Predicting dead space ventilation in critically ill patients using clinically available data

Prognostic value of time-varying dead space estimates in mechanically ventilated patients with acute respiratory distress syndrome

Background

The dead space fraction (VD/VT) has proven to be a powerful predictor of higher mortality in acute respiratory distress syndrome (ARDS). However, its measurement relies on expired carbon dioxide, limiting its widespread application in clinical practice. Several estimates employing routine variables have been found to be reliable substitutes for direct measurement of VD/VT. In this study, we evaluated the prognostic value of these dead space estimates obtained in the first 7 days following the initiation of ventilation.

Methods

This retrospective observational study was conducted using data from the Chinese database in intensive care (CDIC). Eligible participants were adult ARDS patients receiving invasive mechanical ventilation while in the intensive care unit between 1st January 2014 and 31st March 2021. We collected data during the first 7 days of ventilation to calculate various dead space estimates, including ventilatory ratio (VR), corrected minute ventilation (V ˙ Ecorr ), VD/VT (Harris-Benedict), VD/VT (Siddiki estimate), and VD/VT (Penn State estimate) longitudinally. A time-dependent Cox model was used to handle these time-varying estimates.

Results

A total of 392 patients (median age 66 [interquartile range: 55-77] years, median SOFA score 9 [interquartile range: 7-12]) were finally included in our analysis, among whom 132 (33.7%) patients died within 28 days of admission. VR (hazard ratio [HR]=1.04 per 0.1 increase, 95% confidence interval [CI]: 1.01 to 1.06; P=0.013), V ˙ Ecorr (HR=1.08 per 1 increase, 95% CI: 1.04 to 1.12; P < 0.001), VD/VT (Harris-Benedict) (HR=1.25 per 0.1 increase, 95% CI: 1.06 to 1.47; P=0.006), and VD/VT (Penn State estimate) (HR=1.22 per 0.1 increase, 95% CI: 1.04 to 1.44; P=0.017) remained significant after adjustment, while VD/VT (Siddiki estimate) (HR=1.10 per 0.1 increase, 95% CI: 1.00 to 1.20; P=0.058) did not. Given a large number of negative values, VD/VT (Siddiki estimate) and VD/VT (Penn State estimate) were not recommended as reliable substitutes. Long-term exposure to VR >1.3, V ˙ Ecorr >7.53, and VD/VT (Harris-Benedict) >0.59 was independently associated with an increased risk of mortality in ARDS patients. These findings were validated in the fluid and catheter treatment trial (FACTT) database.

Conclusions

In cases where VD/VT cannot be measured directly, early time-varying estimates of VD/VT such as VR, V ˙ Ecorr , and VD/VT (Harris-Benedict) can be considered for predicting mortality in ARDS patients, offering a rapid bedside application.

Impact of prone position on dead-space fraction in COVID-19 related acute respiratory distress syndrome

INTRODUCTION

COVID-19 Related Acute Respiratory Syndrome (C-ARDS) is characterized by a mismatch between respiratory mechanics and hypoxemia, suggesting increased dead-space fraction (DSF). Prone position is a cornerstone treatment of ARDS under invasive mechanical ventilation reducing mortality. We sought to investigate the impact of prone position on DSF in C-ARDS in a cohort of patients receiving invasive mechanical ventilation.

METHODS

we retrospectively analysed data from 85 invasively mechanically ventilated patients with C-ARDS in supine and in prone positions, hospitalized in Intensive Care Unit (Reims University Hospital), between November, 1st 2020 and November, 1st 2022. DSF was estimated via 3 formulas usable at patients' bedside, based on partial pressure of carbon dioxide (PaCO2) and end-tidal carbon dioxide (EtCO2).

RESULTS

there was no difference of DSF between supine and prone position, using the 3 formulas. According to Enghoff, Frankenfield and Gattinoni equations, DSF in supine vs. prone position was in median respectively [IQR]: 0.29 [0.13-0.45] vs. 0.31 [0.19-0.51] (p = 0.37), 0.5 [0.48-0.52] vs. 0.51 [0.49-0.53] (p = 0.43), and 0.71 [0.55-0.87] vs. 0.69 [0.57-0.81], (p = 0.32).

CONCLUSION

prone position did not change DSF in C-ARDS.

The Use of Alveolar Dead Space Fraction to Predict Postoperative Outcomes after Pediatric Cardiac Surgery: A Retrospective Study

Patients with congenital heart disease (CHD) that have surgical repair with cardiopulmonary bypass (CPB) reflect a unique population with multiple pulmonary and systemic factors that may contribute to increased alveolar dead space and low cardiac output syndrome. This study aimed to assess and compare changes in the alveolar dead space fraction (AVDSf) in the immediate postoperative period with outcomes in children with CHD who underwent repair on CPB. A single-center retrospective review study of critically ill children with CHD, younger than 18 years of age admitted to the Pediatric Intensive Care Unit (PICU) after undergoing surgical repair on CPB and received invasive mechanical ventilation for at least 24 h. One hundred and two patients were included in the study. Over the first 24 h, mean AVDSf was significantly higher in patients who had longer hospital length of stay (LOS) (> 21 days) p = 0.02, and longer duration of invasive mechanical ventilation (DMV) (> 170 h) p = 0.01. Cross-sectional analyses at 23-24 h revealed that AVDSf > 0.25 predicts mortality and DMV (p = 0.03 and P = 0.02 respectively); however, it did not predict prolonged hospital LOS. For every 0.1 increase in the AVDSf, the odds of mortality, DMV, and hospital LOS increased by 4.9 [95% CI = 1.45-16.60, p = 0.002], 2.06 [95% CI = 1.14-3.71, p = 0.01], and 1.43[95% CI = 0.84-2.45, p = 0.184], respectively. The area under the ROC curve at 23-24 h for AVDSf was 0.868 to predict mortality as an outcome. AVDSf > 0.25 at 23-24 h postoperatively was an independent predictor of mortality with sensitivity and specificity of 83% and 80%, respectively and was superior to other commonly used surrogates of cardiac output. In the immediate postoperative period of pediatric patients with CHD, high AVDSf is associated with longer hospital length of stay and duration of invasive mechanical ventilation. Increased AVDSf values at 23-24 h postoperatively is associated with mortality in patients with CHD exposed to CPB.

Alveolar Dead-Space Fraction and Arterial Saturation Predict Postoperative Course in Fontan Patients

OBJECTIVES

Fontan surgery, the final surgical stage in single ventricle palliation, redirects systemic venous blood into the pulmonary circulation for gas exchange. A decrease in pulmonary blood flow can lead to major complications and grave outcomes. Alveolar dead-space fraction represents the portion of inhaled air that does not participate in gas exchange and hence quantifies ventilation-perfusion abnormalities in the lung. Increased alveolar dead-space fraction has been associated with prolonged mechanical ventilation and worse outcome after congenital heart surgery. The association of alveolar dead-space fraction with clinical outcomes in patients undergoing Fontan operation has not been reported.

INTERVENTIONS

None.

DESIGN, SETTING, AND PATIENTS

A retrospective charts review of all pediatric patients who underwent Fontan surgery during June 2010-November 2018 in a tertiary-care pediatric hospital. Associations between alveolar dead-space fraction and arterial oxyhemoglobin saturation to a composite outcome (surgical or catheter-based intervention, extracorporeal membrane oxygenation use, prolonged ventilation, prolonged hospital length of stay, or death) were explored. Secondary endpoints were parameters of severity of illness, chest drainage duration, and length of stay.

MEASUREMENTS AND MAIN RESULTS

Of 128 patients undergoing Fontan operation, 34 met criteria for composite outcome. Alveolar dead-space fraction was significantly higher in the composite (0.33 ± 0.14) versus control (0.25 ± 0.26; p = 0.016) group. Alveolar dead-space fraction greater than or equal to 0.29 indicated a 37% increase in risk to meet composite criteria. Admission arterial oxygen saturation was significantly lower in composite versus control group (93.4% vs 97.1%; p = 0.005). Alveolar dead-space fraction was significantly associated with increased durations of mechanical ventilation, ICU length of stay, duration of thoracic drainage, and parameters of severity of illness.

CONCLUSIONS

Alveolar dead-space fraction and arterial saturation may predict complicated postoperative course in patients undergoing the Fontan operation.

Temporal Changes in Ventilator Settings in Patients With Uninjured Lungs: A Systematic Review

In patients with uninjured lungs, increasing evidence indicates that tidal volume (VT) reduction improves outcomes in the intensive care unit (ICU) and in the operating room (OR). However, the degree to which this evidence has translated to clinical changes in ventilator settings for patients with uninjured lungs is unknown. To clarify whether ventilator settings have changed, we searched MEDLINE, Cochrane Central Register of Controlled Trials, and Web of Science for publications on invasive ventilation in ICUs or ORs, excluding those on patients <18 years of age or those with >25% of patients with acute respiratory distress syndrome (ARDS). Our primary end point was temporal change in VT over time. Secondary end points were changes in maximum airway pressure, mean airway pressure, positive end-expiratory pressure, inspiratory oxygen fraction, development of ARDS (ICU studies only), and postoperative pulmonary complications (OR studies only) determined using correlation analysis and linear regression. We identified 96 ICU and 96 OR studies comprising 130,316 patients from 1975 to 2014 and observed that in the ICU, VT size decreased annually by 0.16 mL/kg (-0.19 to -0.12 mL/kg) (P < .001), while positive end-expiratory pressure increased by an average of 0.1 mbar/y (0.02-0.17 mbar/y) (P = .017). In the OR, VT size decreased by 0.09 mL/kg per year (-0.14 to -0.04 mL/kg per year) (P < .001). The change in VTs leveled off in 1995. Other intraoperative ventilator settings did not change in the study period. Incidences of ARDS (ICU studies) and postoperative pulmonary complications (OR studies) also did not change over time. We found that, during a 39-year period, from 1975 to 2014, VTs in clinical studies on mechanical ventilation have decreased significantly in the ICU and in the OR.

Measurement of Dead Space Fraction Upon ICU Admission Predicts Length of Stay and Clinical Outcomes Following Bidirectional Cavopulmonary Anastomosis

OBJECTIVES

Increased alveolar dead space fraction has been associated with prolonged mechanical ventilation and increased mortality in pediatric patients with respiratory failure. The association of alveolar dead space fraction with clinical outcomes in patients undergoing bidirectional cavopulmonary anastomosis for single ventricle congenital heart disease has not been reported. We describe an association of alveolar dead space fraction with postoperative outcomes in patients undergoing bidirectional cavopulmonary anastomosis.

DESIGN

In a retrospective case-control study, we examined for associations between alveolar dead space fraction ([PaCO2 - end-tidal CO2]/PaCO2), arterial oxyhemoglobin saturation, and transpulmonary gradient upon postoperative ICU admission with a composite primary outcome (requirement for surgical or catheter-based intervention, death, or transplant prior to hospital discharge, defining cases) and several secondary endpoints in infants following bidirectional cavopulmonary anastomosis.

SETTINGS

Cardiac ICU in a tertiary care pediatric hospital.

PATIENTS

Patients undergoing bidirectional cavopulmonary anastomosis at our institution between 2011 and 2016.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Of 191 patients undergoing bidirectional cavopulmonary anastomosis, 28 patients were cases and 163 were controls. Alveolar dead space fraction was significantly higher in the case (0.26 ± 0.09) versus control group (0.17 ± 0.09; p < 0.001); alveolar dead space fraction at admission was less than 0.12 in 0% of cases and was greater than 0.28 in 35% of cases. Admission arterial oxyhemoglobin saturation was significantly lower in the case (77% ± 12%) versus control group (83% ± 9%; p < 0.05). Sensitivity and specificity for future case versus control assignment was best when prebidirectional cavopulmonary anastomosis risk factors, admission alveolar dead space fraction (AUC, 0.74), and arterial oxyhemoglobin saturation (AUC, 0.65) were combined in a summarial model (AUC, 0.83). For a given arterial oxyhemoglobin saturation, the odds of becoming a case increased on average by 181% for every 0.1 unit increase in alveolar dead space fraction. Admission alveolar dead space fraction and arterial oxyhemoglobin saturation were linearly associated with prolonged ICU length of stay, hospital length of stay, duration of mechanical ventilation, and duration of thoracic drainage (p < 0.001 for all).

CONCLUSIONS

Following bidirectional cavopulmonary anastomosis, alveolar dead space fraction in excess of 0.28 or arterial oxyhemoglobin saturation less than 78% upon ICU admission indicates an increased likelihood of requiring intervention prior to hospital discharge. Increasing alveolar dead space fraction and decreasing arterial oxyhemoglobin saturation are associated with increased lengths of stay.

Comparison of the pulmonary dead-space fraction derived from ventilator volumetric capnography and a validated equation in the survival prediction of patients with acute respiratory distress syndrome

PURPOSE

This prospective observational study aims to evaluate the accuracy of dead-space fraction derived from the ventilator volumetric capnography (volumetric CO₂) or a prediction equation to predict the survival of mechanically ventilated patients with acute respiratory distress syndrome (ARDS).

METHODS

Consecutive VD/VT measurements were obtained based upon a prediction equation validated by Frankenfield et al for dead-space ventilation fraction: VD/VT = 0.320 + 0.0106 (PaCO₂-ETCO₂)⁺ 0.003 (RR)⁺0.0015 (age) in adult patients who had infection-related severe pneumonia and were confirmed as having ARDS. Here PaCO₂ is the arterial partial pressure of carbon dioxide in mmHg; ETCO₂, the end- tidal carbon dioxide measurement in mmHg; RR, respiratory rate per minute; and age in years. Once the patient had intubation, positive end expiratory pressure was adjusted and after Phigh reached a steady state, VD/VT was measured and recorded as the data for the first day. VD/VT measurement was repeated on days 2, 3, 4, 5 and 6. Meanwhile we collected dead-space fraction directly from the ventilator volu- metric CO₂ and recorded it as Vd/Vt. We analyzed the changes in VD/VT and Vd/Vt over the 6-day period to determine their accuracy in predicting the survival of ARDS patients.

RESULTS

Overall, 46 patients with ARDS met the inclusion criteria and 24 of them died. During the first 6 days of intubation, VD/VT was significantly higher in nonsurvivors on day 4 (0.70 ± 0.01 vs 0.57 ± 0.01), day 5 (0.73 ± 0.01 vs. 0.54 ± 0.01), and day 6 (0.73 ± 0.02 vs. 0.54 ± 0.01) (all p =0.000). Vd/Vt showed no significant difference on days 1e4 but it was much higher in nonsurvivors on day 5 (0.45 ± 0.04 vs. 0.41 ± 0.06) and day 6 (0.47 ± 0.05 vs. 0.40 ± 0.03) (both p=0.008). VD/VT on the fourth day was more accurate to predict survival than Vd/Vt. The area under the receiver-operating characteristic curve for VD/VT and Vd/Vt in evaluating ARDS patients survival was day 4 (0.974 ± 0.093 vs. 0.701 ± 0.023, p = 0.0024) with the 95% confidence interval being 0.857-0.999 vs. 0.525-0.841.

CONCLUSION

Compared with Vd/Vt derived from ventilator volumetric CO₂, VD/VT on day 4 calculated by Frankenfield et al's equation can more accurately predict the survival of ARDS patients.

Higher Dead Space Is Associated With Increased Mortality in Critically Ill Children

OBJECTIVE

Elevated dead space has been consistently associated with increased mortality in adults with respiratory failure. In children, the evidence for this association is more limited. We sought to investigate the association between dead space and mortality in mechanically ventilated children.

DESIGN

Single-center retrospective review.

SETTING

Tertiary care pediatric critical care unit.

PATIENTS

Seven hundred twelve mechanically ventilated children with an arterial catheter.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The end-tidal alveolar dead space fraction ((PaCO2-PETCO2)/PaCO2), a dead space marker, was calculated with each arterial blood gas. The initial end-tidal alveolar dead space fraction (first arterial blood gas after intubation) (per 0.1 unit increase: odds ratio, 1.59; 95% CI, 1.40-1.81) and day 1 mean end-tidal alveolar dead space fraction (odds ratio, 1.95; 95% CI, 1.66-2.30) were associated with mortality. The relationship between both initial and day 1 mean end-tidal alveolar dead space fraction and mortality held in multivariate modeling after controlling for any of the following individually: PaO2/FIO2, oxygenation index, 24-hour maximal inotrope score, and Pediatric Risk of Mortality III (all p<0.01), although end-tidal alveolar dead space fraction was no longer significant after controlling for the combination of oxygenation index, 24-hour maximal inotrope score, and Pediatric Risk of Mortality III. In 217 children with acute hypoxemic respiratory failure, initial end-tidal alveolar dead space fraction (per 0.1 unit increase odds ratio, 1.38; 95% CI, 1.14-1.67) and day 1 mean end-tidal alveolar dead space fraction (per 0.1 unit increase odds ratio, 1.60; 95% CI, 1.27-2.0) were associated with mortality. Day 1 mean end-tidal alveolar dead space fraction remained associated with mortality after controlling individually for any of the following in multivariate models: PaO2/FIO2, oxygenation index, and 24-hour maximal inotrope score (p≤0.02), although end-tidal alveolar dead space fraction was no longer significant after controlling for the combination of oxygenation index, 24-hour maximal inotrope score, and Pediatric Risk of Mortality III.

CONCLUSIONS

Increased dead space is associated with higher mortality in critically ill children, although it is no longer independently associated with mortality after controlling for severity of oxygenation defect, inotrope use, and severity of illness. However, because end-tidal alveolar dead space fraction is easy to calculate at the bedside, it may be useful for risk stratification and severity-of-illness scores.

Alveolar Dead Space Fraction Discriminates Mortality in Pediatric Acute Respiratory Distress Syndrome

OBJECTIVES

Physiologic dead space is associated with mortality in acute respiratory distress syndrome, but its measurement is cumbersome. Alveolar dead space fraction relies on the difference between arterial and end-tidal carbon dioxide (alveolar dead space fraction = (PaCO2 - PetCO2) / PaCO2). We aimed to assess the relationship between alveolar dead space fraction and mortality in a cohort of children meeting criteria for acute respiratory distress syndrome (both the Berlin 2012 and the American-European Consensus Conference 1994 acute lung injury) and pediatric acute respiratory distress syndrome (as defined by the Pediatric Acute Lung Injury Consensus Conference in 2015).

DESIGN

Secondary analysis of a prospective, observational cohort.

SETTING

Tertiary care, university affiliated PICU.

PATIENTS

Invasively ventilated children with pediatric acute respiratory distress syndrome.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Of the 283 children with pediatric acute respiratory distress syndrome, 266 had available PetCO2. Alveolar dead space fraction was lower in survivors (median 0.13; interquartile range, 0.06-0.23) than nonsurvivors (0.31; 0.19-0.42; p < 0.001) at pediatric acute respiratory distress syndrome onset, but not 24 hours after (survivors 0.12 [0.06-0.18], nonsurvivors 0.14 [0.06-0.25], p = 0.430). Alveolar dead space fraction at pediatric acute respiratory distress syndrome onset discriminated mortality with an area under receiver operating characteristic curve of 0.76 (95% CI, 0.66-0.85; p < 0.001), better than either initial oxygenation index or PaO2/FIO2. In multivariate analysis, alveolar dead space fraction at pediatric acute respiratory distress syndrome onset was independently associated with mortality, after adjustment for severity of illness, immunocompromised status, and organ failures.

CONCLUSIONS

Alveolar dead space fraction at pediatric acute respiratory distress syndrome onset discriminates mortality and is independently associated with nonsurvival. Alveolar dead space fraction represents a single, useful, readily obtained clinical biomarker reflective of pulmonary and nonpulmonary variables associated with mortality.

Estimating dead-space fraction for secondary analyses of acute respiratory distress syndrome clinical trials

OBJECTIVES

Pulmonary dead-space fraction is one of few lung-specific independent predictors of mortality from acute respiratory distress syndrome. However, it is not measured routinely in clinical trials and thus altogether ignored in secondary analyses that shape future research directions and clinical practice. This study sought to validate an estimate of dead-space fraction for use in secondary analyses of clinical trials.

DESIGN

Analysis of patient-level data pooled from acute respiratory distress syndrome clinical trials. Four approaches to estimate dead-space fraction were evaluated: three required estimating metabolic rate; one estimated dead-space fraction directly.

SETTING

U.S. academic teaching hospitals.

PATIENTS

Data from 210 patients across three clinical trials were used to compare performance of estimating equations with measured dead-space fraction. A second cohort of 3,135 patients from six clinical trials without measured dead-space fraction was used to confirm whether estimates independently predicted mortality.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Dead-space fraction estimated using the unadjusted Harris-Benedict equation for energy expenditure was unbiased (mean ± SD Harris-Benedict, 0.59 ± 0.13; measured, 0.60 ± 0.12). This estimate predicted measured dead-space fraction to within ±0.10 in 70% of patients and ±0.20 in 95% of patients. Measured dead-space fraction independently predicted mortality (odds ratio, 1.36 per 0.05 increase in dead-space fraction; 95% CI, 1.10-1.68; p < 0.01). The Harris-Benedict estimate closely approximated this association with mortality in the same cohort (odds ratio, 1.55; 95% CI, 1.21-1.98; p < 0.01) and remained independently predictive of death in the larger Acute Respiratory Distress Syndrome Network cohort. Other estimates predicted measured dead-space fraction or its association with mortality less well.

CONCLUSIONS

Dead-space fraction should be measured in future acute respiratory distress syndrome clinical trials to facilitate incorporation into secondary analyses. For analyses where dead-space fraction was not measured, the Harris-Benedict estimate can be used to estimate dead-space fraction and adjust for its association with mortality.

Early management of the severely injured major trauma patient

The major trauma team relies on an efficient, communicative team to ensure patients receive the best quality care. This requires a comprehensive handover, rapid systematic review, and early management of life- and limb-threatening injuries. These multiple injured patients often present with complex conditions in a dynamic situation. The importance of team work, communication, senior decision-making, and documentation cannot be underestimated.

Spontaneous breathing with biphasic positive airway pressure attenuates lung injury in hydrochloric acid-induced acute respiratory distress syndrome

BACKGROUND

It has been proved that spontaneous breathing (SB) with biphasic positive airway pressure (BIPAP) can improve lung aeration in acute respiratory distress syndrome compared with controlled mechanical ventilation. The authors hypothesized that SB with BIPAP would attenuate lung injury in acute respiratory distress syndrome compared with pressure-controlled ventilation.

METHODS

Twenty male New Zealand white rabbits with hydrochloric acid aspiration-induced acute respiratory distress syndrome were randomly ventilated using the BIPAP either with SB (BIPAP plus SB group) or without SB (BIPAP minus SB group) for 5 h. Inspiration pressure was adjusted to maintain the tidal volume at 6 ml/kg. Both groups received the same positive end-expiratory pressure level at 5 cm H2O for hemodynamic goals. Eight healthy animals without ventilatory support served as the control group.

RESULTS

The BIPAP plus SB group presented a lower ratio of dead space ventilation to tidal volume, a lower respiratory rate, and lower minute ventilation. No significant difference in the protein levels of interleukin-6 and interleukin-8 in plasma, bronchoalveolar lavage fluid, and lung tissue were measured between the two experimental groups. However, SB resulted in lower messenger ribonucleic acid levels of interleukin-6 (mean ± SD; 1.8 ± 0.7 vs. 2.6 ± 0.5; P = 0.008) and interleukin-8 (2.2 ± 0.5 vs. 2.9 ± 0.6; P = 0.014) in lung tissues. In addition, lung histopathology revealed less injury in the BIPAP plus SB group (lung injury score, 13.8 ± 4.6 vs. 21.8 ± 5.7; P < 0.05).

CONCLUSION

In hydrochloric acid-induced acute respiratory distress syndrome, SB with BIPAP attenuated lung injury and improved respiratory function compared with controlled ventilation with low tidal volume.

Prediction equation to estimate dead space to tidal volume fraction correlates with mortality in critically ill patients

OBJECTIVE

The measurement of dead space to tidal volume fraction (Vd/Vt) using various methodologies has been shown to be a reliable predictor of mortality in critically ill patients. In this study, we evaluated the correlation of a validated equation using clinically available information to predict calculation of Vd/Vt with clinically relevant outcome parameters in patients requiring mechanical ventilation.

METHODS

Calculations of Vd/Vt were obtained based upon a previously published prediction equation for dead space ventilation fraction: Vd/Vt = 0.320 + 0.0106 (Paco2--end-tidal carbon dioxide measurement) + 0.003 (respiratory rate per minute) + 0.0015 (age in years) on study days 1, 3 to 4, 6 to 9, and 14 after initiation of mechanical ventilation in adult patients who satisfied 1 of the 3 study defined diseases: (1) acute bacterial pneumonia, (2) acute respiratory distress syndrome, or (3) cystic fibrosis.

RESULTS

Using the final/last available time point calculation of Vd/Vt, a significant difference was observed between survivors and nonsurvivors both in relation to mean and median values (56.5% vs 71.2% and 56.0% vs 65.0%, respectively). In addition, sequential analyses of Vd/Vt calculations over time also demonstrated a statistically significant difference between survivors and nonsurvivors for days 6 to 9.

CONCLUSION

In this study-specific population of critically ill patients, the prediction equation of Vd/Vt using clinically available parameters correlates with mortality. In addition, we provide a simple method to estimate Vd/Vt that can be potentially applicable to all critically ill intensive care unit patients.

Energy expenditure in critically ill surgical patients. Comparative analysis of predictive equation and indirect calorimetry

PURPOSE

The aim of this investigation was to compare the resting energy expenditure (REE) calculated by the Harris-Benedict equation (REE HB) with the REE measured by indirect calorimetry (REE IC) in critically ill surgical patients under mechanical ventilation.

METHODS

Thirty patients were included in this work. REE was calculated by the Harris-Benedict equation (REE HB) using real body weight, and it was also measured by indirect calorimetry (REE IC), which was performed for 30 minutes.

RESULTS

REE HB had significant (p < 0.0005) but low correlation (Spearman r = 0.57) with REE IC, with a mean bias of 12 kcal.d-1 and limits of agreement ranging from - 599.7 to 623.7 kcal.d-1 as detected by the Bland-Altman analysis.

CONCLUSION

These findings suggest that REE IC seems to be more appropriate than REE HB for accurate measurement of REE in critically ill surgical patients under mechanical ventilation.

The association between the end tidal alveolar dead space fraction and mortality in pediatric acute hypoxemic respiratory failure

OBJECTIVE

To investigate the relationship of markers of oxygenation, PaO2/FIO2 ratio, SpO2/FIO2 ratio, oxygenation index, oxygen saturation index, and dead space (end tidal alveolar dead space fraction) with mortality in children with acute hypoxemic respiratory failure.

DESIGN

Retrospective.

SETTING

Single-center tertiary care pediatric intensive care unit.

PATIENTS

Ninety-five mechanically ventilated children with a PaO2/FIO2 ratio <300 within 24 hrs of the initiation of mechanical ventilation.

INTERVENTIONS

None.

MAIN RESULTS

The end tidal alveolar dead space fraction, PaO2/FIO2 ratio, SpO2/FIO2 ratio, oxygenation index, and oxygen saturation index were all associated with mortality (p < .02). There was a small correlation between the end tidal alveolar dead space fraction and decreasing PaO2/FIO2 (r2 = .21) and SpO2/FIO2 ratios (r2 = .22), and increasing oxygenation index (r2= .25) and oxygen saturation index (r2 = .24). In multivariate logistic regression modeling, the end tidal alveolar dead space fraction was independently associated with mortality (p < .02). Oxygenation index, oxygen saturation index, and the end tidal alveolar dead space fraction were all acceptable discriminators of mortality with receiver operating characteristic plot area under the curves ≥ 0.7.

CONCLUSIONS

In pediatric acute hypoxemic respiratory failure, easily obtainable pulmonary specific markers of disease severity (SpO2/FIO2 ratio, oxygen saturation index, and the end tidal alveolar dead space fraction) may be useful for the early identification of children at high risk of death. Furthermore, the end tidal alveolar dead space fraction should be considered for risk stratification of children with acute hypoxemic respiratory failure, given that it was independently associated with mortality.