Assessment of lung function in the intensive care unit

Comparison of the spo2/fio2 ratio and the pao2/fio2 ratio in patients with acute lung injury or acute respiratory distress syndrome

INTRODUCTION

Diagnostic criteria for acute lung injury (ALI) and Acute Respiratory Distress syndrome (ARDS) includes acute onset of disease, chest radiograph demonstrating bilateral pulmonary infiltrates, lack of significant left ventricular dysfunction and Pao2/Fio2 (PF) ratio ≤300 for ALI or ≤200 for ARDS. Recent criteria require invasive arterial sampling. The pulse oximetric saturation Spo2/Fio2 (SF) ratio may be a reliable non-invasive alternative to the PF ratio.

METHODS

In this cross-sectional study, we enrolled 70 patients with ALI or ARDS who were admitted in Tabriz children's hospital pediatrics intensive care unit (PICU). Spo2, Fio2, Pao2, charted within 5 minutes of each other and calculated SF and PF were recorded to determine the relationship between SF and PF ratio. SF values were examined as a substitute of PF ratio for diagnosis ARDS and ALI.

RESULTS

The relationship between SF and PF ratio was described by the following regression equation: SF=57+0.61 PF (P<0.001). SF ratios of 181 and 235 corresponded of PF ratio 300 and 200. The SF cutoff of 235 had 57% sensitivity and 100% specificity for diagnosis of ALI. The SF cutoff of 181 had 71% sensitivity and 82% specificity for diagnosis of ARDS.

CONCLUSION

SF ratio is a reliable noninvasive surrogate for PF ratio to identify children with ALI or ARDS with the advantage of replacing invasive arterial blood sampling by non-invasive pulse oximetry.

Non invasive monitoring in mechanically ventilated pediatric patients

Cardiopulmonary monitoring is a key component in the evaluation and management of critically ill patients. Clinicians typically rely on a combination of invasive and non-invasive monitoring to assess cardiac output and adequacy of ventilation. Recent technological advances have led to the introduction: of continuous non-invasive monitors that allow for data to be obtained at the bedside of critically ill patients. These advances help to identify hemodynamic changes and allow for interventions before complications occur. In this manuscript, we highlight several important methods of non-invasive cardiopulmonary monitoring, including capnography, transcutaneous monitoring, pulse oximetry, and near infrared spectroscopy.

Acute lung injury in children: therapeutic practice and feasibility of international clinical trials

OBJECTIVES

To describe mechanical ventilation strategies in acute lung injury and to estimate the number of eligible patients for clinical trials on mechanical ventilation management. In contrast to adult medicine, there are few clinical trials to guide mechanical ventilation management in children with acute lung injury.

DESIGN

A cross-sectional study for six 24-hr periods from June to November 2007.

SETTING

Fifty-nine pediatric intensive care units in 12 countries in North America and Europe.

PATIENTS

We identified children meeting acute lung injury criteria and collected detailed information on illness severity, mechanical ventilatory support, and use of adjunctive therapies.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Of 3823 patients screened, 414 (10.8%) were diagnosed with acute lung injury by their treating physician, but only 165 (4.3%) patients met prestablished inclusion/exclusion criteria to this trial and, therefore, would have been eligible for a clinical trial. Of these, 124 (75.2%) received conventional mechanical ventilation, 27 (16.4%) received high-frequency oscillatory ventilation, and 14 (8.5%) received noninvasive mechanical ventilation. In the conventional mechanical ventilation group, 43.5% were ventilated in a pressure control mode with a mean tidal volume of 8.3 ± 3.3 mL/kg; and there was no clear relationship between positive end-expiratory pressure and Fio2 delivery in the conventional mechanical ventilation group. Use of adjunctive treatments, including nitric oxide, prone positioning, surfactant, hemofiltration, recruitment maneuvers, steroids, bronchodilators, and fluid restriction, was highly variable.

CONCLUSIONS

Our study reveals inconsistent mechanical ventilation practice and use of adjunctive therapies in children with acute lung injury. Pediatric clinical trials assessing mechanical ventilation management are needed to generate evidence to optimize outcomes. We estimate that a large number of centers (∼60) are needed to conduct such trials; it is imperative, therefore, to bring about international collaboration.

Characteristics of children intubated and mechanically ventilated in 16 PICUs

BACKGROUND

When designing multicenter clinical trials, it is important to understand the characteristics of children who have received ventilation in PICUs.

METHODS

This study involved the secondary analysis of an existing data set of all children intubated and mechanically ventilated from 16 US PICUs who were initially screened for a multicenter clinical trial on pediatric acute lung injury (ALI).

RESULTS

A total of 12,213 children between 2 weeks and 18 years of age who were intubated and mechanically ventilated were included, representing 30% of PICU admissions (center range, 20 to 64%). Of the children who received ventilation, 22% had cyanotic congenital heart disease; 26% had respiratory failure but not bilateral pulmonary infiltrates on chest radiograph; 8% had chronic respiratory disease; 7% had upper airway obstruction; and 5% had reactive airway disease. At least 1,457 patients (15%) with respiratory failure lacked an arterial line. Of these patients, 97% had a positive end-expiratory pressure <or= 8 cm H(2)O, and 80% were supported on an Fio(2) of <or= 0.40. Moreover, 104 of 904 patients (12%) with pulse oximetric saturation (Spo(2)) and Fio(2) measurements available would have met the oxygenation criteria for ALI according to Spo(2)/Fio(2) ratio criteria.

CONCLUSIONS

At least 30% of children in a cross-section of US PICUs are endotracheally intubated, and 25% of those with respiratory failure do not fulfill the radiographic criteria for ALI. Although few patients without an indwelling arterial line require more than modest ventilator support, many may still meet the oxygenation criteria for ALI. These findings will facilitate sample size calculations and help to determine feasibility for future trials on pediatric mechanical ventilation.

Comparison of the pulse oximetric saturation/fraction of inspired oxygen ratio and the PaO2/fraction of inspired oxygen ratio in children

BACKGROUND

Although diagnostic criteria for acute lung injury (ALI) and ARDS are clear, invasive arterial sampling is required for computation of Pao(2)/fraction of inspired oxygen (Fio(2)) [PF] ratios. The pulse oximetric saturation (Spo(2))/Fio(2) (SF) ratio may be a reliable noninvasive alternative to the PF ratio for identifying children with lung injury.

METHODS

We electronically queried blood gas measurements from two tertiary care pediatric ICUs (PICUs). Included in the analysis were corresponding measurements of Spo(2), Pao(2), and Fio(2) charted within 15 min of each other when Spo(2) values were between 80% and 97%. Computed PF and SF ratios were compared to identify threshold values for SF ratios that correspond to PF criteria for ALI (< or = 300) and ARDS (< or = 200). Data from one PICU were used for derivation and validated with measurements from the second PICU.

RESULTS

From the 1,298 observations in the derivation data set, SF ratio could be predicted by the regression equation SF = 76 + 0.62 x PF (p < 0.0001, R(2) = 0.61). SF ratios of 263 and 201 corresponded to PF ratios of 300 and 200, respectively. The ALI SF cutoff of 263 had 93% sensitivity and 43% specificity, and the ARDS cutoff of 201 had 84% sensitivity and 78% specificity. Applying these values to the 1,845 observations in the validation data set yielded a sensitivity of 86% and specificity of 47% for ALI and a sensitivity of 68% and specificity of 84% for ARDS.

CONCLUSION

SF ratio is a reliable noninvasive marker for PF ratio to identify children with ALI or ARDS.

Accuracy of displayed values of tidal volume in the pediatric intensive care unit

OBJECTIVES

To assess the accuracy of the expired tidal volumes (VT(E)) displayed by one of the most frequently used ventilators that measures exhaled volume at the expiratory valve.

DESIGN

Prospective study.

SETTING

The intensive care units of a pediatric tertiary referral center in London, UK.

PATIENTS

A total of 56 intubated children aged between 3 wks and 16.6 yrs who were clinically stable and ventilated with a Servo 300 ventilator.

INTERVENTIONS

The CO2SMO Plus respiratory monitor, which measures flow at the airway opening, was validated using calibrated syringes and appropriate tracheal tubes and connections. Simultaneous in vivo recordings of VT(E) from the Servo 300 and CO2SMO Plus were compared before (displayed Servo VT(E)) and after (effective Servo VT(E)) compensating for ventilator circuit compliance.

MEASUREMENTS AND MAIN RESULTS

The in vitro accuracy of the CO2SMO Plus was within +/-5% over a wide range of volumes and measurement conditions. The displayed Servo 300 VT(E) overestimated the true VT(E) by between 2% and 91%. The magnitude of error varied within and between children, according to pressure change (peak inspiratory pressure minus positive end-expiratory pressure), VT(E), and circuit size. Mean (sd) error was 32% (20%) in 40 children with displayed Servo VT(E) of <160 mL and 18% (6%) in 16 subjects with displayed Servo VT(E) of >/=160 mL. After correcting for gas compression, effective VT(E) from the Servo 300 underestimated the true VT(E) by up to 64% in the smallest infants but continued to overestimate by as much as 29% in older children.

CONCLUSIONS

The accuracy of tidal volume values is crucially dependent on the site of measurement. Unless measured at the airway opening, displayed values are an inconsistent and misleading indicator of the true volumes delivered.

Are tidal volume measurements in neonatal pressure-controlled ventilation accurate?

Bedside pulmonary mechanics monitors (PMM) have become useful in ventilatory management in neonates. These monitors are used more frequently due to recent improvements in data-processing capabilities. PMM devices are often part of the ventilator or are separate units. The accuracy and reliability of these systems have not been carefully evaluated. We compared a single ventilatory parameter, tidal volume (V(t)), as measured by several systems. We looked at two freestanding PMMs: the Ventrak Respiratory Monitoring System (Novametrix, Wallingford, CT) and the Bicore CP-100 Neonatal Pulmonary Monitor (Allied Health Care Products, Riverside, CA), and three ventilators with built-in PMM: the VIP Bird Ventilator (Bird Products Corp., Palm Springs, CA), Siemens Servo 300A (Siemens-Elema AB, Solna, Sweden), and Drager Babylog 8000 (Drager, Inc., Chantilly, VA). A calibrated syringe (Hans Rudolph, Inc., Kansas City, MO) was used to deliver tidal volumes of 4, 10, and 20 mL to each ventilator system coupled with a freestanding PMM. After achieving steady state, six consecutive V(t) readings were taken simultaneously from the freestanding PMM and each ventilator. In a second portion of the bench study, we used pressure-control ventilation and measured exhaled tidal volume (V(te)) while ventilating a Bear Test Lung with the same three ventilators. We adjusted peak inspiratory pressure (PIP) under controlled conditions to achieve the three different targeted tidal volumes on the paired freestanding PMM. Again, six V(te) measurements were recorded for each tidal volume. Means and standard deviations were calculated.The percentage difference in measurement of V(t) delivered by calibrated syringe varied greatly, with the greatest discrepancy seen in the smallest tidal volumes, by up to 28%. In pressure control mode, V(te) as measured by the Siemens was significantly overestimated by 20-95%, with the biggest discrepancy at the smallest V(te), particularly when paired with the Bicore PMM. V(te), as measured by the VIP Bird and Drager paired with the Ventrak PMM, had a tendency to underestimate V(t) by up to 25% at the smallest V(te). However, when paired with the Bicore PMM, these same two ventilators read over target by up to 18%. Under controlled laboratory conditions, we demonstrated that true delivered V(te), as measured by the three ventilators and two freestanding PMM, differed markedly. In general, decreasing dynamic compliance of the tubing was not associated with greater inaccuracy in V(te) measurements.

Effect of delayed sternal closure after cardiac surgery on respiratory function in ventilated infants

OBJECTIVE

Studies examining the effect of sternal closure on respiratory function have not been published, and currently there is little evidence to guide ventilation management immediately after closure. The aim of this study was to establish the impact of delayed sternal closure on expired tidal volume, respiratory system compliance, and CO2 elimination immediately after the procedure in infants who had undergone open heart surgery.

DESIGN

Prospective study of respiratory function before and after delayed sternal closure.

SETTING

Cardiac intensive care unit, Great Ormond Street Hospital, London.

PATIENTS

Seventeen infants (median age, 2 wks) with open median sternotomy incisions after cardiac surgery. Data were collected between August 1998 and March 2000.

INTERVENTIONS

Respiratory function was measured continuously for 30 mins before and after delayed sternal closure in paralyzed ventilated infants.

MEASUREMENTS AND RESULTS

Four babies were excluded from the study because they required either immediate increase in ventilation after delayed sternal closure (n = 3) or removal of pericardial blood collection (n = 1). In the remaining 13 infants, expired tidal volume and CO2 elimination decreased significantly (p < .005) by a mean of 17% and 29%, respectively, after sternal closure. In five of the remaining 13 patients, the magnitude of tracheal tube leak increased by > or = 10% after delayed sternal closure, thereby invalidating recorded changes in respiratory system compliance. Of the eight infants in whom there was a minimal change in leak, respiratory system compliance decreased significantly (p < .05) by a mean of 19%.

CONCLUSIONS

This study supports the hypothesis that respiratory function may be compromised after delayed sternal closure and that ventilatory support should be increased to counteract the anticipated decrease in tidal volume. Extra vigilance should be applied in monitoring blood gases after delayed sternal closure to assess clinical responses to sternal closure or changes in ventilatory support. Accurate assessment of change in respiratory system compliance after any therapeutic intervention may be precluded by changes in tracheal tube leak during the procedure.

Acute lung injury: pathophysiology, assessment and current therapy

Acute respiratory distress syndrome (ARDS) is a clinically defined entity describing the severity of diffuse alveolar injury caused by direct or indirect injury to the lung. Pathophysiology, clinical course and outcome of ARDS depend on the underlying cause, the severity of the disease and co-morbidities. Pulmonary function tests show restrictive lung disease, which is characterised by a reduction in lung compliance and functional residual capacity, resulting in marked ventilation-perfusion inequality. Current ventilator strategies aim to minimise ventilator-induced lung injury by targeting mechanical ventilation between the lower and upper inflection point of the pressure volume curve. This includes recruitment manoeuvres and the use of high PEEP to open the atelectatic lung and the use of permissive hypercapnia and the limitation of peak inspiratory pressure below 35 cm H2O to avoid overinflation. The clinical benefit of newer modes of ventilatory support such as inverse ratio ventilation, high frequency oscillatory ventilation, surfactant replacement, prone positioning and inhaled nitric oxide has yet to be determined in children.

Single-breath CO2 analysis as a predictor of lung volume in a healthy animal model during controlled ventilation

OBJECTIVE

To examine the utility of single-breath CO2 analysis as a measure of lung volume.

DESIGN

A prospective, animal cohort study comparing 21 parameters derived from single-breath CO2 analysis with lung volume measurements determined by nitrogen washout in animals during controlled ventilation.

SETTING

An animal laboratory in a university-affiliated medical center.

SUBJECTS

Seven healthy lambs.

INTERVENTIONS

The single-breath CO2 analysis station consists of a mainstream capnometer, a variable orifice pneumotachometer, a signal processor and computer software with capability for both on- and off-line data analysis. Twenty-one derived components of the CO2 expirogram were evaluated as predictors of lung volume. Lung volume was manipulated by 3 cm H2O incremental increases in positive end-expiratory pressure from 0 to 21 cm H2O, and ranged between 147 and 942 mL.

MEASUREMENTS AND MAIN RESULTS

Fifty-five measurements of lung volume were available for comparison with derived variables from the CO2 expirogam. Stepwise linear regression identified four variables that were most predictive of lung volume: a) dynamic lung compliance; b) the slope of phase 3; c) the slope of phase 2 divided by the mixed expired CO2 tension; and d) airway deadspace. The multivariate equation was highly statistically significant and explained 94% of the variance (adjusted r2 =.94, p < .0001). The bias and precision of the calculated lung volume was .00 and 51, respectively. The mean percent difference for the lung volume estimate derived from the single-breath CO2 analysis station was 0.79%.

CONCLUSIONS

Our data indicate that analysis of the CO2 expirogram can yield accurate information about lung volume. Specifically, four variables derived from a plot of expired CO2 concentration vs. expired volume predict changes in lung volume in healthy lambs with an adjusted coefficient of determination of .94. Prospective application of this technology in the setting of lung injury and rapidly changing physiology is essential in determining the clinical usefulness of the technique.