Effects of Hypercapnia and Hypercapnic Acidosis on Hospital Mortality in Mechanically Ventilated Patients

OBJECTIVES

Lung-protective ventilation is used to prevent further lung injury in patients on invasive mechanical ventilation. However, lung-protective ventilation can cause hypercapnia and hypercapnic acidosis. There are no large clinical studies evaluating the effects of hypercapnia and hypercapnic acidosis in patients requiring mechanical ventilation.

DESIGN

Multicenter, binational, retrospective study aimed to assess the impact of compensated hypercapnia and hypercapnic acidosis in patients receiving mechanical ventilation.

SETTINGS

Data were extracted from the Australian and New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database over a 14-year period where 171 ICUs contributed deidentified data.

PATIENTS

Patients were classified into three groups based on a combination of pH and carbon dioxide levels (normocapnia and normal pH, compensated hypercapnia [normal pH with elevated carbon dioxide], and hypercapnic acidosis) during the first 24 hours of ICU stay. Logistic regression analysis was used to identify the independent association of hypercapnia and hypercapnic acidosis with hospital mortality.

INTERVENTIONS

Nil.

MEASUREMENTS AND MAIN RESULTS

A total of 252,812 patients (normocapnia and normal pH, 110,104; compensated hypercapnia, 20,463; and hypercapnic acidosis, 122,245) were included in analysis. Patients with compensated hypercapnia and hypercapnic acidosis had higher Acute Physiology and Chronic Health Evaluation III scores (49.2 vs 53.2 vs 68.6; p < 0.01). The mortality was higher in hypercapnic acidosis patients when compared with other groups, with the lowest mortality in patients with normocapnia and normal pH. After adjusting for severity of illness, the adjusted odds ratio for hospital mortality was higher in hypercapnic acidosis patients (odds ratio, 1.74; 95% CI, 1.62-1.88) and compensated hypercapnia (odds ratio, 1.18; 95% CI, 1.10-1.26) when compared with patients with normocapnia and normal pH (p < 0.001). In patients with hypercapnic acidosis, the mortality increased with increasing PCO2 until 65 mm Hg after which the mortality plateaued.

CONCLUSIONS

Hypercapnic acidosis during the first 24 hours of intensive care admission is more strongly associated with increased hospital mortality than compensated hypercapnia or normocapnia.

[Noninvasive ventilation and acute respiratory failure]

Noninvasive ventilation is effective in acute respiratory failure, in which drug therapy and administration of supplemental oxygen do not suffice and attempts are made to prevent the patient from ending up in invasive respirator therapy. The treatment is suited for acute respiratory failure for instance in cases of exacerbation of chronic obstructive pulmonary disease, in which a disturbance of pulmonary ventilation leads to the accumulation of carbon dioxide and to respiratory acidosis. Disadvantages associated with artificial airways are avoided, number of complications are reduced, hospitalization periods become shorter, mortality decreases and costs are saved.

The use of noninvasive mechanical ventilation in COPD with severe hypercapnic acidosis

STUDY OBJECTIVES

To compare the effect of noninvasive mechanical ventilation (NIV) in severely acidotic with mildly acidotic patients with acute hypercapnic chronic obstructive lung disease (COPD).

DESIGN

Comparison of NIV in consecutively enrolled patients with acute hypercapnic COPD with mild (pH 7.25-7.35) or severe (pH<7.25) acidosis on time to normalise pH and improve PaCO(2), duration of NIV treatment, length of stay in hospital and survival. Results (meadian (IQR)): Twenty-nine patients had 36 episodes of acute hypercapnic respiratory failure: Seventeen with pH<7.25 and 19 with pH 7.25-7.34. Compared with the mildly acidotic group, the severely acidotic group took a similar length of time for pH to normalise and PaCO(2) improve (12 (6-34) vs 12 (4-28)h, respectively, P=0.42), with similar duration of NIV treatment (60 (35-96) vs 68 (36-48)h, respectively, P=0.25) and hospital length of stay (8 (7-18) vs 9 (5-17) days, respectively, P=0.61). Overall survival was 89%, with 95% in the mild and 82% in the severely acidotic groups.

CONCLUSIONS

Noninvasive ventilation is effective in the treatment of patients with severe acidosis due to acute hypercapnic COPD.

Hypercapnic acidosis and mortality in acute lung injury*

OBJECTIVE

We tested the hypothesis that hypercapnic acidosis is associated with reduced mortality rate in patients with acute lung injury independent of changes in mechanical ventilation.

DESIGN

Secondary analysis of randomized clinical trial data using hypothesis-driven multivariate logistic regression.

SETTING

Randomized, multiple-center trial (n = 861) comparing 12 mL/kg to 6 mL/kg predicted body weight tidal volumes previously published by the National Institutes of Health Acute Respiratory Distress Syndrome (ARDS) Network.

PATIENTS

Acute lung injury patients enrolled in a randomized, multiple-center trial (n = 861).

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The adjusted odds ratio and 95% confidence intervals (CI) for 28-day mortality rate associated with hypercapnic acidosis defined as day 1 pH <7.35 and Pa(CO2) >45 mm Hg were 0.14 (95% CI 0.03-0.70, p = .016) in the 12 mL/kg predicted body weight tidal volume group and 1.18 (95% CI 0.59-2.35, p = .639) in the 6 mL/kg predicted body weight tidal volume group. Other definitions of hypercapnic acidosis spanning a range of magnitudes suggest a dose-response association between hypercapnic acidosis and 28-day mortality in the 12 mL/kg predicted body weight tidal volume group. None of our definitions of hypercapnic acidosis were associated with reduction in 28-day mortality in the 6 mL/kg predicted body weight tidal volume group.

CONCLUSIONS

Hypercapnic acidosis was associated with reduced 28-day mortality in the 12 mL/kg predicted body weight tidal volume group after controlling for comorbidities and severity of lung injury. These results are consistent with a protective effect of hypercapnic acidosis against ventilator-associated lung injury that was not found when the further ongoing injury was reduced by 6 mL/kg predicted body weight tidal volumes.

The use of oxygen in acute exacerbations of chronic obstructive pulmonary disease: a prospective audit of pre-hospital and hospital emergency management

Treatment with high-flow oxygen in acute exacerbations of chronic obstructive pulmonary disease (AECOPD) can cause or aggravate acute hypercapnic respiratory failure and adversely affect prognosis. National guidelines for the management of COPD recommend an initial fractional inspired oxygen concentration (FiO2) of no more than 0.28. However, a prospective audit of 101 consecutive episodes of AECOPD demonstrated that oxygen therapy with an FiO2 in excess of 0.28 is common, potentially deleterious and predominantly initiated in the ambulance. Patient awareness, aids to disease identification and ambulance protocols are likely to hold the key to improvement in the acute care of these patients.

[Can subpartal fetal acidosis be avoided? Investigations on the complex causes of intrauterine asphyxia]

OBJECTIVE

The extent to which faulty medical treatment and defective apparatus are concomitant causes for the development of subpartal acidosis was investigated in a retrospective study. At the same time, the incidence of acidosis in the Lippe-Detmold Hospital, Department of Gynaecology and Obstetrics, its morbidity and mortality were analysed.

METHODS

Data from all case histories of neonates with acidosis (pH in the umbilical artery < 7.100) who were born between 1 st January 1992 and 31 st December 1998 at the Department of Gynecology and Obstetrics at the Lippe-Detmold Hospital were evaluated electronically. Analytical measurements of blood gases (pH, pCO 2 and pO 2 ) in umbilical artery and venous blood were available from all cases. The base excess was corrected by computations according to Siggaard-Andersen and R. Zander for the actual oxygen saturation. The delivery cardiotocograms (CTG) were appraised qualitatively. Equipment defects, mistakes on the part of doctors and/or midwives (including the head of the department) were recorded after critical analysis of each individual case and documented in accordance with a key. The neonatal data were taken from the files of the paediatric division of the hospital (head of the department Dr. K. Wesseler).

RESULTS

In seven years under report, 9.876 babies were born, 156 (1.58 %) of whom showed a pH of less than 7.100 in the umbilical arterial blood. The mean actual pH value was 7.047 +/- 0.058, and the oxygen-corrected base excess was - 16.3 +/- 3.2 mmol/l. Correction of the base excess resulted in a numerical lowering by about 2.0 mmol/l. The rate of premature births was 17.4 %. One newborn baby died of hypoxic shock (0.67 %). 94 % of these neonates could be discharged in a healthy condition. 4.6 % still showed symptoms on discharge. Disorders of respiratory adaptation were the most prominent feature in morbidity from acidosis (about 18 %). Only two babies showed neonatal convulsions. Renal, cardiac and haemostaseological complications were rarely observed. Only 5 (3.9 %) of the 128 neonates with available recordings did not show any pathological changes in the CTG. 26.4 % of all acidoses had to be designated as "pure fate". In a further 35 % medical mistakes could not be discerned. Consequently, 61.4 % of the acidoses had to be designated as "unavoidable". In the remaining roughly 40 %, inadequate cardiotocographical knowledge, inattentiveness, defective equipment etc. clearly played a causal role. In the severe cases (pH in the umbilical artery < 7.000), medical mistakes were much more frequent (50 %).

CONCLUSIONS

Three-fifths of all subpartal acidoses in this study have a fateful nature, i.e. they cannot be prevented even by optimal professional management in good time. About two-fifths are avoidable if appropriate equipment and trained staff are available around the clock. Use of cardiotocography alone enables the threat of asphyxia to be detected in 97 % of the cases. The short-term prognosis of subpartal acidosis is good provided very low pH values (< 6.900) can be avoided. In perinatological studies, the base excess value should be corrected by computation. "Quality control" worthy of the name should include critical single-case analysis at least in severe acidosis.

Effect of alkalinization and fluids on survival in acute, non-hypoxic respiratory acidosis

Effects of intravenous normal saline, sodium bicarbonate, hypertonic saline, and Tromethamine were studied in 21 pigs and 60 rats subjected to acute, severe respiratory acidosis. Transient multiphasic alterations of systemic arterial pressures were seen with boluses of each agent-especially sodium bicarbonate and hypertonic saline-but any improvements in hemodynamic variables were transient. At the dose given, sodium bicarbonate significantly increased PaCO2 while decreasing hydrogen ion (H+) accumulation, whereas tromethamine buffered pH without significantly increasing PaCO2. However, no change of either arterial or venous H+ or PCO2 could be identified which rapidly produced death. Survival times were statistically equivalent among all groups. Therefore, intravenous treatment of respiratory acidosis with fluids or alkalinizing agents appears neither helpful nor harmful.

Benzyl alcohol toxicity in a neonatal intensive care unit. Incidence, symptomatology, and mortality

We unknowingly "screened" all NICU infants for elevated levels of serum benzyl alcohol (Bz-OH) over a three-month period. The fortuitous "screening" procedure resulted from the interference by Bz-OH with a routine blood CO2 assay used for all infants; validity was proved by (1) replication of the interference pattern with Bz-OH or benzoic acid; (2) confirmation of elevated benzoic acid levels in serum in two of the four screening positive infants tested but not in control infants, and (3) disappearance of the interference patterns when Bz-OH solutions were discontinued in affected infants. Screened Bz-OH-positive infants were compared to screened negative control infants, matched for weight (less than 1000 g), severity of RDS (on respirators), exposure to Bz-OH, and survival for longer than 48 hours. Intraventricular hemorrhage (IVH), metabolic acidosis appearing prior to IVH, and mortality were increased in Bz-OH-positive infants (P less than .05 in each case); hyperbilirubinemia and thrombocytopenia were not. Gasping respirations were not a major symptom. It is concluded that Bz-OH poisoning was a major cause of morbidity and mortality in NICU infants weighing less than 1000 g at birth during the three-month screening period. A retrospective review of patient records covering a 16-month period showed significant improvement in the survival rate of infants weighing less than 1000 g following the discontinuation of Bz-OH solutions.

Evolution of the acid-base status in cardiac arrest

In a study of the evolution of acid-base status in 26 patients who had cardiopulmonary arrest in the operating room, it appeared that:The determination of acid-base status within the first hour post-cardiac arrest is useful in differentiating final survivors from non-survivors. Respiratory or combined acidosis carries a poor prognosis not evidenced for metabolic acidosis. Late respiratory complications are more frequent in patients with initial combined acidosis. Treatment should be instituted on the basis of frequent determinations of acidbase status, since accurate diagnosis of degree and type of acidosis cannot be done on clinical grounds only. Recovery of consciousness is influenced by the type and severity of acidosis, less so by duration of arrest; and that high pCO(2) is associated frequently with unconsciousness after recovery of circulatory function.