Regional venous–arterial CO2 to arterial–venous O2 content difference ratio in experimental circulatory shock and hypoxia

Background Venous–arterial carbon dioxide (CO2) to arterial–venous oxygen (O2) content difference ratio (Cv-aCO2/Ca-vO2) > 1 is supposed to be both sensitive and specific for anaerobic metabolism. What regional hemodynamic and metabolic parameters determine the ratio has not been clarified. Objectives To address determinants of systemic and renal, spleen, gut and liver Cv-aCO2/Ca-vO2. Methods Post hoc analysis of original data from published experimental studies aimed to address effects of different fluid resuscitation strategies on oxygen transport, lactate metabolism and organ dysfunction in fecal peritonitis and endotoxin infusion, and from animals in cardiac tamponade or hypoxic hypoxia. Systemic and regional hemodynamics, blood flow, lactate uptake, carbon dioxide and oxygen-derived variables were determined. Generalized estimating equations (GEE) were fit to assess contributors to systemic and regional Cv-aCO2/Ca-vO2. Results Median (range) of pooled systemic Cv-aCO2/Ca-vO2 in 64 pigs was 1.02 (0.02 to 3.84). While parameters reflecting regional lactate exchange were variably associated with the respective regional Cv-aCO2/Ca-vO2 ratios, only regional ratios were independently correlated with systemic ratio: renal Cv-aCO2 /Ca-vO2 (β = 0.148, 95% CI 0.062 to 0.234; p = 0.001), spleen Cv-aCO2/Ca-vO2 (β = 0.065, 95% CI 0.002 to 0.127; p = 0.042), gut Cv-aCO2/Ca-vO2 (β = 0.117, 95% CI 0.025 to 0.209; p = 0.013), liver Cv-aCO2/Ca-vO2 (β = − 0.159, 95% CI − 0.297 to − 0.022; p = 0.023), hepatosplanchnic Cv-aCO2/Ca-vO2 (β = 0.495, 95% CI 0.205 to 0.786; p = 0.001). Conclusion In a mixed set of animals in different shock forms or during hypoxic injury, hepatosplanchnic Cv-aCO2/Ca-vO2 ratio had the strongest independent association with systemic Cv-aCO2/Ca-vO2, while no independent association was demonstrated for lactate or hemodynamic variables.

Hypercapnia is essential to reduce the cerebral oxidative metabolism during extreme apnea in humans

The cerebral metabolic rate of oxygen (CMRO2) is reduced during apnea that yields profound hypoxia and hypercapnia. In this study, to dissociate the impact of hypoxia and hypercapnia on the reduction in CMRO2, 11 breath-hold competitors completed three apneas under: (a) normal conditions (NM), yielding severe hypercapnia and hypoxemia, (b) with prior hyperventilation (HV), yielding severe hypoxemia only, and (c) with prior 100% oxygen breathing (HX), yielding the greatest level of hypercapnia, but in the absence of hypoxemia. The CMRO2 was calculated from the product of cerebral blood flow (ultrasound) and the radial artery-jugular venous oxygen content difference (cannulation). Secondary measures included net-cerebral glucose/lactate exchange and nonoxidative metabolism. Reductions in CMRO2 were largest in the HX condition (−44 ± 15%, p < 0.05), with the most severe hypercapnia (PaCO2 = 58 ± 5 mmHg) but maintained oxygen saturation. The CMRO2 was reduced by 24 ± 27% in NM ( p = 0.05), but unchanged in the HV apnea where hypercapnia was absent. A net-cerebral lactate release was observed at the end of apnea in the HV and NM condition, but not in the HX apnea (main effect p < 0.05). These novel data support hypercapnia/pH as a key mechanism mediating reductions in CMRO2 during apnea, and show that severe hypoxemia stimulates lactate release from the brain.

Repeated sprint performance and metabolic recovery curves: effects of aerobic and anaerobic characteristics

To examine the influence of aerobic and anaerobic indices on repeated sprint (RS) performance and ability (RSA), 8 sprinters (SPR), 8 endurance runners (END), and 8 active participants (ACT) performed the following tests: (i) incremental test; (ii) 1-min test to determine first decay time constant of pulmonary oxygen uptake off-kinetics and parameters related to anaerobic energy supply, lactate exchange, and removal abilities from blood lactate kinetics; and (iii) RS test (ten 35-m sprints, departing every 20 s) to determine best (RSbest) and mean (RSmean) sprint times and percentage of sprint decrement (%Dec). While SPR had a 98%–100% likelihood of having the fastest RSbest (Cohen’s d of 1.8 and 1.4 for ACT and END, respectively) and RSmean (2.1 and 0.9 for ACT and END, respectively), END presented a 97%–100% likelihood of having the lowest %Dec (0.9 and 2.2 for ACT and SPR, respectively). RSmean was very largely correlated with RSbest (r = 0.85) and moderately correlated with estimates of anaerobic energy supply (r = –0.40 to –0.49). RSmean adjusted for RSbest (which indirectly reflects RSA) was largely correlated with lactate exchange ability (r = 0.55). Our results confirm the importance of locomotor- and anaerobic-related variables to RS performance, and highlight the importance of disposal of selected metabolic by-products to RSA.