Tracy L. Thurman

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.

Accuracy of small tidal volume measurement comparing two ventilator airway sensors

Goals of modern mechanical ventilation in infants focus on preventing over-distention by limiting tidal volume. Accurate measurement of these volumes is essential. We hypothesized that tidal volume accuracy differs dependent upon the type of airway sensor utilized in tidal volumes less than 10 mL. Intubated, sedated Sprague Dawley rats (n = 40) were ventilated utilizing both control and support ventilator modes. Accuracy of volume delivery was compared between a fixed orifice flow sensor (FOF) and a hot wire anemometer (HWA) to a Hans Rudolph linear pneumotachograph positioned at the patient wye. Rats median weight was 476 grams (range 370-544), tidal volume (V T ) 3.5 mL (1.2-11.4), f 50 (18-102), and PIP 9.5 cm H 2 O (1-34). Across all modes, bias and precision were HWA -0.76, 1.09; FOF 0.22, 0.61. This study confirms that there are differences in the accuracy of small tidal volumes measured with a FOF as compared to a HWA. Utilizing a FOF, control modes exhibit improved precision and decreased bias as compared to support modes.

Application of mid-frequency ventilation in an animal model of lung injury: a pilot study.

BACKGROUND: Mid-frequency ventilation (MFV) is a mode of pressure control ventilation based on an optimal targeting scheme that maximizes alveolar ventilation and minimizes tidal volume (VT). This study was designed to compare the effects of conventional mechanical ventilation using a lung-protective strategy with MFV in a porcine model of lung injury. Our hypothesis was that MFV can maximize ventilation at higher frequencies without adverse consequences. We compared ventilation and hemodynamic outcomes between conventional ventilation and MFV. METHODS: This was a prospective study of 6 live Yorkshire pigs (10 ± 0.5 kg). The animals were subjected to lung injury induced by saline lavage and injurious conventional mechanical ventilation. Baseline conventional pressure control continuous mandatory ventilation was applied with VT = 6 mL/kg and PEEP determined using a decremental PEEP trial. A manual decision support algorithm was used to implement MFV using the same conventional ventilator. We measured PaCO2, PaO2, end-tidal carbon dioxide, cardiac output, arterial and venous blood oxygen saturation, pulmonary and systemic vascular pressures, and lactic acid. RESULTS: The MFV algorithm produced the same minute ventilation as conventional ventilation but with lower VT (−1 ± 0.7 mL/kg) and higher frequency (32.1 ± 6.8 vs 55.7 ± 15.8 breaths/min, P < .002). There were no differences between conventional ventilation and MFV for mean airway pressures (16.1 ± 1.3 vs 16.4 ± 2 cm H2O, P = .75) even when auto-PEEP was higher (0.6 ± 0.9 vs 2.4 ± 1.1 cm H2O, P = .02). There were no significant differences in any hemodynamic measurements, although heart rate was higher during MFV. CONCLUSIONS: In this pilot study, we demonstrate that MFV allows the use of higher breathing frequencies and lower VT than conventional ventilation to maximize alveolar ventilation. We describe the ventilatory or hemodynamic effects of MFV. We also demonstrate that the application of a decision support algorithm to manage MFV is feasible.

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.

Effect of ventilator mode on patient-ventilator synchrony and work of breathing in neonatal pigs

Patient‐ventilator asynchrony can result in increased work of breathing (WOB) and need for increased sedation, as well as respiratory muscle fatigue and prolonged mechanical ventilation. Different ventilator modes may result in varying degrees of asynchrony and WOB.

Reliability of Displayed Tidal Volume in Healthy and Surfactant-Depleted Piglets

BACKGROUND: Volutrauma has been established as the key factor in ventilator-induced lung injury and can only be avoided if tidal volume (VT) is accurately displayed and delivered. The purpose of this study was to investigate the accuracy of displayed exhaled VT in a ventilator commonly used in small infants with or without a proximal flow sensor and using 3 methods to achieve a target VT in both a healthy and lung-injured neonatal pig model. METHODS: This was a prospective animal study utilizing 8 male pigs, approximately 2.0 kg (range 1.8–2.2 kg). Intubated, sedated, neonatal pigs were studied with both healthy and injured lungs using the Servo-i ventilator. In pressure-regulated volume control, both with and without a proximal flow sensor, we used 3 methods to set VT: (1) circuit compliance compensation (CCC) on, set VT 6–8 mL/kg; (2) CCC off, calculated VT using the manufacturers circuit compliance factor; and (3) CCC off, set VT 10–12 mL/kg to approximate a target VT of 6–8 mL/kg. Ventilator-displayed exhaled VT measurements were compared with exhaled VT measured at the airway opening by a calibrated pneumotachograph. Bland-Altman plots were constructed to show the level of agreement between the two. RESULTS: CCC improved accuracy and precision of displayed exhaled VT when the sensor was not used, more markedly in the lung-injured model. Without CCC, the sensor improved accuracy and precision of displayed exhaled VT, again more markedly in the lung-injured model. CONCLUSIONS: When the Servo-i ventilator is used in neonates, CCC or the in-line sensor should be employed due to the large positive bias and imprecision seen with CCC off and no sensor in-line.

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.