Theoretical analysis of oxygen transport during hypothermia

Bidirectional alterations in brain temperature profoundly modulate spatiotemporal neurovascular responses in-vivo

Neurovascular coupling (NVC) is a mechanism that, amongst other known and latent critical functions, ensures activated brain regions are adequately supplied with oxygen and glucose. This biological phenomenon underpins non-invasive perfusion-related neuroimaging techniques and recent reports have implicated NVC impairment in several neurodegenerative disorders. Yet, much remains unknown regarding NVC in health and disease, and only recently has there been burgeoning recognition of a close interplay with brain thermodynamics. Accordingly, we developed a novel multi-modal approach to systematically modulate cortical temperature and interrogate the spatiotemporal dynamics of sensory-evoked NVC. We show that changes in cortical temperature profoundly and intricately modulate NVC, with low temperatures associated with diminished oxygen delivery, and high temperatures inducing a distinct vascular oscillation. These observations provide novel insights into the relationship between NVC and brain thermodynamics, with important implications for brain-temperature related therapies, functional biomarkers of elevated brain temperature, and in-vivo methods to study neurovascular coupling.

Ectothermy and cardiac shunts profoundly slow the equilibration of inhaled anaesthetics in a multi-compartment model

The use of inhalational anaesthesia is ubiquitous in terrestrial vertebrates. Given the dependence of these agents on delivery by the cardiorespiratory system, we developed a new computational model predicting equilibration of inhaled anaesthetics in mammalian and ectotherm conditions including the ability of reptiles to maintain vascular shunts. A multi-compartment model was constructed from simultaneously-solved equations, verified by comparison to the literature for endo and ectotherm physiology. The time to 90% equilibration of anaesthetic in arterial blood (t₉₀) is predicted and used to compare anaesthetics and physiologies. The five to tenfold lower cardiac output and minute ventilation of ectothermic vertebrates is predicted to slow equilibration times by five to ten times leading to 90% equilibration in ectotherm arterial blood of over 200 min, compounded by reduction in body temperature, and the extent of right-to-left vascular shunts. The impact of these findings is also influenced by the solubility coefficient of the anaesthetic, such that at net right-to-left shunt fractions of over 0.8, sevoflurane loses the advantage of faster equilibration, in comparison with isoflurane. We explore clinical strategies to regulate anaesthetic uptake in ectotherms by managing convectional flow especially by supportive ventilation and reduction of the right-to-left shunt.

Oxygen Consumption by Warm Ischemia-Injured Porcine Kidneys in Hypothermic Static and Machine Preservation

Static cold storage (SCS) and hypothermic machine perfusion (HMP) are currently standard methods for renal grafts clinical preservation. Both methods are predominantly implemented without the active delivery of oxygen, even for donation after circulatory death-like kidneys. However, even under severe hypothermia (4°C-6°C), kidneys can consume oxygen and produce ATP. What is not established, though, is to what extent and how SCS and HMP compare in terms of oxygen. Using a porcine preclinical model of renal warm ischemia (WI) to compare SCS and HMP methods, we continuously monitored and quantified oxygen level and consumption along preservation; we also determined prepreservation and postpreservation cortical ATP level; values were given as median and [min; max] range. One-hour WI reduced ATP by ∼90% (from 3.3 [1.7; 4.5] mmol/L tissue in Controls). Oxygen consumption (QO₂, μmol/min per 100 g) was determined from initial solution PO₂ decrease (SCS and HMP) and from arterio-venous difference (HMP). In SCS and HMP, PO₂ decreased rapidly (t_1/2 ∼1 h) from atmospheric levels to 52.9 [38.0; 65.9] and 8.2 [3.0, 16.0] mmHg, respectively. In HMP, QO₂ was 2.7 [0.4; 3.9] versus 0.5 [0.0; 1.3] in SCS (P < 0.05); postpreservation ATP amounted to 5.8 [3.2; 6.5] in HMP versus 0.1 [0.0; 0.2] in SCS. Despite hypothermic conditions in SCS or HMP, donation after circulatory death-like renal grafts require oxygen. Increased oxygen consumption, restored ATP level, and improved histological profile in HMP might explain the established HMP superiority over SCS. These results establish a rational basis for the use of oxygen in hypothermic preservation. Optimal levels required for preservation and graft-type variants remain to be determined.

Hemodilution with liposome-encapsulated low-oxygen-affinity hemoglobin does not attenuate hypothermic cerebral ischemia in rats

Hypothermia decreases cerebral metabolism and increases hemoglobin oxygen affinity. A hypothesis that the reversal of increased oxygen affinity would further attenuate hypothermic cerebral ischemia was tested by evaluating the effects of liposome-encapsulated hemoglobin (LipoHb) with low oxygen affinity (P50 = 40-50 mmHg) on hypothermic incomplete cerebral ischemia. Wistar rats were randomly assigned to one of the following two groups: (A) exchange transfusion with LipoHb solution (Hb = 6 g/dl) (LipoHb, n = 5), (B) no exchange transfusion (control, n = 5). After surface cooling to 22 degrees C, forebrain ischemia was induced for 15 min by bilateral carotid artery occlusion combined with a decrease in the mean arterial pressure (MAP) to 40 mmHg. (31)P-magnetic resonance spectroscopy was performed during ischemia and 45 min of reperfusion. After reperfusion, MAP was significantly higher in the control group than in the LipoHb group (P < 0.01), although there were no significant differences during ischemia. Intracellular pH and phosphocreatine (PCr) levels decreased during ischemia and returned to the preischemic level in both groups following reperfusion. The LipoHb group had a significantly larger decrease and smaller recovery in PCr than the control group (P < 0.0001). Althouth beta-adenosine triphosphate decreased during ischemia in the LipoHb group, it increased in the control group (P < 0.0001). Inorganic phosphate (Pi) increased during ischemia and decreased to the normal value after reperfusion. The LipoHb group experienced a significantly larger production of Pi than the control group (P = 0.02). Hemodilution with high-P50 LipoHb does not reduce ischemic energy depletion induced by hypothermic incomplete forebrain ischemia in rats.

Les variations thermiques modifient les paramètres des gaz du sang : quelles conséquences en pratique clinique ?

OBJECTIVE

To understand changes in blood gases results with core temperature.

METHODS

Analysis from two case reports.

RESULTS

Hypothermia induces a decrease in PaCO(2) with a related increase in pH, thus a physiologic alkalosis. Decrease in PaCO(2) is due to an increase of gas solubility and a decrease of peripheral consumption that can be estimated from comparison between corrected and non-corrected for temperature blood gases. For O(2), variations of temperature induce variations of solubility but also of haemoglobin affinity for O(2). During hyperthermia, haemoglobin affinity for O(2) is decreased with a decreased SvO(2) for a same PvO(2). SvO(2) ischemic or therapeutic thresholds are thus modified with core temperature.

CONCLUSION

Blood gases cannot be understood without patient core temperature. Physiologic variations of PaCO(2) and pH must probably be tolerated. Ischemic threshold should be estimated on PvO(2), not only on PvO(2).

Protective effects of moderate hypothermia on phosphoenergetic metabolism in rat liver during gradual hypoxia studied by in vivo 31P nuclear magnetic resonance spectroscopy

BACKGROUND

Hepatic hypoxia during resuscitation and other critically ill conditions is a serious cause of acute hepatic failure. Measurement of the ATP concentration is a sensitive method to evaluate the extent of hypoxic damage in the liver. Hypothermia has been shown to attenuate organ injury in hypoxia. Our aim was to evaluate the effects of moderate hypothermia on the hepatic energy metabolism in rats during gradual hypoxia using (31)P nuclear magnetic resonance (NMR) spectroscopy.

MATERIALS AND METHODS

The rats were divided into two groups: a normothermia group (n = 8, rectal temperature at 37-37.5 degrees C) and a hypothermia group (n = 8, rectal temperature at 30-32 degrees C). The fraction of inhaled oxygen (F(I)O(2)) was reduced gradually (0.5, 0.2, 0.15, 0.125, 0.1, 1.0) and rectal temperature was regulated using a water perfusion mat under the rat body. Phosphoenergetic metabolism of the liver was evaluated from the changes in peak areas of beta-adenosine triphosphate (ATP) and inorganic phosphate (P(i)) in (31)P NMR spectra. Intracellular pH (pH(i)) was calculated from the chemical shifts between P(i) and alpha-ATP peaks.

RESULTS

In the normothermia group, beta-ATP decreased markedly and P(i) increased during hypoxia, while in the hypothermia group, both beta-ATP and P(i) changed only slightly from the initial state during hypoxia. During hypoxia, the minimal changes in beta-ATP were 18% and 80%, and the maximal changes in P(i) were 270% and 160% in the normothermia and hypothermia groups, respectively. Significant difference between the two groups was observed during hypoxia. The recoveries of beta-ATP and P(i) were more complete in the hypothermia group. The decrease in pH(i) during hypoxia was less in the hypothermia group.

CONCLUSIONS

During gradual hypoxia, beta-ATP decreased, P(i) increased, and pH(i) decreased in the rat liver, depending on the oxygen concentration. These changes were more prominent in the NT group than in the HT group. We conclude that moderate hypothermia effectively protects high energy phosphoenergetic metabolites in rat liver during gradual hypoxia as compared to normothermia.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Mild hypothermia alters the oxygen consumption/delivery relationship by decreasing the slope of the supply-dependent line

OBJECTIVE

To evaluate the effect of mild hypothermia on the relationship between systemic oxygen consumption and oxygen delivery.

DESIGN

Prospective animal study.

SETTING

University research laboratory.

SUBJECTS

Anesthetized and ventilated rabbits.

INTERVENTIONS

Rabbits were subjected to stepwise cardiac tamponade to reduce oxygen delivery while body temperature was maintained at 34 degrees C (group H, n = 8) or 39 degrees C (group N, n = 8).

MEASUREMENTS AND MAIN RESULTS

The oxygen consumption/oxygen delivery relationship was analyzed by the dual-line method. The slope of the supply-dependent line was significantly decreased in group H (y = 0.57x + 1.3) compared with that in group N (y = 0.72x + 1.7), indicating that the ability of tissues to extract oxygen was impaired during hypothermia. Consequently, the proportion of the supply-independent area over the entire range of oxygen delivery was decreased in response to hypothermia.

CONCLUSION

The potential for tissue hypoxia is likely to be increased during hypothermia when the circulation becomes unstable and oxygen delivery decreases.

Pharmacological correction of hypothermic P(50) shift does not alter outcome from focal cerebral ischemia in rats

Hypothermia decreases the arterial PO(2) at which hemoglobin is 50% saturated (P(50)), increasing hemoglobin O(2)-binding affinity. We used RSR13, a synthetic allosteric modifier of hemoglobin that increases P(50), to study the role of altered hemoglobin O(2)-binding affinity in mild hypothermic neuroprotection. RSR13 (150 mg/kg iv) restored P(50) to normothermic values. Rats underwent 70 min of middle cerebral artery occlusion (MCAO) at 30.0, 34.0, or 37.5 degrees C with hemoglobin saturation held at 98-100%. The 34.0 degrees C group received RSR13 or vehicle before ischemia. After 7 days of recovery, infarct volumes were reduced in all hypothermic groups, without evidence of a detrimental effect on infarct size or neurological score as a result of P(50) correction. To examine for a beneficial effect of P(50) correction, ischemia duration was increased to 120 min in rats maintained at 34.0 degrees C. Correction of P(50) by RSR13 did not alter cerebral infarct sizes or neurological scores. The decrease in P(50), caused by mild hypothermia, could not be associated with infarct size or neurological deficit resulting from ischemic brain hypoxia in rats.

Relationship between brain temperature, brain chemistry and oxygen delivery after severe human head injury: the effect of mild hypothermia

We studied brain temperature and the effect of mild hypothermia in 58 patients after severe head injury (SHI). Brain tissue oxygen tension (ptiO2), carbon dioxide tension (ptiCO2), tissuie pH (pHti) and temperature (T.br) were measured using a multiparameter probe. Microdialysis was performed to measure glucose, lactate, glutamate, and aspartate in the extracellular fluid. Mild hypothermia (34 degrees-36 degrees C) was employed in 33 selected patients who had persistent increased intracranial pressure (ICP > 20 mmHg). Mild induced hypothermia decreased brain oxygen significantly from 33 +/- 24 mmHg to 30 +/- 22 mmHg (p < 0.05). The ptiCO2 (46 +/- 8 mmHg) was also significantly lower during mild hypothermia (40.4 +/- 4.0 mmHg), p < 0.0001). The pHti increased from 7.13 +/- 0.15 to 7.24 +/- 0.10 (p < 0.0001) under hypothermic conditions. Induced hypothermia may protect patients from secondary ischemic events by lowering the critical ptiO2 threshold, reducing anaerobic metabolism, and decreasing the release of excitatory aminoacids. However, patients with spontaneous brain hypothermia on admission (Tbr < 36.0 degrees C) showed significantly higher levels of glutamate as well as lactate, compared to all other patients, and had a worse outcome. Spontaneous brain hypothermia carries a poor prognosis, and was characterized by markedly abnormal brain metabolic indices.

Cardiopulmonary bypass and oxygen consumption: oxygen delivery and hemodynamics

BACKGROUND

This study was undertaken to investigate the relations between whole body oxygen consumption (VO2), oxygen delivery (DO2), and hemodynamic variables during cardiopulmonary bypass.

METHODS

One hundred one patients were studied during cooling, hypothermia, and rewarming. Oxygen consumption, DO2, hemodynamics, and DO2crit were measured at these times.

RESULTS

There was a direct linear relation between DO2 and VO2 during all three times. No relation between VO2 and hemodynamics was detected during cooling; during hypothermia, an inverse linear relation with peripheral arterial resistance was found. Finally, during rewarming, there was a direct relation with pump flow rate, and an inverse relation with arterial pressure and arterial resistance. The same relations among the variables were found at delivery levels above or below DO2crit.

CONCLUSIONS

During cardiopulmonary bypass there is a direct linear relation between DO2 and VO2; the relations with hemodynamic variables depend on the phases of cardiopulmonary bypass. This suggests that increasing delivery levels may recruit and perfuse more vascular beds, and higher delivery levels are advisable during perfusion. During rewarming and hypothermia, lower arterial resistances are also desirable to optimize VO2.

Hypothermia

Hypothermia in the hospitalized adult may be a primary process, as in exposure, or a result of a multitude of disease processes or iatrogenic factors. The condition affects virtually every metabolic process in the body. A thorough understanding of the pathophysiology of hypothermia enables the clinician to differentiate between the hypothermic syndrome and underlying illness and can assist in the detection and management of clinical sequelae. A reliable patient history is the most helpful diagnostic tool, but careful physical examination and laboratory studies are also important for detection of primary or secondary illness.

The brain uses mostly dissolved oxygen during profoundly hypothermic cardiopulmonary bypass

BACKGROUND

During profoundly hypothermic cardiopulmonary bypass, cerebral venous oxygen saturation increases (eg, to 98% at 15 degrees C). We reanalyzed results of clinical studies to learn why.

METHODS

One hundred sixty-eight cerebral oxygen transport measurements were available from 96 infants and children undergoing profoundly hypothermic cardiopulmonary bypass during repair of congenital heart defects.

RESULTS

Dissolved oxygen accounted for 2% to 17% of arterial oxygen content, depending on the arterial oxygen partial pressure and hemoglobin concentration. The fraction of the cerebral metabolic rate for oxygen obtained from dissolved oxygen depended on pump flow, temperature, hemoglobin concentration, and arterial oxygen partial pressure (all p < 10(-3)). For "full-flow" cardiopulmonary bypass, temperatures less than 18 degrees C, and arterial oxygen partial pressure measurements more than 180 mm Hg, the mean +/- standard deviation of the fraction of cerebral metabolic rate for oxygen obtained from dissolved oxygen equaled 77% +/- 19%.

CONCLUSIONS

Dissolved oxygen satisfies most of the brain's oxygen requirements during profound hypothermic cardiopulmonary bypass. This result reflects four properties of profound hypothermic cardiopulmonary bypass: (1) increases in hemoglobin's oxygen affinity due to profound hypothermia (which impairs oxygen transfer from hemoglobin to cerebral tissue), (2) use of hemodilution, (3) use of high arterial oxygen partial pressure, and (4) low cerebral metabolic rate of oxygen.

Insensitivity of VO2max to hemoglobin-P50 as sea level and altitude

O2 uptake in the lungs, and therefore arterial oxygenation, is favored by a low Hb-P50 but this inhibits tissue O2 extraction, raising the question of optimal P50 during maximal exercise when VO2 is limited by O2 supply. Using a model of the lungs and muscles connected by the circulation so that O2 transport is simultaneously considered in both sites, sensitivity of VO2max to P50 was determined at three altitudes: PB = 760, 464 and 253 Torr, encompassing sea level and Mt. Everest. At Pb = 760 Torr, VO2max peaked at P50 = 30 Torr, was 99.4% (of that at 30) at normal P50 (26.8) and exceeded 95% (of that at P50 = 30) over the P50 range from 24 to 43 Torr. Optimal P50 at PB = 464 Torr was 25 Torr, VO2max at P50 = 26.8 was reduced only 0.1% and exceeded 95% over the P50 range from 19 to 41 Torr. At Pb = 253, optimal P50 was 20 Torr, VO2max at P50 = 26.8 was reduced only 0.4% and exceeded 95% over the wide P50 range from 15 to 47 Torr. While this analysis shows that technically, optimal P50 falls with altitude, VO2max is insensitive to P50 over a range that widens with altitude, so that P50 changes over a realistic interval would not measurably affect VO2max.

Oxygen status algorithm, version 3, with some applications

The Oxygen Status Algorithm is a computer program which uses measurements from a pH & blood gas analyser and a hemoximeter to calculate the oxygen status and the acid-base status of the arterial blood. Version 3 features on-line data collection from the analyser; storage of up to 2000 patient cases in a Lotus 123 file format; printing of a Cumulated Patient Report in addition to the Patient Status Report; combination of arterial and mixed venous data for calculation of the shunt and the oxygen consumption rate (when cardiac output is keyed in); calculation of reference values for fetal haemoglobin for newborns (when gestational age is keyed in). Examples of applications answer the following questions: 1) Does hyperventilation improve the oxygen supply to the tissues? No, for a normal person a slight hypoventilation with a pCO2 of 8.5 kPa provides a maximal oxygen extraction tension. 2) What is the optimal hyperventilation at the top of Mt. Everest (ambient pressure 33 kPa)? Hyperventilation to a pCO2 of about 1.4 kPa provides a maximal oxygen extraction tension of 2.4 kPa for an unacclimatized person. 3) Which change in haemoglobin oxygen affinity would be equivalent to a decrease in arterial pO2 to 6.3 kPa? The oxygen extraction tension would decrease to 4.0 kPa and the same value would be caused by a decrease in half-saturation tension to 2.8 kPa, a decrease which could be due to a moderate alkalaemia (pH = 7.54) combined with a moderately decreased 2,3-diphosphoglycerate concentration (3.4 mmol/L). 4) Is temperature correction of the measured pO2 and pCO2 to the actual body temperature needed? Yes, for example, omitting temperature correction even when the patient temperature is only slightly decreased to 36 degrees C would result in a negative value for the calculated arterio-venous shunt fraction when the actual value, using temperature correction, is 11%. 5) Does the alpha-stat approach of pCO2 and pH regulation in hypothermia, where pH is allowed to rise as in blood in vitro, cause a fall in mixed venous pO2 below the critical value? No, although the mixed venous pO2 will be lower than with the pH-stat approach (constant pH at body temperature), it remains above the critical mixed venous pO2 level. The program is intended for clinical routine use as well as teaching purposes. It has context sensitive help as well as an extensive help index. A number of "demo" cases are provided with annotations in a separate file.

Retrograde cerebral perfusion during hypothermic circulatory arrest reduces neurologic morbidity

Hypothermic circulatory arrest has become an accepted technique for a variety of cardiac and complex aortic operations. However, prolonged periods (> 45 min) of hypothermic circulatory arrest in older patients is associated with marginal cerebral protection and an increased incidence of adverse neurologic events. In an effort to minimize such morbidity, we used a technique of retrograde cerebral perfusion with continuous monitoring of cerebral hemoglobin oxygen saturation during hypothermic circulatory arrest in 35 patients who underwent thoracic aortic operations or resection of intracardiac tumor. There were 27 men and 8 women (mean age 60 years, range 21 to 83 years). Sixteen patients had acute dissection, 6 had contained rupture of a thoracic aortic aneurysm, 10 had either a chronic dissection or aneurysm, and 3 had hypernephromas extending into the heart. Six patients underwent root replacement by means of an open technique for their distal anastomosis, 7 underwent root and partial arch replacement, 12 had root and total arch replacement, 7 had total arch replacement, and 3 had resection of tumor in the heart and retrohepatic vena cava. Seven patients had simultaneous coronary artery bypass grafting, 3 had replacement of one of the arch vessels, and 2 patients had a cesarean section. Sixteen cases were emergency, 6 urgent, and 13 elective. Nine (26%) were reoperations. Thirty-four patients underwent the procedure via a median sternotomy and one patient through a posterolateral thoracotomy. The mean retrograde cerebral perfusion time was 63 minutes (range 35 to 128 minutes), with 30 (86%) patients having more than 45 minutes, 12 (34%) having more than 65 minutes, and 4 (11%) having more than 90 minutes. There was 1 operative death caused by a preoperative myocardial infarction from an aortic dissection, and there were 2 late deaths (multiple organ failure and ruptured total aortic aneurysm). One patient had a stroke with a residual right hemiplegia and a pronounced aphasia. There were no other significant neurologic events or reoperations for bleeding. The average length of stay for patients having elective operations was 11 days and for those having emergency operations, 27 days. At a mean follow-up of 6 months all surviving patients (91%) are well. Hypothermic circulatory arrest is a relatively simple technique that provides a bloodless field and good visualization without the need for aortic crossclamps. Moreover, retrograde cerebral perfusion with continuous monitoring of cerebral oxygen saturation extends the "safe" time for hypothermic circulatory arrest, allowing ample opportunity to perform complicated cardiac and aortic operations with reduced risk of adverse neurologic events.

Cerebral oxygen metabolism during hypothermic circulatory arrest in humans

Profound hypothermia with circulatory arrest is an important surgical adjuvant that allows protected cessation of cerebral blood flow for a brief period. In seven patients undergoing this procedure, continuous spectroscopic measurement of cerebral hemoglobin oxygen saturation was performed. Circulatory arrest at 18 degrees C was associated with a significant progressive desaturation (p < 0.01) of residual cerebral hemoglobin. Arrest time varied based on operative complexity (range 10 to 65 minutes), and a negative linear correlation between arrest time (y) and oxygen saturation (x) was noted (y = -0.87 x + 64). Five patients whose saturation remained above 35% had no neurological injury attributable to hypoxia. One patient (Hunt and Hess Grade 0) whose saturation fell below 35% had evidence of a global hypoxic injury at postmortem examination. Spectroscopically measured cerebral hemoglobin saturation (cerebral oximetry) may be used to monitor metabolic activity during circulatory arrest. Although the clinical utility of such monitoring cannot be established at this time, the potential may exist to prolong the safe duration of induced circulatory arrest for cerebral protection.

Effects of induced hypothermia in patients with septic adult respiratory distress syndrome

OBJECTIVE

To test the hypothesis that treatment with hypothermia affects the course of overwhelming acute respiratory failure associated with sepsis.

DESIGN

Concurrent-controlled, prospective study.

SETTING

Adult multidisciplinary ICU in a teaching hospital.

PATIENTS

Nineteen consecutive patients with septic ARDS mechanically ventilated and a P(A-a)O2 > 500 Torr during 36 h on > or = 10 cm H2O of PEEP.

INTERVENTIONS

Patients were assigned to receive conventional treatment (n = 10) or conventional treatment plus mild hypothermia (32-35 degrees C) instituted as a last resort (n = 9).

RESULTS

Hypothermia (33.7 +/- 0.6 degrees C) was associated with a reduction in mortality rate (67% vs. 100%, P < 0.05), P(A-a)O2 (P < 0.001), heart rate (P < 0.001), cardiac index (P < 0.01), and QS/QT (P < 0.01). There were no significant differences in oxygen consumption (VO2) before (243 +/- 74 ml/min) and during treatment with hypothermia (246 +/- 87 ml/min) although O2 extraction increased during hypothermia (26 +/- 6 vs. 30 +/- 6%, P < 0.05).

CONCLUSIONS

This study suggests that hypothermia was effective in improving oxygenation and survival in patients with severe ARDS associated with sepsis, even though VO2 was unchanged.

Oxygen consumption after cardiopulmonary bypass--implications of different measuring methods

OBJECTIVE

To determine whether intra-pulmonary oxygen consumption or whole body oxygen consumption is the main determinant of the hypermetabolic response after cardiopulmonary bypass. Secondly, which method of measuring oxygen consumption best quantifies this hyperdynamic response.

DESIGN

We measured oxygen consumption by analysing respiratory gas (VO2-gas), carbon dioxide excretion (VCO2), and respiratory exchange ratio (RER = VCO2/VO2), and calculated oxygen consumption using the Fick-method (VO2-Fick) and intra-pulmonary oxygen consumption (VO2-gas - VO2-Fick) in patients at fixed times before and after elective cardiac surgery. Next, comparisons were made between methods and also between measurements at different times before and after bypass.

SETTING

University hospital.

PATIENTS

10 elective cardiac surgical patients.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

VO2-gas, VCO2 and RER were measured with an open circuit indirect calorimeter. VO2-Fick was calculated: VO2-Fick = cardiac index x (arterial - mixed venous oxygen content). Intrapulmonary oxygen consumption was calculated as the difference between VO2-gas and VO2-Fick. Both VO2-gas and VO2-Fick were about 20% higher after bypass than after induction of anaesthesia. Absolute values of VO2-gas were about 30% higher than VO2-Fick. Intra-pulmonary oxygen consumption accounted for 32% of whole body oxygen consumption after induction of anaesthesia and did not increase after bypass.

CONCLUSION

Whole body oxygen consumption and not intra-pulmonary oxygen consumption is the main determinant of the hypermetabolic response after bypass. Increased intra-pulmonary oxygen consumption is not related to bypass. VO2-gas best quantifies this hypermetabolic response directly after bypass, and not VO2-Fick, VCO2 or intra-pulmonary oxygen consumption, since VO2-Fick excludes intra-pulmonary oxygen consumption and VCO2 does not reflect metabolism directly after bypass.

Hypothermia with and without end-expiratory pressure in canine oleic acid pulmonary edema

An important goal in managing patients with respiratory failure using mechanical ventilatory support and positive end-expiratory pressure (PEEP) is to optimize tissue oxygen delivery relative to oxygen consumption. To this end, systemic hypothermia has been reported to reduce oxygen consumption. Cooling, however, may antagonize hypoxic pulmonary vasoconstriction and depress cardiac output. To determine whether these potentially adverse effects of cooling on tissue oxygen delivery would outweigh any potential benefits, we studied the effects of systemic hypothermia and end-expiratory pressure on venous admixture, intrapulmonary blood distribution, and oxygenation variables in 40 dogs with oleic acid-induced pulmonary edema of the right lung. The dogs were randomly assigned to four treatment groups of 10 dogs each: normothermia and zero end-expiratory pressure (ZEEP); normothermia and 10 cm H2O PEEP; hypothermia and ZEEP; hypothermia and PEEP. Hypothermia to 31.9 +/- 0.1 degree C (mean +/- SEM) caused no adverse effects on intrapulmonary blood flow distribution (measured by radioactive microspheres) or on venous admixture. Tissue oxygen delivery and arterial oxygenation did not improve with hypothermia, the latter being 109 +/- 13 mm Hg and 70 +/- 8 mm Hg with PEEP and ZEEP, respectively. However, hypothermia significantly reduced oxygen consumption, so that the coefficient of oxygen delivery (i.e., the ratio of oxygen supply to consumption) increased from 2.5 +/- 0.1 to 3.2 +/- 0.2 (p less than 0.01) with ZEEP and from 2.0 +/- 0.1 to 2.6 +/- 0.3 with PEEP (p = 0.016). Thus, although systemic hypothermia failed to improve arterial oxygenation and tissue oxygen delivery, it decreased systemic oxygen demands, thereby improving the oxygen supply-demand balance.

An integrated view of the determinants of maximum oxygen uptake

This paper develops an analysis of the interaction between circulatory convective and tissue diffusive O2 transport that explains how VO2 max is set by the integrated response to each and every step in the O2 transport chain from atmosphere to mitochondria. We suggest that it is not useful to search for "the" rate limiting step determining VO2max; rather, all steps contribute. Beyond these basic concepts, the theory accounts for observations of VO2max at altitude, permits experimentally testable predictions of VO2max under a variety of conditions, explains the concept of critical O2 delivery below which VO2 cannot be maintained, and suggests an analysis to separate the contributions of central convective from peripheral diffusive adaptations to stimuli such as exercise training and chronic altitude exposure.

Decreased critical mixed venous oxygen tension and critical oxygen transport during induced hypothermia in pigs

The effects of hypothermia on oxygen delivery and tolerance to hypoxia were studied in 8 normothermic (36.8 degrees C) and 10 hypothermic (29.3 degrees C) pigs that had been anesthetized and surgically implanted with instruments. Cardiac output (QT), VO2 [oxygen consumption, or QT X C(a-v)O2, where C(a-v)O2 is arteriovenous oxygen content difference], arterial and mixed venous blood gas values, and lactate concentrations were measured as the animals were made progressively hypoxic. Under control, normoxic conditions, mixed venous oxygen tension (PvO2) was 41.4 +/- 2.1 mm Hg (mean +/- SE) in the normothermic animals and 26.1 +/- 1.6 mm Hg in the hypothermic animals; these values are close to those predicted in our previous theoretical analysis. To study tolerance to hypoxia during hypothermia, critical PvO2 and critical total oxygen transport (TOT = QT X CaO2, where CaO2 is oxygen content of arterial blood) were determined by decreasing the inspired oxygen concentration (FIO2) in steps and measuring the point where VO2 and blood lactate levels became PO2 or TOT dependent. Again as predicted, the critical PVO2 was lower in the hypothermic animals (15.5 +/- 1.0 mm Hg at 29.3 degrees C compared with 22.0 +/- 1.4 mm Hg at 36.8 degrees C), but critical venous oxyhemoglobin saturation values were not statistically different at the two temperatures. Critical TOT was also decreased during hypothermia, as was the margin of reserve in both PVO2 and TOT (the difference between the normoxic and the critical values).