Ivar Hejde Gøthgen

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.

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.

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 extractivity 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.

Oxygen uptake and carbon dioxide elimination after acetazolamide in the critically ill.

Acetazolamide, which reversibly inhibits carbonic anhydrase, is a useful diuretic in alkalotic and over-hydrated patients. In two earlier investigations we have consistently found increases in the arterial and venous oxygen saturation and tension when patients were treated with acetazolamide 15 mg·kg-1. A plausible explanation of this phenomenon is that acetazolamide diminishes oxygen consumption. In the present study we measured oxygen uptake in 10 critically ill patients. We found a minor and statistically insignificant decrease in oxygen consumption. Nevertheless SvO2 increased from 0.77 to 0.83 and PvO2 from 5.9 kPa to 6.8 kPa. It is still not possible from this investigation to determine the origin of the improvement in blood oxygenation. The inhibition of carbonic anhydrase caused a CO2 retention of 5.8% of the total CO2 production. An increase in body stores of CO2 of this magnitude is without clinical significance.

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.

Late Patency of Clinical Microvascular Anastomoses to Free Composite Tissue Transplants: I. Angiographical Aspects

In a series of sixty-five free composite tissue transplantations by means of microsurgical techniques, 37 transplantations were performed to recipient sites in the lower extremity in 34 patients. Twenty-eight of these patients with 31 composite transplants were examined by femoral arteriography to assess the patency of 33 arterial anastomotic lines 6-45 months, median 12 months, postoperatively. Angiography showed patency in 32 of 33 arterial anastomotic lines, one arteriography being inconclusive. The results suggest that microsurgical techniques in small vessel anastomosis (ext. diam, 3/4-3 1/2 mm) are essential prerequisites for long-term vascular patency.

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.

The oxygen- and acid-base status during hypothermic cardiopulmonary bypass

In ten patients undergoing coronary artery bypass grafting with hypothermic cardiopulmonary bypass (CPB) the oxygen- and acid-base status were evaluated. The CPB was performed with the pH-stat approach i.e. constant pH (=7.40) at the actual body temperature. The mean mixed venous pO2 at 25 °C was 9.9 kPa (range 5.0 to 22.8) and the mean cBASE in the extracellular fluid was -1.9 mmol·l-1. The mean oxygen uptake rate was 2.1 mmol·min-1·m-2 at 25 °C and changed 8 % per°C. The mean ratio between CO2 elimination rate and O2 uptake rate ranged from 0.70 to 0.93. It is concluded that the pH-stat approach to acid-base status during moderate hypothermia and CPB is safe and that the ratio between oxygen uptake and oxygen delivery may be more appropriate than during the α—stat approach, but knowledge of the fundamental mechanisms influencing the transport of oxygen to tissues during periods of reduced body temperature is still lacking.

Reflection spectrophotometry (OxySat-2®) for monitoring oxygen saturation and hematocrit during extracorporeal circulation

Continuous measurement of sO2 and hematocrit during extracorporeal circulation has become feasible using reflection spectrophotometry. We have compared simultaneous measurements with in-line reflection spectrophotometry (OxySat-2® American Bent-ley), and in-vitro transmission spectrophotometry (OSM3 Hemoximeter® Radiometer) on discrete samples.In 8 patients undergoing open heart surgery during hypothermic extracorporeal circulation with hemodilution arterial and venous sO2 and hematocrit were measured continuous in-line in the extracorporeal circuit. Simultaneous and anaerobically collected blood samples were analyzed within one minute in the OSM3. The relationship between hemoglobin concentration (cHb) measured in the OSM3 and hematocrit (Hct) measured by the OxySat-2 was estimated from a mean cell hemoglobin concentration of 20 mmol·l-1, i.e. Hct (%) = 100 cHb/20.There was a systematic difference between the OxySat-2 and the reference method for sO 2

Computing the Oxygen Status of the Blood from Heated Skin pO2

The non-invasive measurements of pO2 (transcutaneous oxygen) has been shown to be different from arterial pO2 due to dependence upon various factors in the skin.1 Recently a mathematical mode2 has been confirmed in adult ICU patients.3 By means of this model, describing the relation between arterial- and skin pO2, other oxygen parameters of the blood can be calculated from skin pO2 (pO2 (S)) measurements and thereby the oxygen status of the blood may be evaluated by three parameters, i.e. the partial pressure of oxygen (pO2), saturation of hemoglobin with oxygen (sO2) and the concentration of total oxygen (ctO2) all obtained non-invasively.