Effect of inspiratory flow waveform on work on endotracheal tubes: a model analysis

Airway Management and Risk Factors for Prolonged Intubation in Patients with Severe Croup

Croup is a common respiratory illness in children with a substantial variation in the severity of symptoms. Most of the patients present with mild symptoms, but patients with severe croup require intensive care unit (ICU) management. The aim of this study was to investigate the airway management of patients with severe croup who required intubation and determine the risk factors for prolonged intubation. We performed an 18-year retrospective observational cohort study at the pediatric ICU of a tertiary children's hospital in Japan. A total of 16 patients with croup who were intubated for upper airway obstruction were included in the study. Most patients (13of 16, 81%) were intubated with an endotracheal tube (ETT) smaller than their age-appropriate size. The median difference in the internal diameter (ID) between the selected ETT and the age-appropriate size was 1.0 mm (interquartile range: 0.5-1.0). Multivariate analysis performed on factors affecting the cumulative incidence of extubation revealed that the difference in ID between the selected ETT and age-appropriate size (mm) significantly reduced the duration of intubation (hazard ratio: 0.092, p  = 0.03). A downsized ETT without a cuff may be recommended for intubation of patients with croup.

The effects of pressure- versus volume-controlled ventilation on ventilator work of breathing

Background

Measurement of work of breathing (WOB) during mechanical ventilation is essential to assess the status and progress of intensive care patients. Increasing ventilator WOB is known as a risk factor for ventilator-induced lung injury (VILI). In addition, the minimization of WOB is crucial to facilitate the weaning process. Several studies have assessed the effects of varying inspiratory flow waveforms on the patient’s WOB during assisted ventilation, but there are few studies on the different effect of inspiratory flow waveforms on ventilator WOB during controlled ventilation.

Methods

In this paper, we analyze the ventilator WOB, termed mechanical work (MW) for three common inspiratory flow waveforms both in normal subjects and COPD patients. We use Rohrer’s equation for the resistance of the endotracheal tube (ETT) and lung airways. The resistance of pulmonary and chest wall tissue are also considered. Then, the resistive MW required to overcome each component of the respiratory resistance is computed for square and sinusoidal waveforms in volume-controlled ventilation (VCV), and decelerating waveform of flow in pressure-controlled ventilation (PCV).

Results

The results indicate that under the constant I:E ratio, a square flow profile best minimizes the MW both in normal subjects and COPD patients. Furthermore, the large I:E ratio may be used to lower MW. The comparison of results shows that ETT and lung airways have the main contribution to resistive MW in normals and COPDs, respectively.

Conclusion

These findings support that for lowering the MW especially in patients with obstructive lung diseases, flow with square waveforms in VCV, are more favorable than decelerating waveform of flow in PCV. Our analysis suggests the square profile is the best choice from the viewpoint of less MW.

Changes in peak inspiratory flow rate and peak airway pressure with endotracheal tube size during chest compression

BACKGROUND

Adequate airway management plays an important role in high-quality cardiopulmonary resuscitation (CPR). Airway management is usually performed using an endotracheal tube (ETT) during CPR. However, no study has assessed the effect of ETT size on the flow rate and airway pressure during CPR.

METHODS

We measured changes in peak inspiratory flow rate (PIFR), peak airway pressure (P_peak), and mean airway pressure (P_mean) according to changes in ETT size (internal diameter 6.0, 7.0, and 8.0 mm) and with or without CPR. A tidal volume of 500 mL was supplied at a rate of 10 times per minute using a mechanical ventilator. Chest compressions were maintained at a constant compression depth and speed using a mechanical chest compression device (LUCAS2, mode: active continuous, chest compression rate: 102±2/minute, chest compression depth 2-2.5 inches).

RESULTS

The median of several respiratory physiological parameters during CPR was significantly different according to the diameter of each ETT (6.0 vs. 8.0 mm): PIFR (32.1 L/min [30.5-35.3] vs. 28.9 L/min [27.5-30.8], P<0.001), P_peak (48.84 cmH₂O [27.46-52.11] vs. 27.45 cmH₂O [22.53-52.57], P<0.001), and P_mean (18.34 cmH₂O [14.61-21.66] vs.13.66 cmH₂O [8.41-19.24], P<0.001).

CONCLUSION

The changes in PIFR, P_peak, and P_mean were related to the internal diameter of ETT, and these values tended to decrease with an increase in ETT size. Higher airway pressures were measured in the CPR group than in the no CPR group.

Rohrer's constant, K2, as a factor of determining inspiratory resistance of common adult endotracheal tubes

The aim of the study was to calculate the in vitro inspiratory resistance (R(ETT)) of adult endotracheal tubes (ETT), via the end-inspiratory occlusion method, and to apply this method in vivo in order to estimate R(ETT) value in real time. By plotting R(ETT) over inspiratory flow (V) and calculating Rohrer's coefficients of linear and nonlinear resistance, K1 and K2 respectively, we determined the resistive behaviour of each ETT. Peak and plateau pressures were recorded at both proximal and distal sites of the ETT after applying a three-second occlusion under constant flow. Distal pressure was obtained via an intraluminal catheter R(ETT) was calculated as (P(peak) - P(plateau))/(V), at both sites. R(ETT) value resulted from the difference R(proximal) - R(distal). Graph R(ETT) over (V) was plotted and Rohrer's constants were calculated by the method of least squares. For ETTs with inner diameter 9.0, 8.5, 8.0, 7.5, 7.0 and 6.5 mm, K2 was 2.42, 3.05, 4.65, 6.01, 9.17 and 12.80 cmH2O/l/s, respectively. The intraluminal catheter increased R(ETT) No.7.0 by an average of 49%. Finally, ten patients with partially obstructed ETTs were tested and K2 in vivo constants found to be higher than their corresponding in vitro values (P value 0.00012). Therefore, knowing the performing size of an ETT may help the clinicians identify ETT obstruction and deal with weaning problems.

Analysis of multiple linear regression algorithms used for respiratory mechanics monitoring during artificial ventilation

Many patients undergo long-term artificial ventilation and their respiratory system mechanics should be monitored to detect changes in the patient's state and to optimize ventilator settings. In this work the most popular algorithms for tracking variations of respiratory resistance (R(rs)) and elastance (E(rs)) over a ventilatory cycle were analysed in terms of systematic and random errors. Additionally, a new approach was proposed and compared to the previous ones. It takes into account an exact description of flow integration by volume-dependent lung compliance. The results of analyses showed advantages of this new approach and enabled to form several suggestions. Algorithms including R(rs) and E(rs) dependencies on airflow and lung volume can be effectively applied only at low levels of noise present in measurement data, otherwise the use of the simplest model with constant parameters is preferable. Additionally, one should avoid including the resistance dependence on airflow alone, since this considerably destroys the retrieved trace of R(rs). Finally, the estimated cyclic trajectories of R(rs) and E(rs) are more sensitive to noise present in pressure than in the flow signal, and the elastance traces are estimated more accurately than the resistance ones.

Detection of expiratory flow limitation during mechanical ventilation: a simulation study

Expiratory flow limitation (EFL) is frequent in mechanically ventilated patients with obstructive pulmonary disease and its prompt detection is important to optimize respiratory assistance. The present study aims to compare by simulation two methods for the detection of flow limitation in intensive care unit: the negative expiratory pressure (NEP) method and the external resistance (DeltaR) method. To this purpose, a non linear dynamic morphometric model of breathing mechanics, derived from the Weibel symmetrical description of lungs, was used to simulate a normal and an obstructive respiratory condition during artificial ventilation. Both methods revealed the presence of EFL in the pathological case. The NEP method seems to promote the collapse of the upper and intermediate airways, so producing an overestimation of the pathology result. On the contrary, during the DeltaR maneuver the same airways increase their radius and, therefore, EFL appears underestimated. The DeltaR method appears less practical with respect to the NEP method, because of the procedure required to select the appropriate resistance degree. Moreover the flow limited portion of expiration estimated by the DeltaR technique sounds rather dependent on the choice of the external resistance level.

A Simulation Study of Expiratory Flow Limitation in Obstructive Patients during Mechanical Ventilation

Although normal lungs may be represented satisfactorily by symmetrical architecture, pathological conditions generally require accounting for asymmetrical branching of the bronchial tree, since lung heterogeneity may be significant in respiratory diseases. In the present study, a recently proposed symmetrical dynamic morphometric model of the human lung, based on Weibel's regular dichotomy, was adapted to simulate different physiopathological scenarios of lung heterogeneity. The asymmetrical architecture was mimicked by modeling different conductive airway compartments below the main bronchi, each compartment being characterized by regular branching. The respiratory zone and chest wall were described by a Voigt body and a constant elastance, respectively. Simulation results allowed us to investigate the influence of the main mechanisms involved in expiratory flow limitation and dynamic hyperinflation in mechanically ventilated COPD patients. In brief, they showed that convective gas acceleration plays a key role in reproducing a negative relationship between driving pressure and expiratory flow. Moreover, reduced lung elastance due to emphysema resulted in a remarkable increase in dynamic hyperinflation, although it did not significantly modify expiratory flow limitation. Finally, the presence of a normal lung compartment masked pathological behaviors, preventing standard techniques from revealing expiratory flow limitation in affected compartments.

A Dynamic Morphometric Model of the Normal Lung for Studying Expiratory Flow Limitation in Mechanical Ventilation

A nonlinear dynamic morphometric model of breathing mechanics during artificial ventilation is described. On the basis of the Weibel symmetrical representation of the tracheo-bronchial tree, the model accurately accounts for the geometrical and mechanical characteristics of the conductive zone and packs the respiratory zone into a viscoelastic Voigt body. The model also accounts for the main mechanisms limiting expiratory flow (wave speed limitation and viscous flow limitation), in order to reproduce satisfactorily, under dynamic conditions, the expiratory flow limitation phenomenon occurring in normal subjects when the difference between alveolar pressure and tracheal pressure (driving pressure) is high. Several expirations characterized by different levels of driving pressure are simulated and expiratory flow limitation is detected by plotting the isovolume pressure-flow curves. The model is used to study the time course of resistance and total cross-sectional area as well as the ratio of fluid velocity to wave speed (speed index), in conductive airway generations. The results highlight that the coupling between dissipative pressure losses and airway compliance leads to onset of expiratory flow limitation in normal lungs when driving pressure is increased significantly by applying a subatmospheric pressure to the outlet of the ventilator expiratory channel; wave speed limitation becomes predominant at still higher driving pressures.

Nonlinear model for mechanical ventilation of human lungs

A complex nonlinear model for mechanical ventilation, its computer implementation and validation are presented. The model includes the morphometry-based symmetrical structure of the 23 airway generations, dynamic properties of the respiratory system, as well as the description of a ventilator. Distributed character of airway mechanical properties is taken into account when determining airway inertance, resistance and compliance, including turbulence of flow, airway collapsing and the wave speed theory. In effect, the airway parameters vary within the ventilatory cycle and their values are nonlinear functions of control signals. Results of simulations corresponding to normal conditions and airway narrowing are consistent with the published experimental data. The model enables investigations on how specific pathological changes influence the signals and physiological variables during mechanical ventilation, as well as testing known and developing new algorithms tracking time-variability of the respiratory parameters.