[Treatment of accidental hypothermia]. / Behandling af accidentel hypotermi.

New knowledge about accidental hypothermia acquired in recent years may simplify treatment and aid the evaluation of prognosis. Evidence of death or severe collapse due to the feared afterdrop has not been published. Afterdrop is a phenomenon of conductive heat loss. Evaluation of rewarming techniques shows that results from forced air rewarming techniques are equivalent to or better than results from invasive rewarming methods, except for rewarming with cardiopulmonary bypass. In hypothermia the most important differential diagnosis is death. Patients who are cold and could be resuscitated must be differentiated from patients, who are cold because they are dead. Experience from abroad has shown that extreme hyperkalaemia may be a useful diagnostic tool.

[Treatment of accidental hypothermia]. / Behandling af accidentel hypotermi.

New knowledge about accidental hypothermia acquired in recent years may simplify treatment and aid the evaluation of prognosis. Evidence of death or severe collapse due to the feared afterdrop has not been published. Afterdrop is a phenomenon of conductive heat loss. Evaluation of rewarming techniques shows that results from forced air rewarming techniques are equivalent to or better than results from invasive rewarming methods, except for rewarming with cardiopulmonary bypass. In hypothermia the most important differential diagnosis is death. Patients who are cold and could be resuscitated must be differentiated from patients, who are cold because they are dead. Experience from abroad has shown that extreme hyperkalaemia may be a useful diagnostic tool.

Oxygen status of arterial and mixed venous blood.

OBJECTIVES: To describe system requirements for determination of the oxygen status of the blood using the oxygen status algorithm, a computer program. To define the oxygen extractivity, a term we propose, of the arterial blood and the oxygen extraction tension. To describe the different causes of tissue hypoxia, and the clinical interpretation of mixed venous oxygen tension and oxygen consumption rate. DATA SOURCES: Previous physiological and clinical studies related to oxygen status of the blood. DATA SYNTHESIS: The oxygen status algorithm calculates the oxygen extraction tension and generates the oxygen graph as an aid in interpreting oxygen status of the patient. A cybernetic scheme explains the causes of tissue hypoxia and forms the basis for the interpretation of changes in the mixed venous oxygen tension. A diagram with the mixed venous oxygen tension on the abscissa and the oxygen consumption rate on the ordinate illustrates the oxygen flux dependent oxygen consumption rate. A graph shows the relationship between mixed venous oxygen tension and oxygen delivery. CONCLUSIONS: The oxygen status of arterial blood comprises three groups of quantities related to arterial oxygen tension, hemoglobin oxygen capacity, and hemoglobin oxygen affinity. Disturbances in one of these groups may be compensated by opposite changes in one or both of the other. The oxygen extraction tension indicates the degree of compensation, and mixed venous oxygen tension is the key parameter in evaluating the presence of a state of oxygen flux-dependent oxidative metabolism.

Classes of tissue hypoxia.

We identify eight causes of tissue hypoxia, falling into three classes, A, B, and C, depending upon the effect on the critical mixed venous pO2 and the optimal oxygen consumption rate. The critical mixed venous pO2 is the value above which the oxygen consumption rate is optimal and independent of the mixed venous pO2 and below which the oxygen consumption rate decreases towards zero. Class A hypoxia: primary decrease in mixed venous pO2. Causes: 1) ischaemic hypoxia (decrease in cardiac output), 2) low-extractivity hypoxia (decrease in oxygen extraction tension, px). Class B hypoxia: primary increase in critical mixed venous pO2. Causes: 1) shunt hypoxia (increased a-v shunting), 2) dysperfusion hypoxia (increased diffusion length from erythrocytes to mitochondria and/or decreased total capillary endothelial diffusion area, e.g., tissue oedema, microembolism), 3) histotoxic hypoxia (inhibition of the cytochrome chain). Class C hypoxia: primary increase in optimal oxygen consumption rate. Causes: 1) uncoupling hypoxia (uncoupling of the ATP formation associated with O2 reduction), 2) hypermetabolic hypoxia (increased energy metabolism, e.g., due to hyperthermia).

Oxygen and acid-base parameters of arterial and mixed venous blood, relevant versus redundant.

A complete pH and blood gas analysis of arterial and mixed venous blood may comprise more than forty different quantities. We have selected sixteen, including patient temperature. The arterial oxygen tension group includes the oxygen tension, fraction of oxygen in inspired air, and fraction of mixed venous blood in the arterial (total physiological veno-arterial shunting). The haemoglobin oxygen capacity group includes effective haemoglobin concentration and fractions of carboxy- and methaemoglobin. The haemoglobin oxygen affinity group includes half-saturation tension and estimated 2,3-diphosphoglycerate concentration of erythrocytes. In a neonatal care unit fraction of fetal haemoglobin needs to be included. The arterial oxygen extra-activity is measured as the oxygen extraction tension, which indicates the degree of compensation among the oxygen tension, capacity, and affinity. The mixed venous group includes mixed venous oxygen tension, and, when measured, cardiac output, and oxygen consumption rate. The acid-base status includes blood pH, arterial carbon dioxide tension, and extracellular base excess. Other quantities such as haemoglobin oxygen saturation, respiratory index, total oxygen concentration (oxygen content), oxygen extraction fraction, oxygen delivery, and several others, provide no essential additional clinical information and are therefore redundant.

Biosensors and bioprobes in anaesthesia and intensive care. From in vitro to in vivo monitoring.

In vitro monitoring is inherently invasive with discrete measurements on blood samples and the results are often delayed an hour or more when the analyses are performed in the central laboratory. The delay may be greatly reduced if the analyses are performed near the patient. In vivo monitoring may be non-invasive and may provide continuous real-time data but the accuracy usually does not match that of in vitro measurements. In vivo monitoring therefore finds its application in the detection of trends of change, and it is needed only for quantities that change rapidly and unpredictably and where a suitable therapeutic action is available. In critically ill patients, this applies to the arterial pO2, pCO2, and pH, and the mixed venous pO2. Ideal in vivo monitoring techniques are not available for all these quantities. In the newborn, the arterial pO2 may be monitored with a transcutaneous pO2 electrode. In the adult, the arterial pO2 may be monitored indirectly by monitoring the arterial oxygen saturation with a pulse oximeter and the mixed venous pO2 by monitoring the mixed venous oxygen saturation with a catheter tip sensor. The arterial pCO2 may be monitored with a transcutaneous pCO2 electrode or by capnography, i.e., by monitoring the end-expiratory pCO2. Other in vivo monitoring techniques such as gastric tonometry for the gastric mucosal pH and thoracic impedance measurement have found some routine application, whereas near-infrared spectrometry for oxy- and deoxyhaemoglobin in the brain, and magnetic resonance spectroscopy for tissue ATP are at the stage of research and development.

Ventilation in ARDS and asthma: the optimal blood gas values.

Artificial ventilation of patients with acute respiratory diseases, i.e. ARDS and severe asthma, may involve the risk of pulmonary oxygen toxicity as well as volutrauma. The relationship between ventilator treatment and volutrauma suggests that only in patients with normal lungs the aim of ventilator treatment should be an arterial carbon dioxide tension and pH within the normal ranges. In patients suffering from a lung disease the clinical target must be based not only upon the arterial blood gases but also upon airway pressure and respiratory tidal volume. Thus during artificial ventilation of a patient with an acute pulmonary disease the following arterial pH and pCO2 optima are proposed: pH 7.35, with a range from 7.1 to 7.4; pCO2 is related to pH but an acceptable range is 5-12 kPa. The lowest acceptable fraction of inspired oxygen and thereby the safe lower level of arterial pO2 for an individual patient depends on many factors. The lower limit may be about 3 kPa, but the arterial pO2 should not be evaluated as an isolated parameter. It is related to the general oxygen transport capability of arterial blood, extractable oxygen, cardiac output and the microcirculation.

Arterial oxygen status determined with routine pH/blood gas equipment and multi-wavelength hemoximetry: reference values, precision, and accuracy.

We measured pH, pCO2, pO2, oxygen saturation, total hemoglobin concentration, and fractions of carboxy- and methemoglobin in arterial blood samples from 35 healthy adults. We used a new algorithm to calculate active hemoglobin concentration, total oxygen concentration, actual half-saturation tension, 2,3-diphosphoglycerate concentration, estimated functional shunt, oxygen extraction tension px (for extracting 2.3 mmol of oxygen per liter of blood, values below 4.5 kPa indicating risk of tissue hypoxia), and the oxygen compensation factor Qx (the factor by which the cardiac output should rise to maintain a normal mixed venous pO2 of 5.0 kPa, factors above 1.5 indicating an extra burden on the heart). Analytical precision was evaluated by duplicate determinations. The accuracy of the half-saturation tension was evaluated by comparison with values for simultaneously drawn venous blood, the accuracy of the calculated concentration of 2,3-diphosphoglycerate by comparison with direct enzymatic measurements. We conclude that all the variables may be determined with sufficient accuracy and precision in healthy adults, provided the oxygen saturation is less than 0.97 and the measurements are performed according to the highest state of the art.

Positive correlation between 'the arterial oxygen extraction tension' and mixed venous pO2 but lack of correlation between 'the oxygen compensation factor' and cardiac output in 38 patients.

pH and blood gases were measured in simultaneous samples of arterial blood from the radial artery and mixed venous blood from the pulmonary artery using an ABL300 and OSM3 (Radiometer A/S, Denmark). Cardiac output was measured by thermodilution. The patients were suffering from chronic obstructive pulmonary disease or adult respiratory distress syndrome. The data indicate that patients respond to a decreased arterial oxygen availability by allowing the mixed venous pO2 to fall rather than by increasing the cardiac output to maintain a normal mixed venous pO2. In other words, the arterial oxygen extraction tension and the oxygen compensation factor were both highly correlated to the mixed venous pO2 but unrelated to the cardiac index. For this reason the arterial oxygen extraction tension appears to be a more relevant parameter of the overall arterial oxygen availability than the oxygen compensation factor. Comparison of the arterial and mixed venous data confirms the accuracy of the Oxygen Status Algorithm for calculating the various oxygen parameters, including the p50, the estimated 2,3-diphosphoglycerate concentration, and the estimated physiological shunt, on the basis of a single arterial blood sample.

Variations in the hemoglobin-oxygen dissociation curve in 10079 arterial blood samples.

A multicenter study including 10079 arterial blood gas measurements were used to describe the clinical variation in the hemoglobin-oxygen dissociation curve i.e. the relationship between measured values of oxygen tension (pO2) versus oxygen saturation (sO2) and the concentration of total oxygen (ctO2). Very large variations in the actual in vivo hemoglobin-oxygen dissociation curve were found. At pO2 = 8 +/- 0.5 kPa the sO2 range was 69.7% to 99.4%, and at sO2 = 90 +/- 2% the pO2 extremes were 3.82 and 18.3 kPa. The actual p50 varied from 2.15 to 6.44 kPa. Arterial pO2 versus oxygen content i.e. at pO2 = 8 +/- 0.5 kPa the total oxygen concentration ranged from 2.04 to 10.76 mmol/L. The results indicate that it is essential to know the actual position of the hemoglobin-oxygen dissociation curve, as well as the hemoglobin concentration in the individual patient, for correct interpretation of pO2 or sO2 in arterial blood.

The oxygen status of the arterial blood revised: relevant oxygen parameters for monitoring the arterial oxygen availability.

The new generation of very accurate multi-wavelength oximeters, e.g. OSM3, for in vitro measurement of the hemoglobin oxygen saturation, total hemoglobin concentration, and carboxy- and methemoglobin fractions opens new aspects of oxygen monitoring. Combined with the data from the blood gas analyzer (e.g. ABL300) these very accurate measurements allow the calculation of several derived oxygen parameters on the basis of a set of newly developed calculation algorithms. The traditional parameters obtained from an arterial sample are the oxygen tension (pO2) and the hemoglobin oxygen saturation (sO2). Clinical examples illustrate that the pO2 and the sO2 even in combination may give misleading information. The new algorithm calculates three extra oxygen parameters. 1) The oxygen extraction tension, px, defined as the tension required to extract 2.3 mmol of oxygen per liter blood. It signals the mixed venous pO2 level on the assumption that the arterio-venous oxygen difference is normal (2.3 mmol/L). 2) The concentration of extractable oxygen, cx, defined as the concentration of oxygen extracted at a tension of 5.0 kPa. 3) The oxygen compensation factor, Qx, derived as (2.3 mmol/L)/cx. It may be interpreted as the increase in cardiac output necessary to maintain a normal mixed venous pO2 of 5 kPa. These three parameters indicate the oxygen availability of the blood and summarize important properties of the arterial blood in relation to oxygen supply of the tissues, including the arterial pO2, the 'active' hemoglobin concentration (equivalent to the oxygen capacity), and the hemoglobin oxygen affinity (p50). The set of data measured with the blood gas analyzer, e.g. the ABL300 combined with the data measured with the OSM3 contains much more information than is routinely utilized. This information is extracted and summarized by our calculation algorithm. Omitting the calculation of the extra oxygen parameters involves a risk of losing valuable information.

Changes in pneumonectomy-space gas tensions.

The development of a simple and reliable method for measurement of the partial pressures of the atmospheric gases offers the possibility of both basic and clinical examination of the air in natural as well as pathological or iatrogenic cavities. From measurements in nine patients a plot of the changes in pO2 and pCO2 in the pneumonectomy space from the end of the thoracotomy to the establishment of equilibrium with the blood gases was made. pCO2 equilibrated faster than pO2 (6- and 50 h respectively). The equilibrium difference between arterial pO2 and pneumonectomy space pO2 was 6.5 kPa (2.5-12.3 kPa) and we propose that measuring this difference may be a sensitive method for the diagnosis of bronchopleural fistula. During the study period one of the patients developed a bronchopleural fistula. The suspicion was based on X-ray findings and was supported by gas analysis from the pneumonectomy space, and conclusively confirmed by bronchoscopy.

Fiber-optic chemical sensors (Gas-Stat) for blood gas monitoring during hypothermic extracorporeal circulation.

Measurements of pO2, pCO2 and pH by optical fluorescence microsensing technology has recently become available for monitoring blood gases during extracorporeal circulation ECC). We have compared simultaneous measurements with fiber-optic sensors (Gas-Stat, Bentley) and electrochemical sensors (ABL-4, Radiometer) on discrete samples. In 10 patients undergoing coronary artery bypass grafting during hypothermic (25 degrees C) ECC and hemodilution (hemoglobin concentration 4 mmol.l-1) arterial and venous pO2, pCO2 and pH were measured in-line in the extracorporeal circuit at the actual blood temperature. Simultaneous and anaerobically collected blood samples in glass syringes were analyzed within five minutes at 37 degrees C in the ABL-4. Linear regression analysis of the values at actual temperature shows the following equations: Gas-Stat = Y, ABL-4 = X: pO2 (kPa): Y = 1.04 X + 0.5 r = 0.95 n = 136; pCO2 (kPa): Y = 0.71 X + 1.5 r = 0.79 n = 136; pH: Y = 0.788 X + 1.590 r = 0.76 n = 136. The advantage of the Gas-Stat is continuous monitoring of blood gas parameters during ECC. The present study shows that measurements of pO2, pCO2 and pH with fiber-optic chemical sensors may be reliable. The differences between the two principles of measurement may be due to unknown factors interfering with the in-line measurements or to variations in sensitivity and stability of the individual sensor.