Accuracy of tidal volume delivery by paediatric intensive care ventilators: A bench-model study.

BACKGROUND: Tidal volume (Vt) delivery during mechanical ventilation is influenced by gas compression, humidity, and temperature. OBJECTIVES: This bench study aimed at assessing the accuracy of Vt delivery by paediatric intensive care ventilators according to the humidification system. Secondary objectives were to assess the following: (i) the accuracy of Vt delivery in ventilators with an integrated Y-piece pneumotachograph and (ii) the ability of ventilators to deliver and maintain a preset positive end-expiratory pressure. METHODS: Six latest-generation intensive care ventilators equipped with a paediatric mode were tested on the ASL5000 test lung in four simulated paediatric bench models (full-term neonate, infant, preschool-age chile, and school-age child), under volume-controlled mode with a heated humidifier (HH) or a heat moisture exchanger, with various loading conditions. Three ventilators equipped with a Y-piece pneumotachograph were tested with or without the pneumotachograph in the neonatal and infant models. "Accurate Vt" delivery was defined as a volume error (percentage of the preset Vt under body temperature and pressure and saturated water vapour conditions) being ≤10 % of the absolute preset value. RESULTS: Vt accuracy varied significantly across ventilators but was acceptable in almost all the ventilators and all the models, except the neonatal model. The humidification system had an impact on Vt delivery in the majority of the tested conditions (p < 0.05). The use of an HH was associated with a better Vt accuracy in four ventilators (V500, V800, R860, and ServoU) and allowed to achieve an acceptable level of volume error in the neonatal model as compared to the use of heat moisture exchanger. The use of an integrated pneumotachograph was associated with lower volume error in only one ventilator (p < 0.01). All the tested ventilators were able to maintain adequate positive end-expiratory pressure levels. CONCLUSION: The humidification system affects Vt accuracy of paediatric intensive care ventilators, especially in the youngest patients for whom the HH should be preferred.

Influence of different noninvasive oxygenation support devices on tidal volume.

BACKGROUND: Multiple devices are available for noninvasive oxygenation support, including non-rebreather oxygen mask (O2-mask), high-flow oxygen through nasal cannula (HFNC), continuous positive airway pressure (CPAP), mask noninvasive ventilation (Mask-NIV) and helmet NIV (Helmet-NIV). As tidal volume is a key determinant of efficacy and safety during ventilatory support, we assessed whether it was influenced by the type of noninvasive oxygenation device. METHODS: A bench study using a manikin with a realistic face connected to a lung simulator was performed. Six conditions were assessed: no device, O2-mask, HFNC, CPAP, Mask-NIV and Helmet-NIV. Three respiratory mechanics were simulated (normal, obstructive, restrictive), at three simulated efforts (low, moderate, respiratory distress). Flow was recorded at the lung simulator inlet and mouth pressure into the manikin mouth. The same devices were evaluated on healthy volunteers with tidal volume assessed by electrical impedance tomography (EIT). RESULTS: Tidal volume was significantly influenced by oxygenation devices in bench model. As compared to O2-mask, HFNC and CPAP delivered significantly lower tidal volumes (440 ± 352 mL, 414 ± 333 mL and 377 ± 297 mL, respectively), while Mask-NIV or Helmet-NIV were associated with significantly higher tidal volumes (690 ± 321 mL and 652 ± 366 mL, respectively). Tidal volume was strongly correlated with the specific effect of each device on mouth pressure during inspiration: HFNC and CPAP were characterized by a negative PTPmouth (- 0.3 [- 0.8 to - 0.2] and - 0.7 [- 2.2 to - 0.5] cmH2O.sec/cycle, respectively), while Helmet-NIV and Mask-NIV were associated with a positive PTPmouth (4.5 [4.1-4.6] and 6.1 [5.9-7.1] cmH2O.sec/cycle, respectively). Tidal volume was also significantly influenced by oxygenation devices in healthy volunteers, with similar tidal volumes between O2-mask and CPAP (644 [571-764] and 648 [586-770] mL) but higher with HFNC, Mask-NIV and Helmet-NIV (819 [609-918], 1110 [661-1305] and 1086 [833-1243] mL). CONCLUSIONS: Tidal volume is significantly influenced by noninvasive oxygenation support devices, with a strong correlation with the pressure variation generated into the mouth during inspiration. NIV was associated with the highest tidal volumes and CPAP with the lowest ones. Clinical studies are needed to clarify the clinical implications of these effects.

Dual titration of minute ventilation and sweep gas flow to control carbon dioxide variations in patients on venovenous extracorporeal membrane oxygenation.

BACKGROUND: The implantation of venovenous extracorporeal membrane oxygenation (VV-ECMO) support to manage severe acute respiratory distress syndrome generates large variations in carbon dioxide partial pressure (PaCO2) that are associated with intracranial bleeding. We assessed the feasibility and efficacy of a pragmatic protocol for progressive dual titration of sweep gas flow and minute ventilation after VV-ECMO implantation in order to limit significant PaCO2 variations. PATIENTS AND METHODS: A protocol for dual titration of sweep gas flow and minute ventilation following VV-ECMO implantation was implemented in our unit in September 2020. In this single-centre retrospective before-after study, we included patients who required VV-ECMO from March, 2020 to May, 2021, which corresponds to two time periods: from March to August, 2020 (control group) and from September, 2020 to May, 2021 (protocol group). The primary endpoint was the mean absolute change in PaCO2 in consecutive arterial blood gases samples drawn over the first 12 h following VV-ECMO implantation. Secondary endpoints included large (> 25 mmHg) initial variations in PaCO2, intracranial bleedings and mortality in both groups. RESULTS: Fifty-one patients required VV-ECMO in our unit during the study period, including 24 in the control group and 27 in the protocol group. The protocol was proved feasible. The 12-h mean absolute change in PaCO2 was significantly lower in patients of the protocol group as compared with their counterparts (7 mmHg [6-12] vs. 12 mmHg [6-24], p = 0.007). Patients of the protocol group experienced less large initial variations in PaCO2 immediately after ECMO implantation (7% vs. 29%, p = 0.04) and less intracranial bleeding (4% vs. 25%, p = 0.04). Mortality was similar in both groups (35% vs. 46%, p = 0.42). CONCLUSION: Implementation of our protocol for dual titration of minute ventilation and sweep gas flow was feasible and associated with less initial PaCO2 variation than usual care. It was also associated with less intracranial bleeding.

Impact of cardiac output and alveolar ventilation in estimating ventilation/perfusion mismatch in ARDS using electrical impedance tomography.

INTRODUCTION: Electrical impedance tomography (EIT) can be used to assess ventilation/perfusion (V/Q) mismatch within the lungs. Several methods have been proposed, some of them neglecting the absolute value of alveolar ventilation (VA) and cardiac output (QC). Whether this omission results in acceptable bias is unknown. METHODS: Pixel-level V/Q maps of 25 ARDS patients were computed once considering (absolute V/Q map) and once neglecting (relative V/Q map) the value of QC and VA. Previously published indices of V/Q mismatch were computed using absolute V/Q maps and relative V/Q maps. Indices computed with relative V/Q maps were compared to their counterparts computed using absolute V/Q maps. RESULTS: Among 21 patients with ratio of alveolar ventilation to cardiac output (VA/QC) > 1, relative shunt fraction was significantly higher than absolute shunt fraction [37% (24-66) vs 19% (11-46), respectively, p < 0.001], while relative dead space fraction was significantly lower than absolute dead space fraction [40% (22-49) vs 58% (46-84), respectively, p < 0.001]. Relative wasted ventilation was significantly lower than the absolute wasted ventilation [16% (11-27) vs 29% (19-35), respectively, p < 0.001], while relative wasted perfusion was significantly higher than absolute wasted perfusion [18% (11-23) vs 11% (7-19), respectively, p < 0.001]. The opposite findings were retrieved in the four patients with VA/QC < 1. CONCLUSION: Neglecting cardiac output and alveolar ventilation when assessing V/Q mismatch indices using EIT in ARDS patients results in significant bias, whose direction depends on the VA/QC ratio value.

Driving pressure-guided ventilation decreases the mechanical power compared to predicted body weight-guided ventilation in the Acute Respiratory Distress Syndrome.

BACKGROUND: Whether targeting the driving pressure (∆P) when adjusting the tidal volume in mechanically ventilated patients with the acute respiratory distress syndrome (ARDS) may decrease the risk of ventilator-induced lung injury remains a matter of research. In this study, we assessed the effect of a ∆P-guided ventilation on the mechanical power. METHODS: We prospectively included adult patients with moderate-to-severe ARDS. Positive end expiratory pressure was set by the attending physician and kept constant during the study. Tidal volume was first adjusted to target 6 ml/kg of predicted body weight (PBW-guided ventilation) and subsequently modified within a range from 4 to 10 ml/kg PBW to target a ∆P between 12 and 14 cm H2O. The respiratory rate was then re-adjusted within a range from 12 to 40 breaths/min until EtCO2 returned to its baseline value (∆P-guided ventilation). Mechanical power was computed at each step. RESULTS: Fifty-one patients were included between December 2019 and May 2021. ∆P-guided ventilation was feasible in all but one patient. The ∆P during PBW-guided ventilation was already within the target range of ∆P-guided ventilation in five (10%) patients, above in nine (18%) and below in 36 (72%). The change from PBW- to ∆P-guided ventilation was thus accompanied by an overall increase in tidal volume from 6.1 mL/kg PBW [5.9-6.2] to 7.7 ml/kg PBW [6.2-8.7], while respiratory rate was decreased from 29 breaths/min [26-32] to 21 breaths/min [16-28] (p < 0.001 for all comparisons). ∆P-guided ventilation was accompanied by a significant decrease in mechanical power from 31.5 J/min [28-35.7] to 28.8 J/min [24.6-32.6] (p < 0.001), representing a relative decrease of 7% [0-16]. With ∆P-guided ventilation, the PaO2/FiO2 ratio increased and the ventilatory ratio decreased. CONCLUSION: As compared to a conventional PBW-guided ventilation, a ∆P-guided ventilation strategy targeting a ∆P between 12 and 14 cm H2O required to change the tidal volume in 90% of the patients. Such ∆P-guided ventilation significantly reduced the mechanical power. Whether this physiological observation could be associated with clinical benefit should be assessed in clinical trials.

Refractory ineffective triggering during pressure support ventilation: effect of proportional assist ventilation with load-adjustable gain factors.

BACKGROUND: Ineffective triggering is frequent during pressure support ventilation (PSV) and may persist despite ventilator adjustment, leading to refractory asynchrony. We aimed to assess the effect of proportional assist ventilation with load-adjustable gain factors (PAV+) on the occurrence of refractory ineffective triggering. DESIGN: Observational assessment followed by prospective cross-over physiological study. SETTING: Academic medical ICU. PATIENTS: Ineffective triggering was detected during PSV by a twice-daily inspection of the ventilator's screen. The impact of pressure support level (PSL) adjustments on the occurrence of asynchrony was recorded. Patients experiencing refractory ineffective triggering, defined as persisting asynchrony at the lowest tolerated PSL, were included in the physiological study. INTERVENTIONS: Physiological study: Flow, airway, and esophageal pressures were continuously recorded during 10 min under PSV with the lowest tolerated PSL, and then under PAV+ with the gain adjusted to target a muscle pressure between 5 and 10 cmH2O. MEASUREMENTS: Primary endpoint was the comparison of asynchrony index between PSV and PAV+ after PSL and gain adjustments. RESULTS: Among 36 patients identified having ineffective triggering under PSV, 21 (58%) exhibited refractory ineffective triggering. The lowest tolerated PSL was higher in patients with refractory asynchrony as compared to patients with non-refractory ineffective triggering. Twelve out of the 21 patients with refractory ineffective triggering were included in the physiological study. The median lowest tolerated PSL was 17 cmH2O [12-18] with a PEEP of 7 cmH2O [5-8] and FiO2 of 40% [39-42]. The median gain during PAV+ was 73% [65-80]. The asynchrony index was significantly lower during PAV+ than PSV (2.7% [1.0-5.4] vs. 22.7% [10.3-40.1], p < 0.001) and consistently decreased in every patient with PAV+. Esophageal pressure-time product (PTPes) did not significantly differ between the two modes (107 cmH2O/s/min [79-131] under PSV vs. 149 cmH2O/s/min [129-170] under PAV+, p = 0.092), but the proportion of PTPes lost in ineffective triggering was significantly lower with PAV+ (2 cmH2O/s/min [1-6] vs. 8 cmH2O/s/min [3-30], p = 0.012). CONCLUSIONS: Among patients with ineffective triggering under PSV, PSL adjustment failed to eliminate asynchrony in 58% of them (21 of 36 patients). In these patients with refractory ineffective triggering, switching from PSV to PAV+ significantly reduced or even suppressed the incidence of asynchrony.