Electrical impedance tomography in the clinical assessment of lung volumes following recruitment manoeuvres

Background: Mechanical ventilation has dramatically improved outcomes in critically ill patients with respiratory failure. Minimizing volumes and higher positive end-expiratory pressures can further improve patient outcomes. Recruitment manoeuvres which can be used to individualize positive end-expiratory pressure may not improve outcome unless recruitable tissue is present. Existing methods of assessing if lung tissue is recruitable have a variety of limitations. Electrical impedance tomography (EIT) is a new technology that may be able to assess whether or not lung tissue is recruitable at the bedside. Objectives: This review will assess the growing body of evidence that EIT is a promising technique which may help the clinician to optimize ventilation, while minimizing injury. We will review how the device works, the data supporting its use, and potential uses for the physical therapist in the critical care environment. Major findings: EIT is a technique of injecting current through tissue, and measuring the difference between an array of electrodes. The difference relates to the changes of volume within the chest cavity–either blood or gas. It is reproducible, non-radiative, and real-time–allowing immediate and repeated imaging in the sickest of patients, who may require high levels of peep and recruitment manoeuvres. Conclusions: This paper has demonstrated that with an understanding of the strengths and limitations of the device, EIT can be used successfully at the bedside by clinicians to guide recruitment and other clinical techniques

Effect of frequency on pressure cost of ventilation and gas exchange in newborns receiving high-frequency oscillatory ventilation

BackgroundWe hypothesized that ventilating at the resonant frequency of the respiratory system optimizes gas exchange while limiting the mechanical stress to the lung in newborns receiving high-frequency oscillatory ventilation (HFOV). We characterized the frequency dependence of oscillatory mechanics, gas exchange, and pressure transmission during HFOV.MethodsWe studied 13 newborn infants with a median (interquartile range) gestational age of 29.3 (26.4-30.4) weeks and body weight of 1.00 (0.84-1.43) kg. Different frequencies (5, 8, 10, 12, and 15 Hz) were tested, keeping carbon dioxide diffusion coefficient (DCO2) constant. Oscillatory mechanics and transcutaneous blood gas were measured at each frequency. The attenuation of pressure swings (ΔP) from the airways opening to the distal end of the tracheal tube (TT) and to the alveolar compartment was mathematically estimated.ResultsBlood gases were unaffected by frequency. The mean (SD) resonant frequency was 16.6 (3.5) Hz. Damping of ΔP increased with frequency and with lung compliance. ΔP at the distal end of the TT was insensitive to frequency, whereas ΔP at the peripheral level decreased with frequency.ConclusionThere is no optimal frequency for gas exchange when DCO2 is held constant. Greater attenuation of oscillatory pressure at higher frequencies offers more protection from barotrauma, especially in patients with poor compliance.Pediatric Research advance online publication, 26 July 2017; doi:10.1038/pr.2017.151

High frequency oscillatory ventilation for respiratory failure due to RSV bronchiolitis

OBJECTIVE: To describe the time course of high frequency oscillatory ventilation (HFOV) in respiratory syncytial virus (RSV) bronchiolitis. DESIGN: Retrospective charts review. SETTING: A tertiary paediatric intensive care unit. PATIENTS AND PARTICIPANTS: Infants with respiratory failure due to RSV infection. INTERVENTION: HFOV. MEASUREMENTS AND RESULTS: Pattern of lung disease, ventilatory settings, blood gases, infant's vital parameters, sedation and analgesia during the periods of conventional mechanical ventilation (CMV, 6 infants), after initiation of HFOV (HFOVi, 9 infants), in the middle of its course (HFOVm), at the end (HFOVe) and after extubation (Post-Extub) were compared. All infants showed a predominant overexpanded lung pattern. Mean airway pressure was raised from a mean (SD) 12.5 (2.0) during CMV to 18.9 (2.7) cmH(2)O during HFOVi (P < 0.05), then decreased to 11.1(1.3) at HFOVe (P < 0.05). Mean FiO(2) was reduced from 0.68 (0.18) (CMV) to 0.59 (0.14) (HFOVi) then to 0.29 (0.06) (P < 0.05) at HFOVe and mean peak to peak pressure from 44.9 (12.4) cmH(2)O (HFOVi) to 21.1 (7.7) P < 0.05 (HFOVe) while mean (SD) PaCO(2) showed a trend to decrease from 72 (22) (CMV) to 47 (8) mmHg (HFVOe) and mean infants respiratory rate a trend to increase from 20 (11) (HFOVi) to 34 (14) (HFOVe) breaths/min. With usual doses of sedatives and opiates, no infant was paralysed and all were extubated to CPAP or supplemental oxygen after a mean of 120 h. CONCLUSION: RSV induced respiratory failure with hypercapnia can be managed with HFOV using high mean airway pressure and large pressure swings while preserving spontaneous breathing

Optimal mean airway pressure during high-frequency oscillatory ventilation determined by measurement of respiratory system reactance.

The aims of the present study were (i) to characterize the relationship between mean airway pressure (PAW) and reactance measured at 5 Hz (reactance of the respiratory system (X RS), forced oscillation technique) and (ii) to compare optimal PAW (P opt) defined by X RS, oxygenation, lung volume (VL), and tidal volume (VT) in preterm lambs receiving high-frequency oscillatory ventilation (HFOV).Nine 132-d gestation lambs were commenced on HFOV at PAW of 14 cmH2O (P start). PAW was increased stepwise to a maximum pressure (P max) and subsequently sequentially decreased to the closing pressure (Pcl, oxygenation deteriorated) or a minimum of 6 cmH2O, using an oxygenation-based recruitment maneuver. X RS, regional V L (electrical impedance tomography), and V T were measured immediately after (t 0 min) and 2 min after (t 2 min) each PAW decrement. P opt defined by oxygenation, X RS, V L, and V T were determined.The PAW-X RS and PAW-VT relationships were dome shaped with a maximum at Pcl+6 cmH2O, the same point as P opt defined by VL. Below Pcl+6 cmH2O, X RS became unstable between t 0 min and t 2 min and was associated with derecruitment in the dependent lung. P opt, as defined by oxygenation, was lower than the P opt defined by X RS, V L, or V T.X RS has the potential as a bedside tool for optimizing PAW during HFOV