Effects of acid-base abnormalities on blood capacity of transporting CO(2): adverse effect of metabolic acidosis

Venous Minus Arterial Carbon Dioxide Gradients in the Monitoring of Tissue Perfusion and Oxygenation: A Narrative Review

According to Fick's principle, the total uptake of (or release of) a substance by tissues is the product of blood flow and the difference between the arterial and the venous concentration of the substance. Therefore, the mixed or central venous minus arterial CO₂ content difference depends on cardiac output (CO). Assuming a linear relationship between CO₂ content and partial pressure, central or mixed venous minus arterial PCO₂ differences (P_cv-aCO₂ and P_mv-aCO₂) are directly related to CO. Nevertheless, this relationship is affected by alterations in the CO₂Hb dissociation curve induced by metabolic acidosis, hemodilution, the Haldane effect, and changes in CO₂ production (VCO₂). In addition, P_cv-aCO₂ and P_mv-aCO₂ are not interchangeable. Despite these confounders, CO is a main determinant of P_cv-aCO₂. Since in a study performed in septic shock patients, P_mv-aCO₂ was correlated with changes in sublingual microcirculation but not with those in CO, it has been proposed as a monitor for microcirculation. The respiratory quotient (RQ)-RQ = VCO₂/O₂ consumption-sharply increases in anaerobic situations induced by exercise or critical reductions in O₂ transport. This results from anaerobic VCO₂ secondary to bicarbonate buffering of anaerobically generated protons. The measurement of RQ requires expired gas analysis by a metabolic cart, which is not usually available. Thus, some studies have suggested that the ratio of P_cv-aCO₂ to arterial minus central venous O₂ content (P_cv-aCO₂/C_a-cvO₂) might be a surrogate for RQ and tissue oxygenation. In this review, we analyze the physiologic determinants of P_cv-aCO₂ and P_cv-aCO₂/C_a-cvO₂ and their potential usefulness and limitations for the monitoring of critically ill patients. We discuss compelling evidence showing that they are misleading surrogates for tissue perfusion and oxygenation, mainly because they are systemic variables that fail to track regional changes. In addition, they are strongly dependent on changes in the CO₂Hb dissociation curve, regardless of changes in systemic and microvascular perfusion and oxygenation.

Serum bicarbonate may replace the arterial base deficit in the trauma intensive care unit

OBJECTIVE

Arterial base deficit (BD) is a commonly used marker of injury severity and endpoint of resuscitation but requires an arterial puncture and blood gas analysis. Serum bicarbonate (HCO3) is routinely obtained as part of the chemistry panel on most admissions. We hypothesized that serum HCO3 strongly correlates with arterial BD and provides equivalent predictive information.

METHODS

All trauma ICU admissions from 1996 to 2004 with simultaneously obtained serum chemistry panels and arterial blood gases were identified. Correlation between BD and HCO3 was analyzed by using linear regression, and predictive abilities for acidoses and mortality were compared using the area under the respective receiver operating characteristic curve (AUC). Separate analyses were done for the entire dataset and the subset of ICU admission laboratory values.

RESULTS

We identified 3,102 patients with 50,311 matched pairs of laboratory data. Serum HCO3 showed a significant linear correlation with BD for all laboratory sets (r = 0.85, P < .01) and admission laboratory values only (r = 0.80, P < .01). Serum HCO3 reliably predicted the presence of significant metabolic acidoses (BD >5), with an AUC of 0.96 (P < .01), which clearly outperformed pH (AUC = 0.83), anion gap (AUC = 0.7), and lactate (AUC = 0.73). The mean admission BD among survivors was 2.5 versus 5.2 for nonsurvivors (P < .01), and the mean HCO3 was 17.7 versus 19.8 (P < .01). The admission HCO3 identified nonsurvivors as accurately as BD (AUCs of 0.66 and 0.68) and more accurately than either pH (AUC = 0.53) or anion gap (AUC = 0.6).

CONCLUSION

Serum HCO3 measurement shows a strong linear correlation and similar predictive ability compared with the arterial BD. Serum HCO3 may be safely and accurately substituted for arterial BD measurement in critically injured patients.

Comparison of two methods to assess blood CO2 equilibration curve in mechanically ventilated patients

In order to compare two mathematical methods to assess the blood CO2 equilibration curve from a single blood gas analysis [Loeppky, J.A., Luft, U.C., Fletcher, E.R., 1983. Quantitative description of whole blood CO2 dissociation curve and Haldane effect. Resp. Physiol. 51, 167-181; Giovannini, I., Chiarla, C., Boldrini, G., Castagneto, M., 1993. Calculation of venoarterial CO2 concentration difference. J. Appl. Physiol. 74, 959-964], arterial and central venous blood gas analyses and oximetry were performed before and after ventilatory resetting, at constant arterial O2 saturation, in 12 mechanically ventilated patients. CO2 equilibration curves obtained from basal arterial blood gas analyses were used to predict arterial CO2 content after ventilatory resetting and vice versa. Internal consistency was very good for both methods and comparable. Method 2 also yielded excellent predictions of changes of arterial pH associated with ventilatory resetting. In determining Haldane effect, method 2 yielded very stable results within the expected range of values, while method 1 yielded a wider spread of results. Method 2 appeared more suitable to determine the Haldane effect in the conditions of the study, probably due to an approach minimizing the effect of potential sources of inaccuracy.

Increased blood flow prevents intramucosal acidosis in sheep endotoxemia: a controlled study

Introduction

Increased intramucosal–arterial carbon dioxide tension (PCO₂) difference (ΔPCO₂) is common in experimental endotoxemia. However, its meaning remains controversial because it has been ascribed to hypoperfusion of intestinal villi or to cytopathic hypoxia. Our hypothesis was that increased blood flow could prevent the increase in ΔPCO₂.

Methods

In 19 anesthetized and mechanically ventilated sheep, we measured cardiac output, superior mesenteric blood flow, lactate, gases, hemoglobin and oxygen saturations in arterial, mixed venous and mesenteric venous blood, and ileal intramucosal PCO₂ by saline tonometry. Intestinal oxygen transport and consumption were calculated. After basal measurements, sheep were assigned to the following groups, for 120 min: (1) sham (n = 6), (2) normal blood flow (n = 7) and (3) increased blood flow (n = 6). Escherichia coli lipopolysaccharide (5 μg/kg) was injected in the last two groups. Saline solution was used to maintain blood flood at basal levels in the sham and normal blood flow groups, or to increase it to about 50% of basal in the increased blood flow group.

Results

In the normal blood flow group, systemic and intestinal oxygen transport and consumption were preserved, but ΔPCO₂ increased (basal versus 120 min endotoxemia, 7 ± 4 versus 19 ± 4 mmHg; P < 0.001) and metabolic acidosis with a high anion gap ensued (arterial pH 7.39 versus 7.35; anion gap 15 ± 3 versus 18 ± 2 mmol/l; P < 0.001 for both). Increased blood flow prevented the elevation in ΔPCO₂ (5 ± 7 versus 9 ± 6 mmHg; P = not significant). However, anion-gap metabolic acidosis was deeper (7.42 versus 7.25; 16 ± 3 versus 22 ± 3 mmol/l; P < 0.001 for both).

Conclusions

In this model of endotoxemia, intramucosal acidosis was corrected by increased blood flow and so might follow tissue hypoperfusion. In contrast, anion-gap metabolic acidosis was left uncorrected and even worsened with aggressive volume expansion. These results point to different mechanisms generating both alterations.

Intramucosal-Arterial Pco2 Gradient Does Not Reflect Intestinal Dysoxia in Anemic Hypoxia

BACKGROUND

An increase in intramucosal-arterial Pco2 gradient (DeltaPco2) might be caused by tissue hypoxia or by diminished blood flow. Our hypothesis was that DeltaPco2 should not be altered in anemic hypoxia with preserved blood flow.

METHODS

In 18 anesthetized, mechanically ventilated sheep, oxygen transport was stepwise reduced by hemorrhage (hypovolemia, n = 9) or by hemorrhage and simultaneous dextran infusion (hemodilution, n = 9).

RESULTS

Hypovolemia and hemodilution produced comparable decreases in systemic and intestinal oxygen transport and uptake. However, mixed venoarterial and mesenteric venoarterial Pco2 gradients and DeltaPco2 were significantly higher in hypovolemia than in hemodilution (25 +/- 5 vs. 10 +/- 2 mm Hg; 21 +/- 6 vs. 10 +/- 5 mm Hg; and 41 +/- 18 vs. 14 +/- 9 mm Hg, respectively; p < 0.01).

CONCLUSION

DeltaPco2 did not reflect intestinal dysoxia during Vo2/Do2 dependency attributable to hemodilution. Blood flow seems to be the main determinant of DeltaPco2.

Acid–base and electrolyte analysis in critically ill patients: are we ready for the new millennium?

PURPOSE OF REVIEW

Disorders of acid-base and electrolytes are commonly seen in critically ill patients. The presence of these disorders typically signals the development of an underlying pathology. These disturbances can be severe and are often associated with worse outcome. Indeed, metabolic acidosis is one of the ways we quantify organ failure. Although acid-base and electrolyte disorders may be a result of the underlying pathophysiology (eg, renal failure, respiratory failure, shock), they may also result from the way in which we manage critically ill patients.

RECENT FINDINGS

The application of the physical-chemical approach to acid-base analysis has led to recent developments in the identification and quantification and understanding of mechanisms for acid-base disorders commonly found in critically ill patients. Examples include a better understanding of the role of electrolytes (especially sodium and chloride) and weak acids in the pathophysiology of acid-base disorders, the implication of acid-base derangements on the inflammatory process and organ perfusion, and the importance of resuscitation fluid composition.

SUMMARY

By adopting a physical-chemical approach to acid-base analysis we are gaining insight to the complexities of acid-base disorders and how their treatments may affect outcome.