wave infarction and 84% (47 of 56) had symptoms typical of acute MI. Mortality was 29% (16 of 56) in this group. Only one patient did not receive thrombolytic therapy due to diabetic retinopathy. CONCLUSIONS Significantly fewer diabetic patients received thrombolytic therapy compared with patients without diabetes. The main reason diabetic patients did not receive thrombolytic therapy was late arrival to the hospital.

Simulation of Pulmonary Pathophysiology During Spontaneous Breathing

This paper presents a functional model of lung mechanics including a non-linear alveolar pressure volume curve and representation of the work of respiratory muscles during breathing. The model is used to simulate the response to forced inspiration and expiration, and these simulations compared to the standard results of lung function tests routinely performed in departments of lung medicine. The model can simulate the characteristics of inspiratory and expiratory flow profiles seen in normal subjects, and in patients with obstructive or restrictive diseases

The intelligent ventilator project: application of physiological models in decision support

This paper describes progress in a model-based approach to building a decision support system for mechanical ventilation. It highlights that the process of building models promotes generation of ideas and describes three systems resulting from this process, i.e. for assessing pulmonary gas exchange, calculating arterial acid-base status; and optimizing mechanical ventilation. Each system is presented and its current status and impact reviewed.

An automated method for measuring static pressure–volume curves of the respiratory system and its application in healthy lungs and after lung damage by oleic acid infusion

Elastic pressure/volume (PV) curves of the respiratory system have attracted increasing interest, because they may be helpful to optimize ventilator settings in patients undergoing mechanical ventilation. Clinically applicable methods need to be fast, use routinely available equipment, draw the inspiratory and expiratory PV curve limbs, separate the resistive and viscoelastic properties of the respiratory system from the elastic properties, and provide reproducible measurements. This paper presents a computer-controlled method for rapid measurements of static PV curves using a long inflation-deflation with pauses, and its evaluation in six pigs before and after lung damage caused by oleic acid. The method is fast, i.e. 20.5 +/- 1.9 s (mean +/- SD) in healthy lungs and 17.7 +/- 4.1 s in diseased lungs, this including inspiratory and expiratory pauses of 1.1 s duration. In addition the only equipment used was a clinical ventilator and a PC. For healthy and damaged lungs expiratory PV curve limbs were very reproducible and were at higher volume than the inspiratory limbs, indicating hysteresis. For damaged lungs inspiratory PV limbs were reproducible. For healthy lungs the inspiratory limbs were reproducible but only after the first inflation-deflation. It is possible that during the first inflation alveoli are recruited which are not derecruited on deflation, shifting the inspiratory limb of the PV curve. The paused long inflation-deflation technique provides a quick, automated measurement of static PV curves on both inspiratory and expiratory limbs using routinely available equipment in the intensive care unit.