Acute hypercapnia increases the oxygen-carrying capacity of the blood in ventilated dogs

Cardiovascular effects of intravenous pimobendan in dogs with acute respiratory acidosis

OBJECTIVE

Acidosis decreases myocardial contractile and myofibrillar responsiveness by reducing the calcium sensitivity of contractile proteins, which could reduce the effectiveness of pimobendan. We aimed to assess the cardiovascular effects of pimobendan in dogs subjected to acute respiratory acidosis.

DESIGN

Randomized crossover study with a 2-week washout period.

SETTING

University Laboratory.

ANIMALS

Six healthy research Beagle dogs.

INTERVENTIONS

Anesthetized dogs were administered 2 doses of IV pimobendan during conditions of eucapnia (Paco2 35-40 mm Hg) and hypercapnia (Paco2 90-110 mm Hg). Eucapnia was maintained by positive pressure ventilation and hypercapnia was induced by adding exogenous CO2 to the anesthesia circuit. Heart rate (HR), systemic arterial blood pressure, cardiac output (CO), systemic and pulmonary vascular resistance (SVR and PVR, respectively), and pulmonary arterial pressure (PAP) were measured at baseline and 60 min after administering 0.125 mg/kg (low) and 0.25 mg/kg (high) pimobendan intravenously. Blood gas and biochemical analyses were performed at baseline and at the end of the experiment.

MEASUREMENTS AND MAIN RESULTS

The median baseline blood pH was 7.41 (range: 7.33-7.45) and 7.03 (range: 6.98-7.09) under conditions of eucapnia and hypercapnia, respectively. The serum concentrations of epinephrine and norepinephrine and the HR, CO, and PAP were higher, and SVR was lower at baseline in hypercapnic dogs. Pimobendan dose-dependently increased CO in eucapnia (baseline: 3.6 ± 0.2 L/kg/m2 [mean ± SE], low: 5.0 ± 0.4 L/kg/m2 , high: 5.8 ± 0.5 L/kg/m2 , P < 0.001) and hypercapnia (baseline: 4.9 ± 0.5 L/kg/m2 , low: 5.8 ± 0.5 L/kg/m2 , high: 6.2 ± 0.5 L/kg/m2 , P < 0.001), and increased HR and decreased SVR and PVR under both conditions (P < 0.001). In hypercapnia, the degree of increase or decrease of these cardiovascular measurements (except for PAP) by pimobendan was less than that in the eucapnic dogs.

CONCLUSIONS

Pimobendan maintains function as an inodilator in anesthetized dogs with induced respiratory acidosis.

The Association of Oxygenation, Carbon Dioxide Removal, and Mechanical Ventilation Practices on Survival During Venoarterial Extracorporeal Membrane Oxygenation

Introduction: Oxygenation and carbon dioxide removal during venoarterial extracorporeal membrane oxygenation (VA ECMO) depend on a complex interplay of ECMO blood and gas flows, native lung and cardiac function as well as the mechanical ventilation strategy applied. Objective: To determine the association of oxygenation, carbon dioxide removal, and mechanical ventilation practices with in-hospital mortality in patients who received VA ECMO. Methods: Single center, retrospective cohort study. All consecutive patients who received VA ECMO in a tertiary ECMO referral center over a 5-year period were included. Data on demographics, ECMO and ventilator support details, and blood gas parameters for the duration of ECMO were collected. A multivariable logistic time-series regression model with in-hospital mortality as the primary outcome variable was used to analyse the data with significant factors at the univariate level entered into the multivariable regression model. Results: Overall, 52 patients underwent VA ECMO: 26/52 (50%) survived to hospital discharge. The median PaO2 for the duration of ECMO support was 146 mmHg [IQR 131-188] and PaCO2 was 37.2 mmHg [IQR 35.3, 39.9]. Patients who survived to hospital discharge had a significantly lower median PaO2 (117 [98, 140] vs. 154 [105, 212] mmHg, P = 0.04) and higher median PaCO2 (38.3 [36.1, 41.1] vs. 36.3 [34.5, 37.8] mmHg, p = 0.03). Survivors also had significantly lower median VA ECMO blood flow rate (EBFR, 3.6 [3.3, 4.2] vs. 4.3 [3.8, 5.2] L/min, p = < 0.001) and greater measured minute ventilation (7.04 [5.63, 8.35] vs. 5.32 [4.43, 6.83] L/min, p = 0.01). EBFR, PaO2, PaCO2, and minute ventilation, however, were not independently associated with death in a multivariable analysis. Conclusion: This exploratory analysis in a small group of VA ECMO supported patients demonstrated that hyperoxemia was common during VA ECMO but was not independently associated with increased mortality. Survivors also received lower EBFR and had greater minute ventilation, but this was also not independently associated with survival. These findings highlight that interactions between EBFR, PaO2, and native lung ventilation may be more relevant than their individual association with survival. Further research is indicated to determine the optimal ECMO and ventilator settings on outcomes in VA ECMO.

The role of hypercapnia in acute respiratory failure

The biological effects and physiological consequences of hypercapnia are increasingly understood. The literature on hypercapnia is confusing, and at times contradictory. On the one hand, it may have protective effects through attenuation of pulmonary inflammation and oxidative stress. On the other hand, it may also have deleterious effects through inhibition of alveolar wound repair, reabsorption of alveolar fluid, and alveolar cell proliferation. Besides, hypercapnia has meaningful effects on lung physiology such as airway resistance, lung oxygenation, diaphragm function, and pulmonary vascular tree.

In acute respiratory distress syndrome, lung-protective ventilation strategies using low tidal volume and low airway pressure are strongly advocated as these have strong potential to improve outcome. These strategies may come at a price of hypercapnia and hypercapnic acidosis. One approach is to accept it (permissive hypercapnia); another approach is to treat it through extracorporeal means. At present, it remains uncertain what the best approach is.

Importance of carbon dioxide in the critical patient: Implications at the cellular and clinical levels

Important recent insights have emerged regarding the cellular and molecular role of carbon dioxide (CO₂) and the effects of hypercapnia. The latter may have beneficial effects in patients with acute lung injury, affording reductions in pulmonary inflammation, lessened oxidative alveolar damage, and the regulation of innate immunity and host defenses by inhibiting the expression of inflammatory cytokines. However, other studies suggest that CO₂ can have deleterious effects upon the lung, reducing alveolar wound repair in lung injury, decreasing the rate of reabsorption of alveolar fluid, and inhibiting alveolar cell proliferation. Clearly, hypercapnia has both beneficial and harmful consequences, and it is important to determine the net effect under specific conditions. The purpose of this review is to describe the immunological and physiological effects of carbon dioxide, considering their potential consequences in patients with acute respiratory failure.

Acidosis in the critically ill - balancing risks and benefits to optimize outcome

Acidosis is associated with poor outcome in critical illness. However, acidosis - both hypercapnic and metabolic - has direct effects that can limit tissue injury induced by many causes. There is also a clear potential for off-target harm with acute exposure (for example, raised intracranial pressure, pulmonary hypertension), and with exposure for prolonged periods (for example, increased risk of infection) or at high doses. Ongoing comprehensive determination of molecular, cellular and physiologic impact across a range of representative pathologies will allow us to understand better the risks and benefits of hypercapnia and acidosis during critical illness.

Hypercapnia: is it protective in lung injury?

Hypercapnic acidosis has been regarded as a tolerated side effect of protective lung ventilation strategies. Various in vivo and ex vivo animal studies have shown beneficial effects in acute lung injury setting, but some recent work raised concerns about its anti-inflammatory properties. This mini-review article aims to expand the potential clinical spectrum of hypercapnic acidosis in critically ill patients with lung injury. Despite the proven benefits of hypercapnic acidosis, further safety studies including dose-effect, level-and-onset of anti-inflammatory effect, and safe applicability period need to be performed in various models of lung injury in animals and humans to further elucidate its protective role.

Haemodynamic effects during endoscopic vein harvest of the saphenous vein for off-pump coronary artery bypass grafting surgery

OBJECTIVE

Endoscopic vein harvest (EVH) for coronary artery bypass grafting surgery is performed with carbon dioxide (CO2) insufflation for visualization and dissection. The insufflated CO2 is rapidly absorbed into the body and may influence haemodynamics. However, the haemodynamic changes during EVH have not been clearly defined. This study evaluated the haemodynamic effects during EVH of the saphenous vein for off-pump coronary artery bypass grafting surgery (OPCAB).

METHODS

After fixing the position for harvesting of the left internal mammary artery, EVH of the saphenous vein was performed at a maximum CO2 pressure of 12 mmHg and a flow of 3 l/min. The haemodynamic parameters were measured before and just after the end of endoscopic vein harvest.

RESULTS

One hundred patients were studied. The end-tidal CO2 pressure (P(ET)CO2, 35.0 +/- 2.7 vs. 52.0 +/- 6.2 mmHg), partial pressure of arterial CO2 (PaCO2, 35.1 +/- 3.1 vs. 52.5 +/- 4.3 mmHg), mixed venous oxygen saturation (SvO2, 75.6 +/- 4.1 vs. 82.0 +/- 1.6%), cardiac index (2.7 +/- 0.6 vs. 3.3 +/- 0.6 l/min/m2), and cerebral oxygen saturation (ScO2, left: 63.5 +/- 7.9 vs. 73.3 +/- 8.4; right: 62.2 +/- 8.0 vs. 72.3 +/- 6.3%) differed significantly between before and after CO2 insufflation, whereas mean systemic blood pressure, mean pulmonary artery blood pressure, central venous pressure, heart rate, partial pressure of arterial oxygen, and peak inspiratory pressure did not differ significantly between before and after CO2 insufflation.

CONCLUSIONS

EVH, at a maximum CO2 pressure of 12 mmHg and a flow of 3 l/min, of the saphenous vein for OPCAB was associated with hypercarbia and a tolerable range of hypercarbia (PaCO2 < 60 mmHg) increased the cardiac index and ScO2 without any complications.

Permissive hypercapnia

Mechanical ventilation using high tidal volume (VT) and transpulmonary pressure can damage the lung, causing ventilator-induced lung injury. Permissive hypercapnia, a ventilatory strategy for acute respiratory failure in which the lungs are ventilated with a low inspiratory volume and pressure, has been accepted progressively in critical care for adult, pediatric, and neonatal patients requiring mechanical ventilation and is one of the central components of current protective ventilatory strategies.

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.

Alteration of the piglet diaphragm contractility in vivo and its recovery after acute hypercapnia

BACKGROUND

The effects of hypercapnic acidosis on the diaphragm and its recovery to normocapnia have been poorly evaluated. The authors studied diaphragmatic contractility facing acute variations of arterial carbon dioxide tension (Paco2) and evaluated the contractile function at 60 min after normocapnia recovery.

METHODS

Thirteen piglets weighing 15-20 kg were anesthetized, ventilated, and separated into two groups: a control group (n = 5) evaluated in normocapnia (time-control experiments) and a hypercapnia group (n = 8) in which animals were acutely and shortly exposed to five consecutive ranges of Paco2 (40, 50, 70, 90, and 110 mmHg). Then carbon dioxide insufflation was stopped. Diaphragmatic contractility was assessed by measuring transdiaphragmatic pressure variations obtained after bilateral transjugular phrenic nerve pacing at increased frequencies (20-120 Hz). For each level of arterial pressure of carbon dioxide, pressure-frequency curves were obtained in vivo by phrenic nerve pacing.

RESULTS

In the hypercapnia group, mean +/- SD transdiaphragmatic pressure significantly decreased from 41 +/- 3 to 29 +/- 3 cm H2O (P < 0.05) between the first (40 mmHg) and fifth (116 mmHg) stages of capnia at the frequency of 100 Hz stimulation. The observed alteration of the contractile force was proportional to the level of Paco2 (r = 0.61, P < 0.01). Normocapnia recuperation allowed a partial recovery of the diaphragmatic contractile force (80% of the baseline value) at 60 min after carbon dioxide insufflation interruption.

CONCLUSION

A short exposure to respiratory acidosis decreased diaphragmatic contractility proportionally to the degree of hypercapnia, and this alteration was only partially reversed at 60 min after exposure.

Outcomes and prognostic factors for severe community-acquired pneumonia that requires mechanical ventilation

BACKGROUND

Community-acquired pneumonia (CAP) remains a common and serious condition worldwide. The mortality from severe CAP remains high, and this has reached 50% in some series. This study was conducted to determine the mortality and predictors that contribute to in-hospital mortality for patients who exhibit CAP and acute respiratory failure that requires mechanical ventilation.

METHODS

We retrospectively reviewed the medical records of 85 patients with severe CAP as a primary cause of acute respiratory failure, and this required mechanical ventilation in a setting of the medical intensive care unit (ICU) of a tertiary university hospital between 2000 and 2003.

RESULTS

The overall in-hospital mortality was 56% (48/85). A Cox-proportional hazard model revealed that the independent predictive factors of in-hospital mortality included a PaCO2 of less than 45 mmHg (p<0.001, relative risk [RR]: 4.73; 95% confidence interval [CI]: 2.16-10.33), a first 24-hour urine output of less than 1.5 L (p=0.006, RR: 2.46, 95% CI: 1.29-4.66) and a high APACHE II score (p=0.004, RR: 1.09, 95% CI: 1.03-1.16).

CONCLUSIONS

Acute respiratory failure caused by severe CAP and that necessitates mechanical ventilation is associated with a high mortality rate. Initial hypercapnia and a large urine output favored survival, whereas a high APACHE II score predicted mortality.

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.

Isocapnic hyperventilation increases carbon monoxide elimination and oxygen delivery

Hyperventilation with mixtures of O2 and CO2 has long been known to enhance carbon monoxide (CO) elimination at low HbCO levels in animals and humans. The effect of this therapy on oxygen delivery (DO2) has not been studied. Isocapnic hyperventilation utilizing mechanical ventilation may decrease cardiac output and therefore decrease DO2 while increasing CO elimination. We studied the effects of isocapnic hyperventilation on five adult mechanically ventilated sheep exposed to multiple episodes of severe CO poisoning. Five ventilatory patterns were studied: baseline minute ventilation (RR. VT), twice (2. RR) and four times (4. RR) baseline respiratory rate, and twice (2. VT) and four times (4. VT) baseline tidal volume. The mean carboxyhemoglobin (HbCO) washout half-time (t1/2) was 14.3 +/- 1.6 min for RR. VT, decreasing to 9.5 +/- 0.9 min for 2. RR, 8.0 +/- 0.5 min for 2. VT, 6.2 +/- 0.5 min for 4. RR, and 5.2 +/- 0.5 min for 4. VT. DO2 was increased during hyperventilation compared with baseline ventilation for 2. VT, 4. RR, and 4. VT ventilatory patterns. Isocapnic hyperventilation, in our animal model, did not alter arterial or pulmonary blood pressures, arterial pH, or cardiac output. Isocapnic hyperventilation is a promising therapy for CO poisoning.

Endothelium-independent and -dependent vasodilation in alkalotic and acidotic piglet lungs

Although significant pulmonary hypertension can occur in patients treated with either hypocapnic alkalosis or "permissive" hypercapnic acidosis, the effects of sustained alkalosis or acidosis on subsequent vasodilator responses have not been established. This study measured the effects of 60-100 min of sustained alkalosis or acidosis on endothelium-independent and -dependent vasodilation with inhaled nitric oxide (iNO) and acetylcholine (ACh) in isolated lungs from 1-week-old piglets. After stabilization, lungs were divided into control (pH 7.40, PaCO(2) 40 torr, n = 5), alkalotic (pH 7.60, PaCO(2) 25 torr, n = 6), or acidotic (pH 7.25, PaCO(2) 65 torr, n = 5) groups and ventilated with 21% O(2) for 40 min. Acute hypoxic pulmonary vasoconstriction (HPV) was then induced with 4-6% O(2). After a stable pressor response had occurred (approximately 20 min), pulmonary artery dose-response relationships to increasing concentrations of iNO were measured. The iNO was then stopped and after a stable hypoxic pressure had again been reestablished (approximately 20 min), dose-responses to increasing concentrations of ACh were measured. Hypoxic pulmonary vascular resistance (PVR) was similar in all groups. Pulmonary artery pressure dose-response relationships to iNO and ACh were blunted in the alkalosis group, suggesting that both endothelium-independent and -dependent vasodilation were reduced during sustained hypocapnic alkalosis. In contrast, sustained acidosis did not alter subsequent vasodilator responses. Future studies must elucidate the mechanisms underlying blunted pulmonary vasodilation during sustained alkalosis and examine the consequences of sustained alkalosis therapy on subsequent vasodilator responses in clinical practice.

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.

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

PURPOSE

We have previously demonstrated an increased arterial O2-carrying capacity in normal ventilated dogs subjected to both acute and prolonged exogenous hypercapnia. In the present study, we tested if arterial hypercapnia, during controlled ventilation, can increase O2-carrying capacity also in rats.

MATERIALS AND METHODS

Twenty young male Sprague Dawley rats were anesthetized (60 mg/kg pentobarbital), tracheostomized, intubated, and one femoral vein and artery were cannulated. Anesthesia and paralysis were maintained using 15 mg/kg/h pentobarbital intravenously, and 2 mg/kg/h vecuronium bromide. The fluid balance (5 mL/kg/h saline), normothermia, and minute volume were maintained. The mean arterial blood pressure and heart rate were continuously monitored. Experiments included the following: (1) a control group, ventilated with normoxic air for 150 minutes (n = 5); (2) mild hypercapnia, a group of eight rats ventilated with normoxic air for 30 minutes and then ventilated with a mixture of normoxic air at 60 mm Hg CO2 (8 kPa) for 1 hour; and (3) severe hypercapnia, a group of seven rats were treated exactly as in group II, except a 90 mm Hg (12 kPa) CO2 during hypercapnia. Gas-exchange profile, arterial hemoglobin (Hb) concentration, arterial Hb-oxygen saturation (Hb-O2), and arterial O2 content were periodically determined during normocapnia and 1 hour of hypercapnia.

RESULTS

Exposures to mild and severe hypercapnia, in rats with maintained ventilation, significantly reduced the arterial O2 content by 20% and 33%, respectively, without significant changes in the arterial Hb concentration (-2%). Severe hypercapnia generated a significant reduction of -14% in the PaO2, but not in PaO2/ FiO2 ratio.

CONCLUSION

Rats subjected to controlled ventilation and permissive hypercapnia, unlike dogs and perhaps humans, show no augmentation of Hb concentration. Hypercapnia in rats also provokes much stronger Bohr effect than in dogs. Hypercapnia-induced Bohr effect in rats is accompanied with extreme desaturations of Hb-O2, and substantial reduction in the O2-carrying capacity. We speculate that the strong hypercapnia-induced Bohr effect in rats may prevent hypoxia at the tissue level. However, to maintain a stable oxygen-carrying capacity in rats used for pulmonary critical care studies with hypercapnia, we suggest to use hyperoxia, with or without a mild hypothermia.

Carbon dioxide and the critically ill--too little of a good thing?

Permissive hypercapnia (acceptance of raised concentrations of carbon dioxide in mechanically ventilated patients) may be associated with increased survival as a result of less ventilator-associated lung injury. Conversely, hypocapnia is associated with many acute illnesses (eg, asthma, systemic inflammatory response syndrome, pulmonary oedema), and is thought to reflect underlying hyperventilation. Accumulating clinical and basic scientific evidence points to an active role for carbon dioxide in organ injury, in which raised concentrations of carbon dioxide are protective, and low concentrations are injurious. We hypothesise that therapeutic hypercapnia might be tested in severely ill patients to see whether supplemental carbon dioxide could reduce the adverse effects of hypocapnia and promote the beneficial effects of hypercapnia. Such an approach could also expand our understanding of the pathogenesis of disorders in which hypocapnia is a constitutive element.