CROSSOVER FROM AUTOMATED TO MANUAL TITRATION OF FiO2 IN THE NICU: IS THERE A TRANSITION LAG?

Effective management of oxygenation of preterm infants in critical care profoundly impacts their outcome. Nurses are challenged to titrate the inspired oxygen in response to constant cardiopulmonary instability. Closed loop control of inspired oxygen based on continuous monitoring of oxygen saturation is just becoming available. Evaluating the relative effectiveness of closed loop control systems is complicated by the wide variability in manual control by nurses. This analysis explored the possibility of a lag in effective control associated with the transition from closed loop to manual control using data from a clinical crossover trial. A short but marked lag phase was detected. It is, however, unlikely to have impact on clinical care or crossover studies. Its presence highlights the anticipative nature of closed loop control as contrasted to the observative nature of manual control

PREVALENCE OF POTENTIALLY CLINICALLY RELEVANT COMPLEX EPISODES OF EXTREME SpO2 DURING MANUAL AND AUTOMATIC CONTROL OF INSPIRED OXYGEN

Continuous monitoring with pulse oximetry is the standard of care for titrating inspired oxygen in the neonatal ICU. However titrating supplemental oxygen to address frequent desaturations is a challenging task for caregivers. Increasing exposure to SpO2 extremes is associated with increasingly poorer long-term outcomes. More recently the prevalence of prolonged episodes at extremes and cluster of short episodes have been reported to be also associated with bad outcomes. We speculated that more complex episodes might also have an impact on outcomes. We defined two sets of these: clusters and swings.  Automatic control of inspired oxygen based on continuous pulse oximetry, is available on many neonatal ventilators. Some have expressed concern that continuous adjustment of inspired oxygen, without observing the infant, might cause instability and thus increased prevalence of clusters and oscillations. The aim of this study was to determine the prevalence of these complex events and determine if they were more common during automated control. To accomplish this we analyzed data of 58 extremely preterm newborns that were ventilated at least 24 hours with manual inspiratory oxygen control and at least 24 hours with automated FiO2 control, in random order. We found that clusters and swings were quite prevalent, that is similar to the prevalence of prolonged episodes that have been shown to be associated with bad outcomes. We also found that these complex events were reduced during automated control, rather than increased. Finally, we suggest that additional research in this area is warranted.

EFFECT OF SAMPLING RATE ON THE ACCURACY OF MEASUREMENT OF NEONATAL OXYGEN SATURATION EXPOSURE

Patient data management systems are becoming commonplace in the ICU. While their intent is to automate patient charting, they provide a readily accessible, large database that would be potentially useful for clinical research and quality improvement projects. We sought to determine if patient data management information could be of value describing neonatal oxygenation saturation exposure (percent time in oxemic-ranges). Our primary measure was the accuracy of 60-second sampling over a 24-hour period, as compared to previously reported results in 23 infants. We found this to be highly accurate. We also conducted a sensitivity analysis using 3 other sampling rates (20, 30, 120 seconds) and 3 other time epochs (4, 12 and 48 hours). We found that sampling frequency and time epoch length impacted accuracy. Nevertheless these combinations could all be useful, if limitations are taken into account in the analysis design.

FREQUENCY AND DURATION OF OXIMETER DROP-OUTS IN THE NICU: AN OBSERVATIONAL STUDY

Oximeters used for continuous monitoring experience periods with no signal. This SpO2drop-out is widely acknowledged and its causes generally understood. This is a prospectively designed analysis of an existing database with the aim of characterizing drop-outs as experienced in the neonatal ICU. The data reflects 116 days of monitoring in seven tertiary care neonatal ICUs in 6 countries.  From the evaluation of 1,396 drop-outs we found that typically the time was minimal with missing SpO2 data, and the episodes were short (median 2.79 minutes per day IQR 0.17-76, median 22 seconds IQR: 15-37, respectively). During during about half of the days there were no prolonged dropouts (1 minute or longer), even so half of the total time spent with no SpO2data were in prolonged episodes (median length 110 seconds IQR 85-150). The predominate factor associated with excessive drop-out time was the number of prolonged episodes rather than their duration. We concluded that the impact of drop-outs during manual control of inspired oxygen primarily impact alarm fatigue, but that during automatic FiO2control they could have an important impact. The relative effectiveness of the fall-back strategies of these automatic control systems ought to be evaluated.

Unloading work of breathing during high-frequency oscillatory ventilation: a bench study

INTRODUCTION: With the 3100B high-frequency oscillatory ventilator (SensorMedics, Yorba Linda, CA, USA), patients' spontaneous breathing efforts result in a high level of imposed work of breathing (WOB). Therefore, spontaneous breathing often has to be suppressed during high-frequency oscillatory ventilation (HFOV). A demand-flow system was designed to reduce imposed WOB. METHODS: An external gas flow controller (demand-flow system) accommodates the ventilator fresh gas flow during spontaneous breathing simulation. A control algorithm detects breathing effort and regulates the demand-flow valve. The effectiveness of this system has been evaluated in a bench test. The Campbell diagram and pressure time product (PTP) are used to quantify the imposed workload. RESULTS: Using the demand-flow system, imposed WOB is considerably reduced. The demand-flow system reduces inspiratory imposed WOB by 30% to 56% and inspiratory imposed PTP by 38% to 59% compared to continuous fresh gas flow. Expiratory imposed WOB was decreased as well by 12% to 49%. In simulations of shallow to normal breathing for an adult, imposed WOB is 0.5 J l(-1 )at maximum. Fluctuations in mean airway pressure on account of spontaneous breathing are markedly reduced. CONCLUSION: The use of the demand-flow system during HFOV results in a reduction of both imposed WOB and fluctuation in mean airway pressure. The level of imposed WOB was reduced to the physiological range of WOB. Potentially, this makes maintenance of spontaneous breathing during HFOV possible and easier in a clinical setting. Early initiation of HFOV seems more possible with this system and the possibility of weaning of patients directly on a high-frequency oscillatory ventilator is not excluded either

Imposed work of breathing during high-frequency oscillatory ventilation: a bench study

INTRODUCTION: The ventilator and the endotracheal tube impose additional workload in mechanically ventilated patients breathing spontaneously. The total work of breathing (WOB) includes elastic and resistive work. In a bench test we assessed the imposed WOB using 3100 A/3100 B SensorMedics high-frequency oscillatory ventilators. METHODS: A computer-controlled piston-driven test lung was used to simulate a spontaneously breathing patient. The test lung was connected to a high-frequency oscillatory ventilation (HFOV) ventilator by an endotracheal tube. The inspiratory and expiratory airway flows and pressures at various places were sampled. The spontaneous breath rate and volume, tube size and ventilator settings were simulated as representative of the newborn to adult range. The fresh gas flow rate was set at a low and a high level. The imposed WOB was calculated using the Campbell diagram. RESULTS: In the simulations for newborns (assumed body weight 3.5 kg) and infants (assumed body weight 10 kg) the imposed WOB (mean ± standard deviation) was 0.22 ± 0.07 and 0.87 ± 0.25 J/l, respectively. Comparison of the imposed WOB in low and high fresh gas flow rate measurements yielded values of 1.63 ± 0.32 and 0.96 ± 0.24 J/l (P = 0.01) in small children (assumed body weight 25 kg), of 1.81 ± 0.30 and 1.10 ± 0.27 J/l (P < 0.001) in large children (assumed body weight 40 kg), and of 1.95 ± 0.31 and 1.12 ± 0.34 J/l (P < 0.01) in adults (assumed body weight 70 kg). High peak inspiratory flow and low fresh gas flow rate significantly increased the imposed WOB. Mean airway pressure in the breathing circuit decreased dramatically during spontaneous breathing, most markedly at the low fresh gas flow rate. This led to ventilator shut-off when the inspiratory flow exceeded the fresh gas flow. CONCLUSION: Spontaneous breathing during HFOV resulted in considerable imposed WOB in pediatric and adult simulations, explaining the discomfort seen in those patients breathing spontaneously during HFOV. The level of imposed WOB was lower in the newborn and infant simulations, explaining why these patients tolerate spontaneous breathing during HFOV well. A high fresh gas flow rate reduced the imposed WOB. These findings suggest the need for a demand flow system based on patient need allowing spontaneous breathing during HFOV

Pulse Oximeter Performance during Rapid Desaturation

The reliability of pulse oximetry is crucial, especially in cases of rapid changes in body oxygenation. In order to evaluate the performance of pulse oximeters during rapidly developing short periods of concurrent hypoxemia and hypercapnia, 13 healthy volunteers underwent 3 breathing phases during outdoor experiments (39 phases in total), monitored simultaneously by five different pulse oximeters. A significant incongruity in values displayed by the tested pulse oximeters was observed, even when the accuracy declared by the manufacturers were considered. In 28.2% of breathing phases, the five used devices did not show any congruent values. The longest uninterrupted congruent period formed 74.4% of total recorded time. Moreover, the congruent periods were rarely observed during the critical desaturation phase of the experiment. The time difference between the moments when the first and the last pulse oximeter showed the typical study endpoint values of SpO2 85% and 75% was 32.1 &plusmn; 23.6 s and 24.7 &plusmn; 19.3 s, respectively. These results suggest that SpO2 might not be a reliable parameter as a study endpoint, or more importantly as a safety limit in outdoor experiments. In the design of future studies, more parameters and continuous clinical assessment should be included

Perlite Has Similar Diffusion Properties for Oxygen and Carbon Dioxide to Snow: Implications for Avalanche Safety Equipment Testing and Breathing Studies

On average, one hundred people die each year under avalanche snow. Despite extensive global research on gas exchange in buried avalanche victims, it remains unclear how the diffusion of respiratory gases affects survival under avalanche snow. This study aims to determine how oxygen and carbon dioxide diffuse through snow, as well as through wet and dry perlite, which may serve as a surrogate for avalanche snow. A custom-made apparatus to study the diffusion of respiratory gases consisted of a plastic cylinder (1200 mm long, ID 300 mm) with 13 gas sampling needles evenly spaced along the axis of the cylinder filled with the tested material. Following 60 min of free diffusion, gas samples were analyzed using a vital signs monitor with a module for respiratory gas analysis (E-CAiOVX, Datex-Ohmeda, GE Healthcare, Chicago, IL, USA). A combination of 16% oxygen, 5% carbon dioxide, and 79% nitrogen was used. The rates of diffusion for both respiratory gases were comparable in snow and both forms of perlite. Oxygen propagated faster than carbon dioxide. Due to similar diffusion characteristics to snow, perlite possesses the potential to stand in as an effective substitute for soft snow for the study of respiratory dynamics, for conducting breathing experiments, and for testing avalanche safety equipment