CO2 dissociation curves of oxygenated whole blood obtained at rest and in exercise

Impact of sweep gas flow on extracorporeal CO2 removal (ECCO2R)

BACKGROUND

Veno-venous extracorporeal carbon dioxide (CO₂) removal (vv-ECCO₂R) is increasingly being used in the setting of acute respiratory failure. Blood flow rates range in clinical practice from 200 mL/min to more than 1500 mL/min, and sweep gas flow rates range from less than 1 to more than 10 L/min. The present porcine model study was aimed at determining the impact of varying sweep gas flow rates on CO₂ removal under different blood flow conditions and membrane lung surface areas.

METHODS

Two different membrane lungs, with surface areas of 0.4 and 0.8m², were used in nine pigs with experimentally-induced hypercapnia. During each experiment, the blood flow was increased stepwise from 300 to 900 mL/min, with further increases up to 1800 mL/min with the larger membrane lung in steps of 300 mL/min. Sweep gas was titrated under each condition from 2 to 8 L/min in steps of 2 L/min. Extracorporeal CO₂ elimination was normalized to a PaCO₂ of 45 mmHg before the membrane lung.

RESULTS

Reversal of hypercapnia was only feasible when blood flow rates above 900 mL/min were used with a membrane lung surface area of at least 0.8m². The membrane lung with a surface of 0.4m² allowed a maximum normalized CO₂ elimination rate of 41 ± 6 mL/min with 8 L/min sweep gas flow and 900 mL blood flow/min. The increase in sweep gas flow from 2 to 8 L/min increased normalized CO₂ elimination from 35 ± 5 to 41 ± 6 with 900 mL blood flow/min, whereas with lower blood flow rates, any increase was less effective, levelling out at 4 L sweep gas flow/min. The membrane lung with a surface area of 0.8m² allowed a maximum normalized CO₂ elimination rate of 101 ± 12 mL/min with increasing influence of sweep gas flow. The delta of normalized CO₂ elimination increased from 4 ± 2 to 26 ± 7 mL/min with blood flow rates being increased from 300 to 1800 mL/min, respectively.

CONCLUSIONS

The influence of sweep gas flow on the CO₂ removal capacity of ECCO₂R systems depends predominantly on blood flow rate and membrane lung surface area. In this model, considerable CO₂ removal occurred only with the larger membrane lung surface of 0.8m² and when blood flow rates of ≥ 900 mL/min were used.

Physiological control of diving behaviour in the Weddell sealLeptonychotes weddelli: a model based on cardiorespiratory control theory

Despite being obligate air breathers, many species of marine mammal are capable of spending most of their lives submerged in water. How they do this has been a subject of intense interest to physiologists for over a century,yet we still do not have a detailed understanding of the physiological mechanisms underlying this behaviour. What are the proximate mechanisms that trigger the 'decisions' to submerge and return to the surface? The present study proposes a model intended to address this question, based on fundamental concepts of cardiorespiratory control. Two basic hypotheses are examined by computer simulation, using a mathematical model of the mammalian cardiorespiratory control system with parameter values for an adult Weddell seal: (1) that the control of diving can be considered to be a respiratory control problem, and (2) that dives are initiated and maintained by disfacilitation of respiratory drive, not inhibition. Computer simulations confirmed the plausibility of these hypotheses. Simulated diving behaviour and physiological responses (ventilation, cardiac output, blood and tissue gas tensions) were consistent with published data from freely diving Weddell seals. Dives up to the estimated aerobic dive limit (ADL, 18-25 min) could be simulated without the need for active inhibition of breathing in this model. This theoretical analysis suggests that the most important physiological adjustments occur during the surface interval phase of the dive cycle and include hyperventilation accompanied by high cardiac output, appropriate regulation of cerebral blood flow and central chemoreceptor threshold shifts. During dives, cardiac output, distribution of peripheral blood flow, splenic contraction and peripheral chemoreflex drives were found to modulate physiological and behavioural responses, but were not essential for simulated dives to occur. The main conclusion from this study is that the central chemoreceptor may be an important mechanism involved in the regulation of diving behaviour, implying that CO2, not O2, is the key regulatory variable in this model. This model includes and extends the ADL concept and suggests an explicit mechanism by which the respiratory control system may play a central role in the regulation of diving behaviour. It is likely that respiratory mechanisms are an important component of a hierarchical behavioural control system and further studies are required to test the qualitative and quantitative validity of the model.

A theoretical study of the effect of circadian rhythms on sleep-induced periodic breathing and apnoea

This study employed a mathematical model of the respiratory control system to test the plausibility of the hypothesis that circadian rhythms in respiratory control can significantly influence respiratory stability at sleep onset. Computer simulations utilized a standardized "normal" sleep onset effect, superimposed upon systematic changes in chemoreflex parameters that mimicked the peaks and troughs of normal and high amplitude circadian rhythms. The analysis predicted that circadian influences may augment sleep-induced periodic breathing in nocturnal sleep compared with daytime naps. Furthermore, increased circadian amplitude of chemoreflex threshold, or absence of a circadian rhythm in peripheral chemosensitivity, each acted to stabilize respiration during daytime sleep onset and promote periodic breathing during nocturnal sleep onset. High amplitude circadian rhythms in respiratory control were predicted to cause an increasing number and duration of obstructive apnoeas from early to late night. It is suggested that the circadian timing system creates a nocturnal window of respiratory vulnerability and that abnormal circadian rhythms could potentially induce nocturnal sleep apnoea, even in individuals with normal sleep mechanisms.

Physiological model of CO2 output during incremental exercise

In this study, a physiological model to explain the pathway of CO2 output during incremental exercise was examined by referring to experimental data. Since CO2 output (VCO2) shows multiple correlations with mixed venous CO2 pressure (PvCO2) and arterial CO2 pressure (PaCO2), the increase in the difference between PvCO2 and PaCO2 was considered to be involved in the increase in VCO2. In order to better understand the influence of CO2 pressure, VCO2 was divided into the expiratory CO2 phase (non-lactic VCO2), which was unrelated to lactic acid increase and the expiratory CO2 phase (excess VCO2), which was related to lactic acid increase. As a result, the non-lactic VCO2 significantly correlated to PvCO2. When non-lactic VCO2 was zero, the value of PvCO2 was 43.7 mmHg. This was higher than the resting PaCO2 value. On the other hand, as PaCO2 showed an almost constant value in the low load phase and showed a low value in the high load phase, it was believed that the low value of PaCO2 was related to the excess VCO2 that appeared in the high load phase. The CO2 excess, which was obtained by adding excess VCO2 in terms of the lapse of exercise time, correlated significantly with an increase in lactate in the blood. Based on the results, a model was constructed to illustrate the pathway of CO2 output. The key points of the model were as follows: (1) the use of the blood CO2 dissociation curve as the vector to transport CO2 from tissue to lungs, (2) the standard value of PaCO2 was established in order to divide non-lactic VCO2 and excess VCO2, (3) the dextroversion of the blood CO2 dissociation curve due to lactic acid was connected to excess VCO2, and (4) a decrease in PaCO2 was related to excess VCO2 derived from tissue.

Measurement of blood CO2 concentration with a conventional PCO2 analyzer

OBJECTIVES

CO2 content can be determined from the Pco2 in an acidified (forces all CO2 into solution) and diluted blood sample. However, Pco2 concentrations measured in conventional blood gas analyzers are only correct for samples with a significant buffer capacity (such as whole blood), so that mixing with the Pco2 in the rinse solution and tubing walls does not significantly change the sample Pco2. This study describes a calibration method and validation data for the Radiometer Medical ABL2 CO2 electrode system to accurately measure unbuffered blood samples used in the determination of blood CO2 content (or other aqueous fluids).

DESIGN

Prospective, criterion standard.

SETTING

Laboratory.

MEASUREMENTS AND MAIN RESULTS

Blood samples (0.4 mL) were acidified and diluted with 0.2 M lactic acid. After measuring Pco2, CO2 content was calculated using the CO2 solubility coefficient and the dilution factor of 20. CO2 content was determined in a series of sodium carbonate (Na2CO3) solutions spanning the physiologic range of CO2 content. Regression of the measured vs. the actual CO2 content data generated a straight line with a slope of 0.796 and y-intercept of 12.5 (r2 = .99; n = 48). These coefficients were successfully used to correct CO2 content determined in blood samples into which graduated amounts of sodium carbonate were added.

CONCLUSIONS

This calibration procedure allows accurate measurement of Pco2 in aqueous samples using the Radiometer ABL2 electrode system, and should be applicable to other blood gas analyzers. Necessary syringes and chemicals are readily available, the method is fast and simple, and the sample volume is small. In the practice of critical care medicine, accurate Pco2 measurement in aqueous acidified and diluted blood provides direct determination of blood CO2 content (useful in calculations of modified Fick cardiac output or tissue CO2 production). Determinations of absolute CO2 content in blood requiring complex methodology are not necessary. In addition, accurate measurement of aqueous gastric Pco2 can help determine gastric pH, which is an important marker of tissue perfusion.