Clinical review: The meaning of acid–base abnormalities in the intensive care unit part I – epidemiology

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Abstract

Acid–base abnormalities are common in critically ill patients. Our ability to describe acid–base disorders must be precise. Small differences in corrections for anion gap, different types of analytical processes, and the basic approach used to diagnose acid–base aberrations can lead to markedly different interpretations and treatment strategies for the same disorder. By applying a quantitive acid–base approach, clinicians are able to account for small changes in ion distribution that may have gone unrecognized with traditional techniques of acid–base analysis. Outcome prediction based on the quantitative approach remains controversial. This is in part due to use of various technologies to measure acid–base variables, administration of fluid or medication that can alter acid–base results, and lack of standardized nomenclature. Without controlling for these factors it is difficult to appreciate the full effect that acid–base disorders have on patient outcomes, ultimately making results of outcome studies hard to compare.

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Most cited references 64

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Early lactate clearance is associated with improved outcome in severe sepsis and septic shock.

H Nguyen, Emanuel P Rivers, Bernhard P Knoblich … (2004)

Serial lactate concentrations can be used to examine disease severity in the intensive care unit. This study examines the clinical utility of the lactate clearance before intensive care unit admission (during the most proximal period of disease presentation) as an indicator of outcome in severe sepsis and septic shock. We hypothesize that a high lactate clearance in 6 hrs is associated with decreased mortality rate. Prospective observational study. An urban emergency department and intensive care unit over a 1-yr period. A convenience cohort of patients with severe sepsis or septic shock. Therapy was initiated in the emergency department and continued in the intensive care unit, including central venous and arterial catheterization, antibiotics, fluid resuscitation, mechanical ventilation, vasopressors, and inotropes when appropriate. Vital signs, laboratory values, and Acute Physiology and Chronic Health Evaluation (APACHE) II score were obtained at hour 0 (emergency department presentation), hour 6, and over the first 72 hrs of hospitalization. Therapy given in the emergency department and intensive care unit was recorded. Lactate clearance was defined as the percent decrease in lactate from emergency department presentation to hour 6. Logistic regression analysis was performed to determine independent variables associated with mortality. One hundred and eleven patients were enrolled with mean age 64.9 +/- 16.7 yrs, emergency department length of stay 6.3 +/- 3.2 hrs, and overall in-hospital mortality rate 42.3%. Baseline APACHE II score was 20.2 +/- 6.8 and lactate 6.9 +/- 4.6 mmol/L. Survivors compared with nonsurvivors had a lactate clearance of 38.1 +/- 34.6 vs. 12.0 +/- 51.6%, respectively (p =.005). Multivariate logistic regression analysis of statistically significant univariate variables showed lactate clearance to have a significant inverse relationship with mortality (p =.04). There was an approximately 11% decrease likelihood of mortality for each 10% increase in lactate clearance. Patients with a lactate clearance> or =10%, relative to patients with a lactate clearance <10%, had a greater decrease in APACHE II score over the 72-hr study period and a lower 60-day mortality rate (p =.007). Lactate clearance early in the hospital course may indicate a resolution of global tissue hypoxia and is associated with decreased mortality rate. Patients with higher lactate clearance after 6 hrs of emergency department intervention have improved outcome compared with those with lower lactate clearance.

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Lactate clearance and survival following injury.

Jessica Hitchcock, Leah Greenspan, David Abramson … (1993)

Previous reports cite optimization of O2 delivery (DO2) to 660 mL/min/m2, O2 consumption (VO2) to 170 mL/min/m2, and cardiac index (CI) of 4.5 L/min as predicting survival. We prospectively evaluated 76 consecutive patients with multiple trauma admitted directly to the ICU from the operating room or emergency department. Patients had serum lactate levels and oxygen transport measured on ICU admission and at 8, 16, 24, 36, and 48 hours. Patients were analyzed with respect to survival (S) versus nonsurvival (NS), lactate clearance to normal (< or = 2 mmol/L) by 24 and 48 hours, hemodynamic optimization as defined above, as well as Injury Severity Score (ISS), ICU stay (LOS), and admission blood pressure. All patients achieved non-flow-dependent VO2. There was no difference in CI, DO2, VO2, or ISS when S was compared with NS. All 27 patients whose lactate level normalized in 24 hours survived. If lactate levels cleared to normal between 24 and 48 hours, the survival rate was 75%. Only 3 of the 22 patients who did not clear their lactate level to normal by 48 hours survived. Ten of the 25 nonsurvivors (40%) achieved the above arbitrary optimization criteria. Fifteen of the survivors never achieved any of these criteria. Optimization alone does not predict survival. However, the time needed to normalize serum lactate levels is an important prognostic factor for survival in severely injured patients.

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Low mortality rate in adult respiratory distress syndrome using low-volume, pressure-limited ventilation with permissive hypercapnia: a prospective study.

Keith Hickling, John Walsh, Seton Henderson … (1994)

To evaluate the outcome in patients with severe adult respiratory distress syndrome (ARDS) managed with limitation of peak inspiratory pressure to 30 to 40 cm H2O, low tidal volumes (4 to 7 mL/kg), spontaneous breathing using synchronized intermittent mandatory ventilation from the start of ventilation, and permissive hypercapnia without the use of bicarbonate to buffer acidosis. Also, to compare hospital mortality rate with that predicted by the Acute Physiology and Chronic Health Evaluation (APACHE) II scoring system and the "ventilator score." A ten-bed general intensive care unit in a university hospital. Prospective, descriptive study. Fifty-three patients with severe ARDS having a lung injury score of > or = 2.5. Data recording. The hospital mortality rate was significantly lower than that predicted by the APACHE II scores (26.4% vs. 53.3%, p = .004), even after correcting the latter for the effect of hypercapnic acidosis (26.4% vs. 51.1%, p = .008). The mortality rate increased with increasing number of organ failures, but was only 43% in patients with > or = 4 organ failures, 20.5% with < or = 3 organ failures, and 6.6% with only respiratory failure. The mean maximum PaCO2 was 66.5 torr (range 38 to 158 torr [8.87 kPa, range 5.07 to 21.07]), and the mean arterial pH at the same time was 7.23 (range 6.79 to 7.45). There was no correlation between the maximum PaCO2 or the corresponding pH and the total respiratory rate at the same time. No pneumothoraces developed during mechanical ventilation. These results lend further support to the hypothesis that limitation of peak inspiratory pressure and reduction of regional lung overdistention by the use of low tidal volumes with permissive hypercapnia may reduce ventilator-induced lung injury and improve outcome in severe ARDS. This hypothesis is supported by a large body of experimental evidence, which also suggests that ventilator-induced lung injury may result in the release of inflammatory mediators, and thus may have the potential to augment the development of multiple organ dysfunction. However, the hypothesis requires testing in a randomized trial as acute hypercapnia could potentially have some adverse as well as beneficial effects.

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Author and article information

Journal

Journal ID (nlm-ta): Crit Care

Title: Critical Care

Publisher: BioMed Central (London )

ISSN (Print): 1364-8535

ISSN (Electronic): 1466-609X

Publication date (Print): 2005

Publication date (Electronic): 10 August 2005

Volume: 9

Issue: 5

Pages: 508-516

Affiliations

[1 ]Assistant Professor, The Virginia Commonwealth University Reanimation Engineering and Shock Center (VCURES) Laboratory, Departments of Anesthesiology/Critical Care and Emergency Medicine, Virginia Commonwealth University Medical Center, Richmond, Virginia, USA

Article

Publisher ID: cc3796

DOI: 10.1186/cc3796

PMC ID: 1297622

PubMed ID: 16277740

SO-VID: e03448d3-ffde-4e41-8982-8fcdb610896d

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Clinical review: The meaning of acid–base abnormalities in the intensive care unit part I – epidemiology

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Most cited references 64

Early lactate clearance is associated with improved outcome in severe sepsis and septic shock.

Lactate clearance and survival following injury.

Low mortality rate in adult respiratory distress syndrome using low-volume, pressure-limited ventilation with permissive hypercapnia: a prospective study.

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