Experimental critical care in ventilated rats: effect of hypercapnia on arterial oxygen-carrying capacity

Improved Long-term Survival with Remote Limb Ischemic Preconditioning in a Rat Fixed-Pressure Hemorrhagic Shock Model

PURPOSE

We investigated whether bilateral, lower limb remote ischemic preconditioning (RIPC) improved long-term survival using a rat model of hemorrhagic shock/resuscitation.

METHODS

Rats were anesthetized, intubated and ventilated, and randomly assigned to RIPC, induced by inflating bilateral pressure cuffs around the femoral arteries to 200 mmHg for 5 min, followed by 5-min release of the cuffs (repeated for 4 cycles), or control group (cuffs were inflated to 30 mmHg). Hemorrhagic shock was induced by withdrawing blood to a fixed mean blood pressure of 30 mmHg for 30 min, followed by 30 min of resuscitation with shed blood. Rats remained anesthetized for 1 h during which hemodynamics were monitored then they were allowed to survive for 6 weeks.

RESULTS

The percentage of estimated total blood volume withdrawn to maintain a level of 30 mmHg was similar in both groups. RIPC significantly increased survival at 6 weeks: 5 of 27 (19%) rats in the control group and 13 of 26 (50%; p = 0.02) rats in the RIPC group survived. Blood pressure was higher in the RIPC group. The diastolic internal dimension of the left ventricle, an indicator of circulating intravascular blood volume, was significantly larger in the RIPC group at 1 h after initiation of resuscitation compared to the control group (p = 0.04). Left ventricular function assessed by fractional shortening was comparable in both groups at 1 h after initiation of resuscitation. Blood urea nitrogen (BUN) was within normal range in the RIPC group (17.3 ± 1.2 mg/dl) but elevated in the control group (22.0 ± 1.7 mg/dl) at 48 h after shock.

CONCLUSIONS

RIPC significantly improved short-term survival in rats that were subjected to hemorrhagic shock, and this benefit was maintained long term. RIPC led to greater circulating intravascular blood volume in the early phase of resuscitation and improved BUN.

T2* and T1 assessment of abdominal tissue response to graded hypoxia and hypercapnia using a controlled gas mixing circuit for small animals

PURPOSE

To characterize T2* and T1 relaxation time response to a wide spectrum of gas challenges in extracranial tissues of healthy rats.

MATERIALS AND METHODS

A range of graded gas mixtures (hyperoxia, hypercapnia, hypoxia, and hypercapnic hypoxia) were delivered through a controlled gas-mixing circuit to mechanically ventilated and intubated rats. Quantitative magnetic resonance imaging (MRI) was performed on a 3T clinical scanner; T2* and T1 maps were computed to determine tissue response in the liver, kidney cortex, and paraspinal muscles. Heart rate and blood oxygen saturation (SaO2 ) were measured through a rodent oximeter and physiological monitor.

RESULTS

T2* decreases consistent with lowered SaO2 measurements were observed for hypercapnia and hypoxia, but decreases were significant only in liver and kidney cortex (P < 0.05) for >10% CO2 and <15% O2 , with the new gas stimulus, hypercapnic hypoxia, producing the greatest T2* decrease. Hyperoxia-related T2* increases were accompanied by negligible increases in SaO2 . T1 generally increased, if at all, in the liver and decreased in the kidney. Significance was observed (P < 0.05) only in kidney for >90% O2 and >5% CO2 .

CONCLUSION

T2* and T1 provide complementary roles for evaluating extracranial tissue response to a broad range of gas challenges. Based on both measured and known physiological responses, our results are consistent with T2* as a sensitive marker of blood oxygen saturation and T1 as a weak marker of blood volume changes. J. Magn. Reson. Imaging 2016;44:305-316.

Hypoxia, not hypercapnia, induces cardiorespiratory failure in rats

Mechanical respiratory loads induce cardiorespiratory failure, presumably by increasing O2 demand concurrently with decreases in O2 availability (decreased PaO2). We tested the hypothesis that asphyxia alone can cause cardiorespiratory failure ("failure") in pentobarbital-anesthetized rats. We also tested the hypothesis that hypoxia, not hypercapnia, is responsible by supplying supplemental O2 during mechanical loading in a separate group of rats. Asphyxia (mean PaO2 and PaCO2 of 43 and 69mmHg, respectively) resulted in failure, evident as a slowing of mean respiratory frequency (133-83breaths/min) and a sudden and large drop in mean arterial pressure (71-47mmHg), after 214±66min (n=16; range 117-355min). Neither respiratory drive nor heart rate decreased, indicating that failure was peripheral, not central. Of 8 rats tested after 3h of asphyxia for the presence in blood of cardiac troponin T, all were positive. In an additional 6 rats, normocapnic hypoxia (mean PaCO2 and PaO2 were 39±2.2 and 41±3.1mmHg, respectively) caused failure after an average 205min (range 181-275min), no different from that of asphyxic rats. In the 6 rats that breathed O2 during an initially moderate inspiratory resistive load, endurances exceeded 7h (failure occurring only because we increased the load after 6h) and tracheal pressure and left ventricular dP/dt were maintained despite supercarbia (PaCO2>150mmHg). Thus, asphyxia alone can induce failure, the failure is due to hypoxia, not hypercapnia, and hypercapnia has minimal effects on cardiac and respiratory muscle function in the presence of hyperoxia.

Retinal tissue oxygen tension imaging in the rat

PURPOSE

To report an imaging technique for measurement of oxygen tension (PO2) in retinal tissue and establish its feasibility for measuring retinal PO2 variations in rat eyes by adjusting the fraction of inspired oxygen (FiO2).

METHODS

A narrow laser line was projected at an angle on the retina, and phosphorescence emission was imaged after intravitreal injection of an oxygen-sensitive molecular probe. A frequency-domain approach was used for phosphorescence lifetime measurements. Retinal PO2 maps were computed from phosphorescence lifetime images, and oxygen profiles through the retinal depth were derived in rats in conditions of 10%, 21%, and 50% FiO2.

RESULTS

Retinal PO2 measurements were repeatable, and variations in outer and inner retina PO2 at different locations along the image were not significant (P>or=0.3). Maximum outer retinal PO2 obtained in 10%, 21%, and 50% FiO2 were significantly different (P<0.0001). Maximum outer retinal PO2 correlated with systemic arterial PO2 (R=0.70; P<0.0001). The slope of the outer retina PO2 profile correlated with maximum outer retinal PO2 (R=0.84; P<0.0001). Mean inner retina PO2 correlated with maximum outer retinal PO2 (R=0.88; P<0.0001).

CONCLUSIONS

A technique has been developed for quantitative mapping of retinal tissue oxygen tension with the potential to enable sequential monitoring of retinal oxygenation in health and disease.

Does hypercapnia ameliorate hyperoxia-induced lung injury in neonatal rats?

Therapeutic hypercapnia (TH), an intentional inhalation of CO(2), has been shown to improve pulmonary function in certain models of lung injury. We tested the null hypothesis that TH does not improve hyperoxic lung injury in neonatal rats. The prospective, randomized study was set at Research laboratory in Children's Hospital. Forty-five newborn rats were randomly assigned to three groups (n = 15/group), and exposed to 96 h of normoxia (FiO(2) = 0.21), hyperoxia (FiO(2) > 0.98), and TH (FiO(2) = 0.95, FiCO(2) = 0.05). Lung histology, wet-weight to dry-weight ratio, and concentrations of pro- and anti-inflammatory cytokines (IL-1beta, IL-6, TNF-alpha, and IL-10) were used to evaluate pulmonary damage. Using a scale of 0-4, the total scores for lungs hypercellularity, inflammation, and hemorrhage was significantly increased from a median value of 1.5 in normoxia to 2.5 in hyperoxia (P < 0.05) and 3.0 with TH (P < 0.001, nonparametric ANOVA). The interstitial space relative to the alveolar space, as a measure of hypercellularity, was increased by 18% during hyperoxia and by 44% with TH compared with normoxia. TH significantly increased the size of the interstitial space by 22% compared with hyperoxia (P < 0.001). The lung wet-weight to dry-weight ratio was increased by 10% in both hyperoxic groups (P < 0.001). Both hyperoxic groups showed significant reductions in the concentration of IL-1beta compared with normoxia (P < 0.001), whereas the ratio of IL-1beta to IL-10 was significantly decreased, indicating an anti-inflammatory trend. TH does not prevent histological manifestations of hyperoxic lung injury in spontaneously breathing neonatal rats and may worsen the outcome.

Physiological effects of a novel immune stimulator drug, (1,4)-α-D-glucan, in rats

The (1,4)-α-D-glucan (α-D-glucan), derived from medicinal plant, Tinospora cordifolia, activates human lymphocytes with downstream synthesis of the pro- and anti-inflammatory cytokines, in vitro. We investigated physiological and immunological effects of a low and a high dose of α-D-glucan (0.5 and 10 mg/kg), in vivo, testing the hypothesis that intravenous administration of α-D-glucan does not affect haemodynamic, respiratory, haematological, and immune responses in normal rats. Male rats (300-400 g) were anaesthetized, tracheostomized, and catheterized in one femoral artery and vein. The mean arterial blood pressure and heart rate were continuously recorded. The baselines for gas exchange, differential blood cell count, and plasma concentration of TNF-α, IL-1β, IL-4, IL-6, and IFN-γ were determined. Rats were then randomly assigned to controls (n = 7), a low dose (0.5 mg/kg; n = 10), and a high dose (10 mg/kg; n = 7) of α-D-glucan for a six 6 hr study period. Gas exchange, differential cell count, plasma concentration of TNF-α, IL-1β, IL-4, IL-6, and IFN-γ, and mean arterial blood pressure values remained within physiological range. Intravenous administration of 10 mg/kg α-D-glucan created tachycardia, associated with hyperventilation, and significant reductions in the blood haemoglobin and haematocrit concentrations. We suggest that these in vivo effects of α-D-glucan should be considered for future clinical and/or experimental trials.

The effects of CO2 on cytokine concentrations in endotoxin-stimulated human whole blood

OBJECTIVES

Hypercapnia is known to modulate inflammation in lungs. However, the effect of hypocapnia and hypercapnia on blood cytokine production during sepsis is not well understood. We hypothesized that CO2 modulates ex vivo inflammatory cytokine production during endotoxin stimulation. To test this hypothesis, we measured the production of pro- and anti-inflammatory cytokines in endotoxin-stimulated human whole blood cultures under hypercapnic, normocapnic, and hypocapnic conditions.

DESIGN

Prospective randomized study.

SETTING

Basic research laboratory.

SUBJECTS

Ten male and 10 female volunteers.

INTERVENTIONS

Venous blood samples, taken from volunteers were cultured at 37 degrees C, under hypocapnic (2% CO2), normocapnic (5% CO2), and hypercapnic (7% CO2) conditions, with and without endotoxin stimulation. After 24 hrs of incubation, each culture's supernatant was analyzed for tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, interleukin-10, and interferon-gamma concentrations by enzyme-linked immunosorbent assay. Data were analyzed using nonparametric repeated measures of analysis of variance followed by Dunn's multiple comparisons test. Analysis of variance with Bonferroni correction was used to compare gender differences in cytokine concentrations. The Pearson test was used to estimate correlation between hydrogen ion and individual cytokine concentrations.

MEASUREMENTS AND MAIN RESULTS

Concentrations of the proinflammatory cytokines tumor necrosis factor-alpha, interleukin-1beta and of the anti-inflammatory cytokine interleukin-10 under hypercapnic condition were significantly decreased (p < 0.05, 0.01, and 0.001, respectively) for both genders when compared with either normocapnic or hypocapnic conditions. Concentrations of tumor necrosis factor-alpha and interleukin-1beta were significantly higher in men. In women, concentrations of interleukin-6 were significantly decreased under hypercapnic condition when compared with hypocapnic condition. An inverse relationship was found between hydrogen ion concentration and concentrations of tumor necrosis factor-alpha and interleukin-10.

CONCLUSIONS

Our results are consistent with the hypothesis that CO2 can affect the production of pro- and anti-inflammatory cytokines after ex vivo stimulation with endotoxin.

Three-dimensional mapping of chorioretinal vascular oxygen tension in the rat

PURPOSE

An optical section phosphorescence lifetime imaging system was developed for three-dimensional mapping of oxygen tension (P(O2)) in chorioretinal vasculatures.

METHODS

A laser line was projected at an oblique angle and scanned on the retina after intravenous injection of an oxygen-sensitive molecular probe to generate phosphorescence optical section images. An automated software algorithm segmented and combined images from spatially adjacent locations to construct depth-displaced en face retinal images. Intravascular P(O2) was measured by determining the phosphorescence lifetime. Three-dimensional chorioretinal P(O2) maps were generated in rat eyes under varying fractions of inspired oxygen.

RESULTS

Under an air-breathing condition, mean P(O2) in the choroid, retinal arteries, capillaries, and veins were 58+/-2 mm Hg, 47+/-2 mm Hg, 44+/-2 mm Hg, and 35+/-2 mm Hg, respectively. The mean arteriovenous P(O2) difference was 12+/-2 mm Hg. With a lower fraction of inspired oxygen, chorioretinal vascular P(O2) and mean arteriovenous P(O2) differences decreased compared with measurements under an air-breathing condition. Retinal venous P(O2) was statistically lower than P(O2) measured in the retinal artery, capillaries, and choroid (P<0.004).

CONCLUSIONS

Three-dimensional mapping of chorioretinal oxygen tension allowed quantitative P(O2) measurements in large retinal blood vessels and in retinal capillaries. This method has the potential to facilitate better understanding of retinal oxygenation in health and disease.

Chorioretinal vascular oxygen tension in spontaneously breathing anesthetized rats

PURPOSE

To establish baseline and variability of oxygen tension (PO(2)) measurements in the choroid, retinal arteries, capillaries, and veins of spontaneously breathing anesthetized rats and determine the effect of a moderate surgical procedure on the chorioretinal PO(2).

METHODS

Our previously established optical section phosphorescence imaging technique was utilized to measure PO(2) in the chorioretinal vasculatures. Imaging was performed in 29 spontaneously breathing rats under ketamine/xylazine anesthesia. In 7 rats, blood was drawn using a surgically implanted femoral arterial catheter and analyzed to determine the systemic arterial PO(2). The PO(2) measurements in 22 rats without surgery (group 1) and 7 surgically instrumented rats (group 2) were statistically compared. The intrasubject variability was calculated by the average standard deviation (SD) of repeated measurements.

RESULTS

The average systemic arterial PO(2) was 52 +/- 7 mm Hg (mean +/- SD) in group 2. In group 1, the average PO(2) measurements in the choroid, retinal arteries, capillaries, and veins were 50 +/- 11, 40 +/- 5, 39 +/- 6, and 30 +/- 5 mm Hg, respectively. No statistically significant PO(2) differences in any of the chorioretinal vasculatures were found between the two groups (p > 0.4). The intrasubject variability was 3 mm Hg in the choroid, retinal arteries, capillaries, and veins.

CONCLUSIONS

Chorioretinal PO(2) measurements in spontaneously breathing anesthetized rats have a relatively low variability, indicating that PO(2) changes due to various physiological alterations can be reliably assessed.

Carbon dioxide attenuates pulmonary impairment resulting from hyperventilation

OBJECTIVE

Deliberate elevation of PaCO2 (therapeutic hypercapnia) protects against lung injury induced by lung reperfusion and severe lung stretch. Conversely, hypocapnic alkalosis causes lung injury and worsens lung reperfusion injury. Alterations in lung surfactant may contribute to ventilator-associated lung injury. The potential for CO2 to contribute to the pathogenesis of ventilator-associated lung injury at clinically relevant tidal volumes is unknown. We hypothesized that: 1) hypocapnia would worsen ventilator-associated lung injury, 2) therapeutic hypercapnia would attenuate ventilator-associated lung injury; and 3) the mechanisms of impaired compliance would be via alteration of surfactant biochemistry.

DESIGN

Randomized, prospective animal study.

SETTING

Research laboratory of university-affiliated hospital.

SUBJECTS

Anesthetized, male New Zealand Rabbits.

INTERVENTIONS

All animals received the same ventilation strategy (tidal volume, 12 mL/kg; positive end-expiratory pressure, 0 cm H2O; rate, 42 breaths/min) and were randomized to receive FiCO2 of 0.00, 0.05, or 0.12 to produce hypocapnia, normocapnia, and hypercapnia, respectively.

MEASUREMENTS AND MAIN RESULTS

Alveolar-arterial oxygen gradient was significantly lower with therapeutic hypercapnia, and peak airway pressure was significantly higher with hypocapnic alkalosis. However, neither static lung compliance nor surfactant chemistry (total surfactant, aggregates, or composition) differed among the groups.

CONCLUSIONS

At clinically relevant tidal volume, CO2 modulates key physiologic indices of lung injury, including alveolar-arterial oxygen gradient and airway pressure, indicating a potential role in the pathogenesis of ventilator-associated lung injury. These effects are surfactant independent.

Hypercapnia increases cerebral tissue oxygen tension in anesthetized rats

PURPOSE

To test the hypotheses that deliberate elevation of PaCO(2) increases cerebral tissue oxygen tension (PBrO(2)) by augmenting PaO(2) and regional cerebral blood flow (rCBF).

METHODS

Anesthetized rats were exposed to increasing levels of inspired oxygen (O(2)) or carbon dioxide (CO(2); 5%, 10% and 15%, n = 6). Mean arterial blood pressure (MAP), PBrO(2) and rCBF were measured continuously. Blood gas analysis and hemoglobin concentrations were determined for each change in inspired gas concentration. Data are presented as mean +/- standard deviation with P < 0.05 taken to be significant.

RESULTS

The PBrO(2) increased in proportion to arterial oxygenation (PaO(2)) when the percentage of inspired O(2) was increased. Proportional increases in PaCO(2) (48.7 +/- 4.9, 72.3 +/- 6.0 and 95.3 +/- 15.4 mmHg), PaO(2) (172.2 +/- 33.1, 191.7 +/- 42.5 and 216.0 +/- 41.8 mmHg), and PBrO(2) (29.1 +/- 9.2, 49.4 +/- 19.5 and 60.5 +/- 23.0 mmHg) were observed when inspired CO(2) concentrations were increased from 0% to 5%, 10% and 15%, respectively, while arterial pH decreased (P < 0.05 for each). Exposure to CO(2) increased rCBF from 1.04 +/- 0.67 to a peak value of 1.49 +/- 0.45 (P < 0.05). Following removal of exogenous CO(2), arterial blood gas values returned to baseline while rCBF and PBrO(2) remained elevated for over 30 min. The hypercapnia induced increase in PBrO(2) was threefold higher than that resulting from a comparable increase in PaO(2) achieved by increasing the inspired O(2) concentration (34.9 +/- 14.5 vs 11.4 +/- 5.0 mmHg, P < 0.05).

CONCLUSION

These data support the hypothesis that the combined effect of increased CBF, PaO(2) and reduced pH collectively contribute to augmenting cerebral PBrO(2) during hypercapnia.

Increased tissue oxygen extraction and acidosis with progressive severity of sepsis

BACKGROUND

Lactic acidosis and increased production of CO(2) are common in septic shock. Presumably, both acidosis and CO(2) enhance the release of oxygen from hemoglobin. The purpose of this study was to assess the relationship of oxygen utilization, CO(2) production, acidosis, and hemoglobin oxygen (Hgb-O(2)) dissociation with progressive severity of sepsis to shock.

MATERIALS AND METHODS

Femoral arterial and vein, hepatic vein, portal vein, and pulmonary artery catheters were placed in 16 anesthetized swine. Organ blood flow was determined by timed injections of colored microspheres. After baseline measurements, Pseudomonas aeruginosa was infused in eight animals. This bacterial slurry was continued inciting a progression of sepsis to shock. Eight animals served as instrumented controls.

RESULTS

With sepsis and shock, there was a progressive decrease in pH and an increase in pCO(2) in plasma with all sampling sites (P < 0.01 septic shock versus baseline versus control). Blood flow to the liver and intestines increased with sepsis (P < 0.01) but then returned to near baseline control values during shock. VO(2) and/or percent O(2) extraction increased with sepsis and septic shock for the whole body and for the liver, intestine and leg (P < 0.01). There was a strong correlation between venous O(2) saturation, acidosis, and pCO(2) to percent O(2) extraction (r > 60; P < 0.0001). However, calculated P(50) values for Hgb-O(2) dissociation remained unchanged.

CONCLUSIONS

This study demonstrates that increased oxygen extraction in severe sepsis is related to a fall in tissue oxygen availability and not related to any allosteric change in Hgb-O(2) dissociation. Therefore, acidosis and hypercapnia do not have a demonstrable effect on altering oxygen availability during sepsis.

Oxygen-carrying capacity during 10 hours of hypercapnia in ventilated dogs

OBJECTIVE

To test if a relatively long-term exogenous hypercapnia, equivalent to those maintained during permissive hypercapnia, can persistently increase oxygen-carrying capacity in ventilated dogs.

DESIGN

Prospective study.

SETTING

Research laboratory in a hospital.

SUBJECTS

Six mongrel dogs (3 males; 3 females).

INTERVENTIONS

The dogs were anesthetized (30 mg/kg pentobarbital, i.v.), intubated, and cannulated in one femoral artery, one femoral vein, and the right jugular vein. The mean arterial blood pressure, heart rate, and mean pulmonary artery pressure were continuously recorded. Anesthesia, fluid balance, and normothermia were maintained. Arterial hypercapnia was generated by the addition of 60 torr dry CO2 (8 kPa) to the inspired air for 10 hrs, continuously. All subjects were paralyzed (vecuronium bromide) and ventilated with room air, while the ventilator settings were kept constant.

MEASUREMENTS AND MAIN RESULTS

Arterial and venous gas exchange profiles, hemoglobin concentration, oxygen saturation, oxygen content, cardiac output, and oxygen consumption were determined, before, during, and after 10 hrs of hypercapnia, periodically. Both hemoglobin concentration and oxygen content were gradually increased during hypercapnia and reached significant levels at 8 and 10 hrs of hypercapnia, respectively. These increases continued up to 2 hrs after termination of hypercapnia. The PaO2/FIO2, as an index of arterial oxygenation, was significantly increased during the first 3 hrs of hypercapnia and then remained at the normoxic level up to 10 hrs of hypercapnia. No significant changes occurred in the mean arterial blood pressure and oxygen consumption. The heart rate and cardiac output were significantly reduced at 4 and 8 hrs of hypercapnia, respectively. The mean pulmonary artery pressure was increased throughout the hypercapnic trial.

CONCLUSIONS

A relatively long-term exogenous hypercapnia can significantly increase oxygen-carrying capacity in normal ventilated dogs. Whether this effect can occur during permissive hypercapnia because of controlled ventilation in patients warrants investigation.