End-tidal and arterial carbon dioxide measurements correlate across all levels of physiologic dead space.

BACKGROUND: End-tidal carbon dioxide (P(ETCO(2))) is a surrogate, noninvasive measurement of arterial carbon dioxide (P(aCO(2))), but the clinical applicability of P(ETCO(2)) in the intensive care unit remains unclear. Available research on the relationship between P(ETCO(2)) and P(aCO(2)) has not taken a detailed assessment of physiologic dead space into consideration. We hypothesized that P(ETCO(2)) would reliably predict P(aCO(2)) across all levels of physiologic dead space, provided that the expected P(ETCO(2))-P(aCO(2)) difference is considered. METHODS: Fifty-six mechanically ventilated pediatric patients (0-17 y old, mean weight 19.5 +/- 24.5 kg) were monitored with volumetric capnography. For every arterial blood gas measurement during routine care, we measured P(ETCO(2)) and calculated the ratio of dead space to tidal volume (V(D)/V(T)). We assessed the P(ETCO(2))-P(aCO(2)) relationship with Pearson's correlation coefficient, in 4 V(D)/V(T) ranges. RESULTS: V(D)/V(T) was 0.7 for 54 measurements (11%). The correlation coefficients between P(ETCO(2)) and P(aCO(2)) were 0.95 (mean difference 0.3 +/- 2.1 mm Hg) for V(D)/V(T) 0.7. CONCLUSIONS: There were strong correlations between P(ETCO(2)) and P(aCO(2)) in all the V(D)/V(T) ranges. The P(ETCO(2))-P(aCO(2)) difference increased predictably with increasing V(D)/V(T).

The endotracheal tube air leak test does not predict extubation outcome in critically ill pediatric patients.

OBJECTIVE: Endotracheal tube air leak pressures are used to predict postextubation upper airway compromise such as stridor, upper airway obstruction, or risk of reintubation. To determine whether the absence of an endotracheal tube air leak (air leak test >/=30 cm H2O) measured during the course of mechanical ventilation predicts extubation failure in infants and children. DESIGN: Prospective, blinded cohort. SETTING: Multidisciplinary pediatric intensive care unit of a university hospital. PATIENTS: Patients younger than or equal to 18 yrs and intubated >/=24 hrs. INTERVENTIONS: The pressure required to produce an audible endotracheal tube air leak was measured within 12 hrs of intubation and extubation. Unless prescribed by the medical care team, patients did not receive neuromuscular blocking agents during air leak test measurements. MEASUREMENTS AND MAIN RESULTS: The need for reintubation (i.e., extubation failure) was recorded during the 24-hr postextubation period. Seventy-four patients were enrolled resulting in 59 observed extubation trials. The extubation failure rate was 15.3% (9 of 59). Seven patients were treated for postextubation stridor. Extubation failure was associated with a longer median length of ventilation, 177 vs. 78 hrs, p = 0.03. Extubation success was associated with the use of postextubation noninvasive ventilation (p = 0.04). The air leak was absent for the duration of mechanical ventilation (i.e., >/=30 cm H2O at intubation and extubation) in ten patients. Absence of the air leak did not predict extubation failure (negative predictive value 27%, 95% confidence interval 6-60). The air leak test was >/=30 cm H2O before extubation in 47% (28 of 59) of patients yet 23 patients extubated successfully (negative predictive value 18%). CONCLUSIONS: An endotracheal tube air leak pressure >/=30 cm H2O measured in the nonparalyzed patient before extubation or for the duration of mechanical ventilation was common and did not predict an increased risk for extubation failure. Pediatric patients who are clinically identified as candidates for an extubation trial but do not have an endotracheal tube air leak may successfully tolerate removal of the endotracheal tube.

Setting positive end-expiratory pressure during jet ventilation to replicate the mean airway pressure of oscillatory ventilation.

BACKGROUND: High-frequency ventilation can be delivered with either oscillatory ventilation (HFOV) or jet ventilation (HFJV). Traditional clinician biases may limit the range of function of these important ventilation modes. We hypothesized that (1) the jet ventilator can be an accurate monitor of mean airway pressure (P (aw)) during HFOV, and (2) a mathematical relationship can be used to determine the positive end-expiratory pressure (PEEP) setting required for HFJV to reproduce the P (aw) of HFOV. METHODS: In phase 1 of our experiment, we used a differential pressure pneumotachometer and a jet adapter in-line between an oscillator circuit and a pediatric lung model to measure P (aw), PEEP, and peak inspiratory pressure (PIP). Thirty-six HFOV setting combinations were studied, in random order. We analyzed the correlation between the pneumotachometer and HFJV measurements. In phase 2 we used the jet as the monitoring device during each of the same 36 combinations of HFOV settings, and recorded P (aw), PIP, and DeltaP. Then, for each combination of settings, the jet ventilator was placed in-line with a conventional ventilator and was set at the same rate and PIP as was monitored during HFOV. To determine the appropriate PEEP setting, we calculated the P (aw) contributed by the PIP, respiratory rate, and inspiratory time set for HFJV, and subtracted this from the goal P (aw). This value was the PEEP predicted for HFJV to match the HFOV P (aw). RESULTS: The correlation coefficient between the pneumotachometer and HFJV measurements was r = 0.99 (mean difference 0.62 +/- 0.30 cm H(2)O, p < 0.001). The predicted and actual PEEP required were highly correlated (r = 0.99, p < 0.001). The mean difference in these values is not statistically significantly different from zero (mean difference 0.25 +/- 1.02 cm H(2)O, p > 0.15). CONCLUSIONS: HFJV is an accurate monitor during HFOV. These measurements can be used to calculate the predicted PEEP necessary to match P (aw) on the 2 ventilators. Replicating the P (aw) with adequate PEEP on HFJV may help simplify transitioning between ventilators when clinically indicated.

Is permissive hypoxemia a beneficial strategy for pediatric acute lung injury?

The adverse effects of high oxygen levels have been widely reported, and clinicians have struggled for many years to find the ideal balance between inspired oxygen levels and acceptable arterial oxygen saturation. However, when asked "what is an acceptable oxygen saturation," one is hard pressed to find a definitive answer. Permissive hypoxemia is a concept similar to the well-described strategy of permissive hypercapnia. It is a strategy that allows the arterial oxygen saturation to be less than normal in an attempt to minimize the amount of artificial support provided to the lungs by mechanical ventilation. It must be noted that this concept is predominantly based on physiology, as data in the medical literature are very limited. Permissive hypoxemia as an approach to acute lung injury remains controversial in the clinical setting.

The role of noninvasive ventilation for acute respiratory failure.

The use of NIV has been shown to facilitate discontinuing ventilatory dependence as well as provide support for adult patients with chronic lung disease without the need for endotracheal intubation. In fact, NIV has recently described as a potential support strategy following extubation failure. Therefore, using NIV as a bridge to liberation from mechanical ventilation may decrease many of the complications associated with long-term use of invasive airway devices as well complications from reinsertion of an artificial airway. Although firm data supporting the use of NIV in the adult population exists, the use of NIV in the pediatric population is based primarily on a series of case studies, retrospective chart reviews, and extrapolation from the adult data. The use of NIV for infants and children remains controversial. The important question to be asked is why there is a lack of randomized controlled trials on NIV in pediatrics? The answer lies somewhere between the lack of equipment designed specifically for pediatrics and the smaller number of patients available compared with adults. Data from the adult population may be more readily adapted to older children; however, it remains difficult to determine the criteria for noninvasive ventilatory use in infants and young children. In fact, this lack of data makes the formulation of firm selection guidelines for infants and children essentially impossible. However, for a select groups of pediatric patients with acute respiratory failure for whom an appropriate noninvasive device with interface is available, a trial of NIV may be seem reasonable to avoid the known negative effects of intubation and invasive mechanical ventilation.

Do all mechanically ventilated pediatric patients require continuous capnography?

With most patients in modern ICUs requiring mechanical ventilation, any technology that may lead to more optimal ventilatory strategies would be invaluable in the management of critically ill patients. The focus of most ventilator strategies is protecting the lung from the deleterious effects of mechanical ventilation. Every effort is made to minimize the duration of mechanical ventilation while optimizing the potential for successful extubation. A concise organized plan based on objective criteria that is adjusted to meet changes in patient status is clearly recommended. Continuous capnographic monitoring provides clinicians with clear, precise, objective data that may prove beneficial in the design and implementation of mechanical ventilatory strategies. There are no clear-cut methods for achieving the optimal ventilator strategy for a specific patient. Although guidelines and management theories exist throughout the medical literature, in practice, they often merely serve as loose guidelines. The dynamic properties of an acutely ill patient make the management of mechanical ventilation an ongoing process requiring clinical assessment and planning by multidisciplinary members of the patient care team. Comprehensive evaluation of ventilatory management strategies and patient responses must be made by a collaborative effort of physicians, respiratory care practitioners, and nurses. An objective, consistent approach to the overall management is essential. Although still controversial, it is the authors' opinion that volumetric capnograph provides the data necessary to establish adequate gas delivery, optimal PEEP, and effective ventilation with the least amount of mechanical assistance, regardless of clinician or institutional preferences.

Carbon dioxide elimination and gas displacement vary with piston position during high-frequency oscillatory ventilation.

INTRODUCTION: Alterations in gas displacement in pediatric patients ventilated with the SensorMedics 3100A high-frequency oscillator are most commonly manipulated by adjusting the amplitude, frequency, and percent inspiratory time. The piston-position-and-displacement indicator is commonly centered and subsequently not adjusted. That practice may limit the clinician's ability to optimize carbon dioxide elimination. We hypothesized that varying the piston position would alter gas displacement and carbon dioxide elimination. METHODS: We conducted an observational study in a tertiary pediatric intensive care unit and a correlated bench study. In the clinical study, 24 patients were ventilated with a SensorMedics 3100A high-frequency oscillator. Transcutaneously measured carbon dioxide ((tCO(2))) values were documented with the piston-position-and-displacement indicator in left, center, and right positions. In the bench study the oscillator was set and maintained at: mean airway pressure 15 cm H(2)O, inspiratory time 33% of respiratory-cycle time, bias flow 20 L/min. A pneumotachometer attached to a respiratory mechanics monitor was placed between the ventilator circuit and a test lung. Data were collected with the piston-position-and-displacement indicator at the left, center, and right positions with frequencies of 4-14 Hz and amplitudes of 25-55 cm H(2)O. Data were collected over a 3-minute time period for each combination of frequency, amplitude, and piston-position-and-displacement-indicator position. We compared the data with repeated-measures analysis of variance. Pairwise comparisons were performed with a 2-tailed Student's test with Bonferroni correction. RESULTS: Among the 24 patients (tCO(2)) was significantly associated with the position of the piston (p < 0.007). In the bench study, gas displacement was higher when the piston-position-and-displacement indicator was positioned to the left (than when at the center position) 91.7% of the time (p < 0.0001). When the piston-position-and-displacement indicator was positioned to the right (as compared to the center position), gas displacement was lower 75% of the time (p < 0.0001). CONCLUSION: Adjusting the oscillator piston alters the volume of gas displaced and provides an additional means for titrating carbon dioxide elimination. .

Successful treatment of acute chest syndrome with high-frequency oscillatory ventilation in pediatric patients.

Severe acute chest syndrome afflicts patients with sickle cell disease and can cause hypoxemia refractory to conventional treatments. Obstructive mucus plugging and the development of acute respiratory distress syndrome may underlie the pathophysiology of refractory hypoxemia in acute chest syndrome. Although high-frequency oscillatory ventilation (HFOV) is well established in the treatment of pediatric acute respiratory distress syndrome, there is no support in the literature for its role in managing hypoxemia in acute chest syndrome. In disease processes with high airways resistance and obstructive mucus plugging, HFOV may predispose to air-trapping and increased morbidity secondary to air leak syndromes. We report the first successful HFOV management of pediatric patients suffering from severe acute chest syndrome and hypoxic respiratory failure. These cases suggest that HFOV should be strongly considered for patients with severe acute chest syndrome that is refractory to conventional mechanical ventilation.