Limitation of measurements of expiratory tidal volume and expiratory compliance under conditions of endotracheal tube leaks

Volume Targeted Ventilation and High Frequency Ventilation as the Primary Modes of Respiratory Support for ELBW Babies: What Does the Evidence Say?

Respiratory management of the extremely low birth weight (ELBW) newborn has evolved over time. Although non-invasive ventilation is being increasingly used for respiratory support in these ELBW infants, invasive ventilation still remains the primary mode in this population. Current ventilators are microprocessor driven and have revolutionized the respiratory support for these neonates synchronizing the baby's breath to ventilator breaths. High frequency ventilators with the delivery of tidal volumes less than the dead space have been introduced to minimize barotrauma and chronic lung disease. Despite these advances, the incidence of chronic lung disease has not decreased. There is still controversy regarding which mode is ideal as the primary mode of ventilation in ELBW infants. The most common modes seem to be pressure targeted conventional ventilation, volume targeted conventional ventilation and high frequency ventilation which includes high frequency oscillatory ventilation, high frequency jet ventilation and high frequency flow interrupter. In recent years, several randomized controlled trials and meta-analyses have compared volume vs. pressure targeted ventilation and high frequency ventilation. While volume targeted ventilation and high frequency ventilation does show promise, substantial practice variability among different centers persists. In this review, we weighed the evidence for each mode and evaluated which modes show promise as the primary support of ventilation in ELBW babies.

Volume-targeted ventilation: one size does not fit all. Evidence-based recommendations for successful use

Despite level 1 evidence for important benefits of volume-targeted ventilation (VTV), many vulnerable extremely preterm infants continue to be exposed to traditional pressure-controlled ventilation. Lack of suitable equipment and a lack of appreciation of the fact that 'one size does NOT fit all' appear to contribute to the slow uptake of VTV. This review attempts to improve clinicians' understanding of the way VTV works and to provide essential information about evidence-based tidal volume (V_T) targets. Focus on underlying lung pathophysiology, individualised ventilator settings and V_Ttargets are keys to successful use of VTV thereby improving important clinical outcomes.

Current methodological and technical limitations of time and volumetric capnography in newborns

Although capnography is a standard tool in mechanically ventilated adult and pediatric patients, it has physiological and technical limitations in neonates. Gas exchange differs between small and adult lungs due to the greater impact of small airways on gas exchange, the higher impact of the apparatus dead space on measurements due to lower tidal volume and the occurrence of air leaks in intubated patients. The high respiratory rate and low tidal volume in newborns, especially those with stiff lungs, require main-stream sensors with fast response times and minimal dead-space or low suction flow when using side-stream measurements. If these technical requirements are not fulfilled, the measured end-tidal CO2 (P et CO 2 ), which should reflect the alveolar CO2 and the calculated airway dead spaces, can be misleading. The aim of this survey is to highlight the current limitations of capnography in very young patients to avoid pitfalls associated with the interpretation of capnographic parameters, and to describe further developments.

Leak around endotracheal tubes in ventilated newborns: an observational study

AIM

The study aims to determine the incidence and magnitude of leak around endotracheal tubes (ETTs) in ventilated babies in our unit, and to identify any relationship between weight, gestation, head position or ETT tip position, and the amount of leak.

METHODS

Ventilated infants in the neonatal unit of the Royal Brisbane and Women's Hospital were observed. Data were collected on ETT size, ventilator settings, body position, head position and amount of leak. Also recorded were weight, post-menstrual age and radiological position of the ETT tip.

RESULTS

Two hundred ninety-six sets of data were collected from 60 infants. At the time of recruitment, infants had a median (interquartile range) post-menstrual age of 32(+3) (28(+4) -36(+3)) years. Minimal leak (defined as <10% measured leak) and a small leak (10-25% leak) were observed on 162 occasions (55%) and 77 occasions (26%), respectively. A large leak of more than 25% was observed on 57 (19%) occasions. The odds of having a large leak with the head turned to the right, or midline, was greater than with the head turned to the left (odds ratio = 2.0; 95% confidence interval: 1.0 to 4.1 and 6.8; 1.4 to 31.5, respectively). Infants lying prone were more likely than those lying supine to have a leak >25% (odds ratio = 2.1; 95% confidence interval: 1.1 to 3.8). There was no association between leak and distance from carina, or weight.

CONCLUSIONS

Few babies in our unit had a large leak around the ETT, and many had minimal leak. A leak was less likely when a baby's head was turned to the left. Further study is required to clarify the reasons for this. Weight and post-menstrual age were not associated with the extent of leak.

Inspiratory capacity at inflation hold in ventilated newborns: a surrogate measure for static compliance of the respiratory system

OBJECTIVE

To study appropriateness of respiratory system compliance calculation using an inflation hold and compare it with ventilator readouts of pressure and tidal volume as well as with measurement of compliance of the respiratory system with the single-breath-single-occlusion technique gained with a standard lung function measurement.

DESIGN

Prospective clinical trial.

SETTING

Level III neonatal unit of a university hospital.

SUBJECTS

Sixty-seven newborns, born prematurely or at term, ventilated for a variety of pathologic conditions.

INTERVENTIONS

A standardized sigh maneuver with a predefined peak inspiratory pressure of 30 cm H2O, termed inspiratory capacity at inflation hold, was applied. Using tidal volume, exhaled from inspiratory pause down to ambient pressure, as displayed by the ventilator, and predefined peak inspiratory pressure, compliance at inspiratory capacity at inflation hold conditions could be calculated as well as ratio of tidal volume and ventilator pressure using tidal volume and differential pressure at baseline ventilator settings: peak inspiratory pressure minus positive end-expiratory pressure.

MEASUREMENTS AND MAIN RESULTS

For the whole cohort, the equation for the regression between tidal volume at inspiratory capacity at inflation hold and compliance of the respiratory system was: compliance of the respiratory system = 0.052 * tidal volume at inspiratory capacity at inflation hold - 0.113, and compliance at inspiratory capacity at inflation hold conditions was closely related to the standard lung function measurement method of compliance of the respiratory system (R = 0.958). In contrast, ratio of tidal volume and ventilator pressure per kilogram calculated from the ventilator readouts and displayed against compliance of the respiratory system per kilogram yielded a broad scatter throughout the whole range of compliance; both were only weakly correlated (R = 0.309) and also the regression line was significantly different from the line of identity (p < .05). Peak inspiratory pressure at study entry did not affect the correlation between compliance at inspiratory capacity at inflation hold conditions and compliance of the respiratory system.

CONCLUSIONS

After a standard sigh maneuver, inspiratory capacity at inflation hold and the derived quantity compliance at inspiratory capacity at inflation hold conditions can be regarded as a valid, accurate, and reliable surrogate measure for standard compliance of the respiratory system in contrast to ratio of tidal volume and ventilator pressure calculated from the ventilator readouts during ongoing mechanical ventilation at respective ventilator settings.

Automated respiratory cycles selection is highly specific and improves respiratory mechanics analysis

OBJECTIVE

Selected optimal respiratory cycles should allow calculation of respiratory mechanic parameters focusing on patient-ventilator interaction. New computer software automatically selecting optimal breaths and respiratory mechanics derived from those cycles are evaluated.

DESIGN

Retrospective study.

SETTING

University level III neonatal intensive care unit.

SUBJECTS

Ten mins synchronized intermittent mandatory ventilation and assist/control ventilation recordings from ten newborns.

INTERVENTION

The ventilator provided respiratory mechanic data (ventilator respiratory cycles) every 10 secs. Pressure, flow, and volume waves and pressure-volume, pressure-flow, and volume-flow loops were reconstructed from continuous pressure-volume recordings. Visual assessment determined assisted leak-free optimal respiratory cycles (selected respiratory cycles). New software graded the quality of cycles (automated respiratory cycles). Respiratory mechanic values were derived from both sets of optimal cycles. We evaluated quality selection and compared mean values and their variability according to ventilatory mode and respiratory mechanic provenance. To assess discriminating power, all 45 "t" values obtained from interpatient comparisons were compared for each respiratory mechanic parameter.

MEASUREMENTS AND MAIN RESULTS

A total of 11,724 breaths are evaluated. Automated respiratory cycle/selected respiratory cycle selections agreement is high: 88% of maximal κ with linear weighting. Specificity and positive predictive values are 0.98 and 0.96, respectively. Averaged values are similar between automated respiratory cycle and ventilator respiratory cycle. C20/C alone is markedly decreased in automated respiratory cycle (1.27 ± 0.37 vs. 1.81 ± 0.67). Tidal volume apparent similarity disappears in assist/control: automated respiratory cycle tidal volume (4.8 ± 1.0 mL/kg) is significantly lower than for ventilator respiratory cycle (5.6 ± 1.8 mL/kg). Coefficients of variation decrease for all automated respiratory cycle parameters in all infants. "t" values from ventilator respiratory cycle data are two to three times higher than ventilator respiratory cycles.

CONCLUSIONS

Automated selection is highly specific. Automated respiratory cycle reflects most the interaction of both ventilator and patient. Improving discriminating power of ventilator monitoring will likely help in assessing disease status and following trends. Averaged parameters derived from automated respiratory cycles are more precise and could be displayed by ventilators to improve real-time fine tuning of ventilator settings.

Tracheal tube airleak in clinical practice and impact on tidal volume measurement in ventilated neonates

OBJECTIVE

To determine the prevalence, size, and factors affecting tracheal tube (TT) leak in clinical practice and their influence on the displayed tidal volume (Vt) in ventilated newborn infants using uncuffed TTs. Monitoring of Vt is important for implementation of lung-protective ventilation strategies but becomes meaningless in the presence of large TT airleaks.

DESIGN

Retrospective clinical study.

SETTING

Neonatal intensive care unit.

PATIENTS

Patient records of 163 neonates ventilated with Babylog 8000 for ≥ 5 hrs with a median (range) gestation age of 31.1 wks (23.3-41.9 wks) and a median birth weight of 1470 g (410-4475 g) were evaluated.

INTERVENTIONS

: Ventilatory settings, TT leak, and Vt were recorded every 3 hrs. The lowest, median, and highest TT leaks were noted on the day the first TT leak (>5%) occurred, the day on which TT leak peaked, and the day of extubation.

MEASUREMENTS AND MAIN RESULTS

A TT leak of >5% was seen in 122 (75%) infants. Neonates with TT leak, compared with those without TT leak, had a longer duration of mechanical ventilation (p < .001), a lower gestational age (p = .004), a reduced birth weight (p = .005), and a higher prevalence of reintubation (p = .003). The greatest TT leak was seen in infants ventilated with a TT of <3-mm diameter. During the entire duration of mechanical ventilation, 42.3% of all neonates experienced at least one TT leak of >40% commonly seen on the third day of mechanical ventilation. Regression analysis showed that a TT leak of 40% indicated that the displayed Vt was underestimated by 1.2 mL/kg (about 24% of target Vt).

CONCLUSIONS

TT leak is highly variable, and TT leak of >40% with clinically relevant Vt errors occurred in nearly half of all ventilated neonates. Preterm infants of low birth weight and with small-diameter TTs ventilated for a long period were at greater risk of TT leak.

Relationship between endotracheal tube leakage and under-reading of tidal volume in neonatal ventilators

AIM

Protective ventilation in neonates requires careful volume monitoring to prevent ventilator-induced lung injury caused by baro/volutrauma and hence chronic lung disease. This study investigated the effect of endotracheal tube (ET) leakage on the displayed tidal volume using an in vitro model.

METHODS

A neonatal lung model was ventilated via a 3 mm ET using three ventilators [Babylog 8000 (BL), Leoni (LE) and Stephanie (ST)]. Tidal volume was measured by each ventilator at the Y-piece and by a pneumotach (CO(2)SMO(+)) in the model. ET leaks were simulated by open tubes of different lengths. PIP (20 cmH(2)O) and PEEP (5 cmH(2)O) were kept constant, and the respiratory rate (RR) was varied between 20/min and 70/min (Ti:Te = 1:1).

RESULTS

Tidal volume displayed by a ventilator decreased independently of RR with increasing leakage up to 21% (BL), 30% (LE) and 33% (ST). However, the volume delivered to the lung was nearly constant. The displayed leakage varied between 0 and 78% and was dependent on RR and leakage resistance. There were distinct differences between the three ventilators in the relationship between displayed leakage and volume error. Accepting a volume error <10% for RR between 20 and 70/min, ET leakage of up to 20% for BL, 12% for LE, but only <5% for ST, was acceptable.

CONCLUSION

Tidal volume underestimation arising from ET leakage depends on ventilator pressures, timing parameters and ventilator-specific algorithms for signal processing. Therefore, neonatologists should be aware of these issues to prevent lung over-inflation when adjusting target volume in the presence of ET leakage.

Automated measurement of the lower inflection point in a pediatric lung model

OBJECTIVE

To determine which flow setting most accurately detects the lower inflection point (Pflex) using an automated constant flow method and varying endotracheal tube (ETT) sizes with and without an airleak in a pediatric lung model.

DESIGN

Interventional laboratory study.

SETTING

Children's hospital research center.

INTERVENTIONS

A pediatric lung model was created with Pflexs of the inspiratory pressure-volume (P-V) curve set at 5 and 10 cm H2O using the ASL 5000 Test Lung (IngMar Medical, Pittsburgh, PA). Three ETT sizes (3.0, 4.0, 5.0 mm) were tested with and without a 25% airleak. P-V curves were obtained using an automated constant flow method at ten different flow rates.

MEASUREMENTS AND MAIN RESULTS

Without an ETT airleak, the lowest flow of 0.5 L/min led to the most accurate determination of Pflex regardless of ETT size or set Pflex (p < 0.001). When a 25% leak was introduced, accuracy of measured Pflex depended on both ETT size (p < 0.001) and flow rate (p < 0.001). Optimum flow rates for Pflex determination were 0.5, 1.0, and 1.5 L/min at Pflex of 5 cm H2O, and 2.0, 3.5, and 4.5 L/min at 10 cm H2O for 3.0, 4.0, and 5.0 mm ETTs, respectively (p < 0.001).

CONCLUSIONS

Estimation of Pflex can be achieved using automated P-V curves with ETTs appropriate for pediatric use, with and without an airleak. ETT size and flow rate affect the accuracy of these measurements when an airleak is present, and use of increased flow rates to create the automated P-V curves can reduce error. These data support the idea that a low-flow technique provides the most accurate determination of Pflex in pediatric patients without a leak around their ETT, whereas increased flows are needed to compensate when an ETT airleak is present.