Shirley J. Holt

Reliability of displayed tidal volume in infants and children during dual-controlled ventilation

Objective: Previous studies have shown a significant difference between ventilator-measured tidal volume and actual-delivered tidal volume. However, these studies used external methods for measurement of compression volume. Our objective was to determine whether tidal volume could be accurately measured at the expiratory valve of a conventional ventilator using internal computer software to compensate for circuit compliance with a dual control mode of ventilation. Design: Clinical study during an 8-month period. Setting: Pediatric intensive care unit. Patients: All patients admitted to the pediatric intensive care unit during the enrollment period who were mechanically ventilated using the Servo I (Maquet, Bridgewater, NJ) were eligible for this study. Interventions: Patients were ventilated using a dual-control mode of ventilatory support and either an infant or adult circuit (with and without circuit compensation). Measurements and Main Results: Tidal volume measured at the endotracheal tube using a pneumotachometer was compared with ventilator-displayed tidal volume. Sixty-eight patients were studied between September 2004 and April 2005. Age range was 2 days to 18 yrs (median, 23 mos) and weight range was 2.3 kg to 103 kg (median, 14.5 kg) with 41 male patients (60%). We found ventilator-displayed tidal volume, without circuit compensation, generally overestimates true-delivered tidal volume and, with circuit compensation, generally underestimates true-delivered tidal volume. However, agreement between tidal volume measured at the patients airway and that measured with and without compensation for circuit compliance was good. The error in both cases, without and with circuit compensation, is relatively greater in infants and small children. Conclusions: There is an underestimation of delivered tidal volume when compensating for circuit volume loss measured at the ventilator. There is no improvement in measured tidal volume using circuit compensation in small infants and children.

Reliability of measured tidal volume in mechanically ventilated young pigs with normal lungs

ObjectiveThis study examined whether volumes can be accurately measured at the expiratory valve of a conventional ventilator using pressure support ventilation and positive end expiratory pressure with software compensation for circuit compliance available in the Servo ί ventilator.Design and settingComparison of two methods for measuring tidal volume in an animal laboratory.SubjectsTwenty healthy, intubated, sedated, spontaneously breathing pigs.InterventionsVolume was measured in ten neonatal-sized and ten pediatric-sized pigs ventilated with the Servo ί ventilator using pressure support ventilation and positive end expiratory pressure with and without circuit compliance compensation. We compared volume measured at the airway opening by pneumotachography to volume measured at the expiratory valve of a conventional ventilator.Measurements and resultsThe use of circuit compliance compensation significantly improved the agreement between the two volume methods in neonatal-sized piglets (concordance correlation coefficient: with circuit compliance compensation, 0.97; without, 0.87, p=0.002). In pediatric-sized pigs there was improvement in agreement between the two measurement methods due to circuit compliance compensation (concordance correlation coefficient with circuit compliance compensation, 0.97; without, 0.88, p=0.027). With circuit compliance compensation off there was positive bias: mean difference (bias) 2.97±0.12 in neonatal-sized and 3.75±0.38 in pediatric-sized pigs.ConclusionsOur results show that volume can be accurately measured at the expiratory valve of a conventional ventilator in neonatal- and pediatric-sized animals.

Neurally triggered breaths have reduced response time, work of breathing, and asynchrony compared with pneumatically triggered breaths in a recovering animal model of lung injury

Objective: Our objective was to compare response time, pressure time product as a reflection of work of breathing, and incidence and type of asynchrony in neurally vs. pneumatically triggered breaths in a spontaneously breathing animal model with resolving lung injury. Design: Prospective animal study. Setting: Experimental laboratory. Subjects: Male Yorkshire pigs. Interventions: Intubated, sedated pigs were ventilated using neurally adjusted ventilatory assist and pressure support ventilation with healthy and sick/recruited lungs. After injury, the lung was recruited using a computer-driven protocol. Respiratory mechanics were determined using a forced oscillation technique, and airway flow and pressure waveforms were acquired using a pneumotachograph. Measurements and Main Results: Waveforms were analyzed for trigger delay, pressure time product, and asynchrony. Trigger delay was defined as the time interval (ms) from initiation of a breath to the beginning of ventilator pressurization. Pressure time product was measured as the area of the pressure curve for animal effort (area A) and ventilator response (area B). Asynchrony was classified according to triggering problems, adequacy of flow delivery, and adequate breath termination. Mean values were compared using the Wilcoxon signed-ranks test (p < .05). Trigger delay (ms) was less in neurally triggered breaths (pressure support ventilation healthy 104 ± 27 vs. neurally adjusted ventilatory assist healthy 72 ± 30, pressure support ventilation sick/recruited 77 ± 18 vs. neurally adjusted ventilatory assist sick/recruited 38 ± 18, p < .01). Pressure time product areas A and B were decreased for neurally triggered breaths compared with pressure support ventilation in both healthy and recruited animals (p ⩽ .02). Overall, the percentage of asynchrony was less for neurally adjusted ventilatory assist breaths in the recruited animals (pressure support ventilation 27% and neurally adjusted ventilatory assist 6%). Conclusions: Neurally triggered breaths have reduced asynchrony, trigger delay, and pressure time product, which may indicate reduced work of breathing associated with less effort to trigger the ventilator and faster response to effort. Further study is required to demonstrate if these differences will lead to decreased days of ventilation and less use of sedation in patients.

Validation of a noninvasive blood pressure monitoring device in normotensive and hypertensive pediatric intensive care patients

Objective. To evaluate the performance and to define limitations of a noninvasive blood pressure monitoring device in the critically ill pediatric population. Method. Patients were included in the study if they were admitted to the Pediatric Intensive Care Unit, were between the ages of 1 month and 18 years with wrist circumferences of ≥ 10 cm, and had an indwelling arterial line. Patients were excluded if their systolic blood pressure differed by ≥ 7.5% between their upper extremities. The measurements were collected simultaneously with those from an arterial line by a computer interfaced with the noninvasive blood pressure monitoring system and the patient’s monitor. Heart rates were calculated from the recorded pulse waveforms of the arterial lines. Comparison analyses were performed via bias and precision plots of the blood pressure and heart rate data in addition to calculation of Pearson’s correlation coefficients and concordance correlation coefficients. As a nonparametric method of comparison, the proportion of measurements that differed by greater than 10% was calculated. Results. Blood pressures and heart rates of 20 patients between the ages of 12 months and 17 years were monitored by a noninvasive blood pressure monitor for 30 min per patient. This data collection resulted in 2015 data points for each blood pressure and heart rate for comparison of methods. Concordance correlation coefficients were the following: systolic blood pressure, 0.93; diastolic blood pressure, 0.93; mean blood pressure, 0.94; and heart rate, 0.85. Conclusions. The noninvasive blood pressure monitor is capable of producing an accurate blood pressure measurement every 12–15 heartbeats in addition to providing a pulse waveform and digital display of the heart rate. Our study showed good agreement between the methods in the normotensive and hypertensive critically ill pediatric population with a wrist circumference limitation defined at ≥ 11 cm.

Comparison of work of breathing between two neonatal ventilators utilizing a neonatal pig model.

Objective The purpose of this study was to determine whether variations in the delivery systems of continuous positive airway pressure between two ventilators would lead to differences in patient work of breathing (WOBp). Design Comparison of two neonatal ventilators with a neonatal pig model. Setting Animal laboratory. Subjects Thirty healthy, intubated, sedated, spontaneously breathing neonatal piglets weighing 1.0–2.0 kg. Interventions Patient work of breathing (WOBp) (gm cm/kg) was measured by using measurements based on an esophageal balloon and a flow transducer. Each breath was analyzed for ventilator response times (in msecs) and negative deflection of pressure. Each animal was studied with the Siemens SV300 and Drager Babylog 8000, on continuous positive airway pressure settings of 0, 3, and 5 cm H2O. Data were analyzed by using Wilcoxon’s Signed Rank Test with significance of p ≤ .05. Measurements and Main Results Comparing ventilators, WOBp was on average 29% higher in the Babylog. Analysis of individual breaths showed that disparity in WOBp was probably related to the automatic availability of 2 cm H2O pressure support ventilation in the SV300. This may also explain the differences in delay time during the start of the inspiratory phase before initiation of gas flow. The mean duration of inspiratory effort was 394 (± 157) msecs in the Babylog and 138 (± 35) msecs in the SV300, a 174% increase in time delay for the Babylog (p = .005). During inspiratory effort, there was >1 cm H2O negative pressure before flow was available with the Babylog. Conclusions In intubated patients, maximum energy expenditure occurs at the initiation of ventilator breaths. WOBp in neonatal pigs was significantly increased. The response time of the ventilators may explain the differences in initiation of flow times and patient work. These differences may have important implications for energy kinetics, weight gain, and duration of mechanical ventilation in preterm neonates.

Comparison of Work of Breathing between Two Neonatal Ventilators utilizing a Neonatal Pig Model † 1667

OBJECTIVE The purpose of this study was to determine whether variations in the delivery systems of continuous positive airway pressure between two ventilators would lead to differences in patient work of breathing (WOBp). DESIGN Comparison of two neonatal ventilators with a neonatal pig model. SETTING Animal laboratory. SUBJECTS Thirty healthy, intubated, sedated, spontaneously breathing neonatal piglets weighing 1.0-2.0 kg. INTERVENTIONS Patient work of breathing (WOBp) (gm cm/kg) was measured by using measurements based on an esophageal balloon and a flow transducer. Each breath was analyzed for ventilator response times (in msecs) and negative deflection of pressure. Each animal was studied with the Siemens SV300 and Drager Babylog 8000, on continuous positive airway pressure settings of 0, 3, and 5 cm H2O. Data were analyzed by using Wilcoxons Signed Rank Test with significance of p <or=.05. MEASUREMENTS AND MAIN RESULTS Comparing ventilators, WOBp was on average 29% higher in the Babylog. Analysis of individual breaths showed that disparity in WOBp was probably related to the automatic availability of 2 cm H2O pressure support ventilation in the SV300. This may also explain the differences in delay time during the start of the inspiratory phase before initiation of gas flow. The mean duration of inspiratory effort was 394 (+/- 157) msecs in the Babylog and 138 (+/- 35) msecs in the SV300, a 174% increase in time delay for the Babylog (p =.005). During inspiratory effort, there was >1 cm H2O negative pressure before flow was available with the Babylog. CONCLUSIONS In intubated patients, maximum energy expenditure occurs at the initiation of ventilator breaths. WOBp in neonatal pigs was significantly increased. The response time of the ventilators may explain the differences in initiation of flow times and patient work. These differences may have important implications for energy kinetics, weight gain, and duration of mechanical ventilation in preterm neonates.

Validation of the accuracy of a transport ventilator utilizing a pediatric animal model.

Objective The objective of this study was to evaluate 2 transport ventilators utilizing both a test lung and a pediatric animal model. Methods Two transport ventilators were utilized for evaluations. A test lung or intubated, sedated pigs (n = 9) with healthy and injured lungs were ventilated using control and support modes. A test lung was used to evaluate alarm responsiveness, FIO2 accuracy, oxygen consumption, and duration of battery power. Pigs were utilized to evaluate the exhalation valve, ventilator response, volume accuracy, and noninvasive functionality. Respiratory mechanics were determined using a forced oscillation technique, and airway flow and pressure waveforms were acquired utilizing a pneumotachograph. Results For both ventilators, FIO2 accuracy was within 10% error. On an E cylinder of oxygen, the EMV+ operated for 3 hours 48 minutes and the LTV 1200 for 1 hour 4 minutes. On battery power, the LTV 1200 ventilated for 6 hours 51 minutes and the EMV+ for 12 hours 8 minutes. Ventilator response time was less (36%), and delta pressure was greater (38%) for the EMV+ utilizing noninvasive ventilation. The percent error for displayed volume was less than 10% for the EMV+. Conclusions In this study, we demonstrate that there are differences between the 2 ventilators in regard to oxygen consumption, duration of battery power, and volume accuracy. Clinicians should be aware of these differences to optimize the choice and use of both ventilators depending on clinical need/setting.

REPEATED MEASUREMENTS OF RESPIRATORY MECHANICS IN DEVELOPING RATS UTILIZING A FORCED OSCILLATION TECHNIQUE

The ability to successfully intubate the trachea of rats repeatedly over time, recover them, and perform repeated measures of changes in respiratory mechanics is important. The authors performed experiments utilizing 2 groups of rats at various ages in their development. Rats in the single-measurement group were studied at 1 age only. Rats in the repeated-measurement group were studied at each age point over time. Measurements of respiratory mechanics were made utilizing a forced-oscillation technique. We found no differences in respiratory mechanics between the 2 groups. Our results demonstrate that developing rats can be studied longitudinally to illustrate maturational changes in respiratory mechanics.