Lung ultrasound predicts non-invasive ventilation outcome in COVID-19 acute respiratory failure: a pilot study

BACKGROUND

The aim of this study is to determine relationships between lung aeration assessed by lung ultrasound (LUS) with non-invasive ventilation (NIMV) outcome, intensive care unit (ICU) admission and mechanical ventilation (MV) needs in COVID-19 respiratory failure.

METHODS

A cohort of adult patients with COVID-19 respiratory failure underwent LUS during initial assessment. A simplified LUS protocol consisting in scanning six areas, three for each side, was adopted. A score from 0 to 3 was assigned to each area. Comprehensive LUS score (LUSsc) was calculated as the sum of the score in all areas. LUSsc, the amount of involved sonographic lung areas (LUSq), the number of lung quadrants radiographically infiltrated and the degree of oxygenation impairment at admission (SpO<inf>2</inf>/FiO2 ratio) were compared to NIMV Outcome, MV needs and ICU admission.

RESULTS

Among 85 patients prospectively included in the analysis, 49 of 61 needed MV. LUSsc and LUSq were higher in patients who required MV (median 12 [IQR 8-14] and median 6 [IQR 4-6], respectively) than in those who did not (6 [IQR 2-9] and 3 [IQR 1-5], respectively), both P<0.001. NIMV trial failed in 26 patients out 36. LUSsc and LUSq were significantly higher in patients who failed NIMV than in those who did not. From ROC analysis, LUSsc ≥12 and LUSq ≥5 gave the best cut-off values for NIMV failure prediction (AUC=0.95, 95%CI 0.83-0.99 and AUC=0.81, 95% CI 0.65-0.91, respectively).

CONCLUSIONS

Our data suggest LUS as a possible tool for identifying patients who are likely to require MV and ICU admission or to fail a NIMV trial.

Diaphragm thickening fraction predicts noninvasive ventilation outcome: a preliminary physiological study

BACKGROUND

A correlation between unsuccessful noninvasive ventilation (NIV) and poor outcome has been suggested in de-novo Acute Respiratory Failure (ARF) patients. Consequently, it is of paramount importance to identify accurate predictors of NIV outcome. The aim of our preliminary study is to evaluate the Diaphragmatic Thickening Fraction (DTF) and the respiratory rate/DTF ratio as predictors of NIV outcome in de-novo ARF patients.

METHODS

Over 36 months, we studied patients admitted to the emergency department with a diagnosis of de-novo ARF and requiring NIV treatment. DTF and respiratory rate/DTF ratio were measured by 2 trained operators at baseline, at 1, 4, 12, 24, 48, 72 and 96 h of NIV treatment and/or until NIV discontinuation or intubation. Receiver operating characteristic (ROC) curve analysis was performed to evaluate the ability of DTF and respiratory rate/DTF ratio to distinguish between patients who were successfully weaned and those who failed.

RESULTS

Eighteen patients were included. We found overall good repeatability of DTF assessment, with Intra-class Correlation Coefficient (ICC) of 0.82 (95% confidence interval 0.72-0.88). The cut-off values of DTF for prediction of NIV failure were < 36.3% and < 37.1% for the operator 1 and 2 (p < 0.0001), respectively. The cut-off value of respiratory rate/DTF ratio for prediction of NIV failure was > 0.6 for both operators (p < 0.0001).

CONCLUSION

DTF and respiratory rate/DTF ratio may both represent valid, feasible and noninvasive tools to predict NIV outcome in patients with de-novo ARF. Trial registration ClinicalTrials.gov Identifier: NCT02976233, registered 26 November 2016.

Physiological Comparison of High-Flow Nasal Cannula and Helmet Noninvasive Ventilation in Acute Hypoxemic Respiratory Failure

Rationale: High-flow nasal cannula (HFNC) and helmet noninvasive ventilation (NIV) are used for the management of acute hypoxemic respiratory failure.Objectives: Physiological comparison of HFNC and helmet NIV in patients with hypoxemia.Methods: Fifteen patients with hypoxemia with Pa_O2/Fi_O2 < 200 mm Hg received helmet NIV (positive end-expiratory pressure ≥ 10 cm H₂O, pressure support = 10-15 cm H₂O) and HFNC (50 L/min) in randomized crossover order. Arterial blood gases, dyspnea, and comfort were recorded. Inspiratory effort was estimated by esophageal pressure (Pes) swings. Pes-simplified pressure-time product and transpulmonary pressure swings were measured.Measurements and Main Results: As compared with HFNC, helmet NIV increased Pa_O2/Fi_O2 (median [interquartile range]: 255 mm Hg [140-299] vs. 138 [101-172]; P = 0.001) and lowered inspiratory effort (7 cm H₂O [4-11] vs. 15 [8-19]; P = 0.001) in all patients. Inspiratory effort reduction by NIV was linearly related to inspiratory effort during HFNC (r = 0.84; P < 0.001). Helmet NIV reduced respiratory rate (24 breaths/min [23-31] vs. 29 [26-32]; P = 0.027), Pes-simplified pressure-time product (93 cm H₂O ⋅ s ⋅ min^-1 [43-138] vs. 200 [168-335]; P = 0.001), and dyspnea (visual analog scale 3 [2-5] vs. 8 [6-9]; P = 0.002), without affecting Pa_CO2 (P = 0.80) and comfort (P = 0.50). In the overall cohort, transpulmonary pressure swings were not different between treatments (NIV = 18 cm H₂O [14-21] vs. HFNC = 15 [8-19]; P = 0.11), but patients exhibiting lower inspiratory effort on HFNC experienced increases in transpulmonary pressure swings with helmet NIV. Higher transpulmonary pressure swings during NIV were associated with subsequent need for intubation.Conclusions: As compared with HFNC in hypoxemic respiratory failure, helmet NIV improves oxygenation, reduces dyspnea, inspiratory effort, and simplified pressure-time product, with similar transpulmonary pressure swings, Pa_CO2, and comfort.

Tidal Volume Lowering by Instrumental Dead Space Reduction in Brain-Injured ARDS Patients: Effects on Respiratory Mechanics, Gas Exchange, and Cerebral Hemodynamics

Background

Limiting tidal volume (V_T), plateau pressure, and driving pressure is essential during the acute respiratory distress syndrome (ARDS), but may be challenging when brain injury coexists due to the risk of hypercapnia. Because lowering dead space enhances CO₂ clearance, we conducted a study to determine whether and to what extent replacing heat and moisture exchangers (HME) with heated humidifiers (HH) facilitate safe V_T lowering in brain-injured patients with ARDS.

Methods

Brain-injured patients (head trauma or spontaneous cerebral hemorrhage with Glasgow Coma Scale at admission < 9) with mild and moderate ARDS received three ventilatory strategies in a sequential order during continuous paralysis: (1) HME with V_T to obtain a PaCO₂ within 30–35 mmHg (HME1); (2) HH with V_T titrated to obtain the same PaCO₂ (HH); and (3) HME1 settings resumed (HME2). Arterial blood gases, static and quasi-static respiratory mechanics, alveolar recruitment by multiple pressure–volume curves, intracranial pressure, cerebral perfusion pressure, mean arterial pressure, and mean flow velocity in the middle cerebral artery by transcranial Doppler were recorded. Dead space was measured and partitioned by volumetric capnography.

Results

Eighteen brain-injured patients were studied: 7 (39%) had mild and 11 (61%) had moderate ARDS. At inclusion, median [interquartile range] PaO₂/FiO₂ was 173 [146–213] and median PEEP was 8 cmH₂O [5–9]. HH allowed to reduce V_T by 120 ml [95% CI: 98–144], V_T/kg predicted body weight by 1.8 ml/kg [95% CI: 1.5–2.1], plateau pressure and driving pressure by 3.7 cmH₂O [2.9–4.3], without affecting PaCO₂, alveolar recruitment, and oxygenation. This was permitted by lower airway (− 84 ml [95% CI: − 79 to − 89]) and total dead space (− 86 ml [95% CI: − 73 to − 98]). Sixteen patients (89%) showed driving pressure equal or lower than 14 cmH₂O while on HH, as compared to 7 (39%) and 8 (44%) during HME1 and HME2 (p < 0.001). No changes in mean arterial pressure, cerebral perfusion pressure, intracranial pressure, and middle cerebral artery mean flow velocity were documented during HH.

Conclusion

The dead space reduction provided by HH allows to safely reduce V_T without modifying PaCO₂ nor cerebral perfusion. This permits to provide a wider proportion of brain-injured ARDS patients with less injurious ventilation.

Physiological effects of high-flow oxygen in tracheostomized patients

Background

High-flow oxygen therapy via nasal cannula (HFOT_NASAL) increases airway pressure, ameliorates oxygenation and reduces work of breathing. High-flow oxygen can be delivered through tracheostomy (HFOT_TRACHEAL), but its physiological effects have not been systematically described. We conducted a cross-over study to elucidate the effects of increasing flow rates of HFOT_TRACHEAL on gas exchange, respiratory rate and endotracheal pressure and to compare lower airway pressure produced by HFOT_NASAL and HFOT_TRACHEAL.

Methods

Twenty-six tracheostomized patients underwent standard oxygen therapy through a conventional heat and moisture exchanger, and then HFOT_TRACHEAL through a heated humidifier, with gas flow set at 10, 30 and 50 L/min. Each step lasted 30 min; gas flow sequence during HFOT_TRACHEAL was randomized. In five patients, measurements were repeated during HFOT_TRACHEAL before tracheostomy decannulation and immediately after during HFOT_NASAL. In each step, arterial blood gases, respiratory rate, and tracheal pressure were measured.

Results

During HFOT_TRACHEAL, PaO₂/FiO₂ ratio and tracheal expiratory pressure slightly increased proportionally to gas flow. The mean [95% confidence interval] expiratory pressure raise induced by 10-L/min increase in flow was 0.2 [0.1–0.2] cmH₂O (ρ = 0.77, p < 0.001). Compared to standard oxygen, HFOT_TRACHEAL limited the negative inspiratory swing in tracheal pressure; at 50 L/min, but not with other settings, HFOT_TRACHEAL increased mean tracheal expiratory pressure by (mean difference [95% CI]) 0.4 [0.3–0.6] cmH₂O, peak tracheal expiratory pressure by 0.4 [0.2–0.6] cmH₂O, improved PaO₂/FiO₂ ratio by 40 [8–71] mmHg, and reduced respiratory rate by 1.9 [0.3–3.6] breaths/min without PaCO₂ changes. As compared to HFOT_TRACHEAL, HFOT_NASAL produced higher tracheal mean and peak expiratory pressure (at 50 L/min, mean difference [95% CI]: 3 [1–5] cmH₂O and 4 [1–7] cmH₂O, respectively).

Conclusions

As compared to standard oxygen, 50 L/min of HFOT_TRACHEAL are needed to improve oxygenation, reduce respiratory rate and provide small degree of positive airway expiratory pressure, which, however, is significantly lower than the one produced by HFOT_NASAL.

Patient self-inflicted lung injury: implications for acute hypoxemic respiratory failure and ARDS patients on non-invasive support

The role of spontaneous breathing among patients with acute hypoxemic respiratory failure and ARDS is debated: while avoidance of intubation with noninvasive ventilation (NIV) or high-flow nasal cannula improves clinical outcome, treatment failure worsens mortality. Recent data suggest patient self-inflicted lung injury (P-SILI) as a possible mechanism aggravating lung damage in these patients. P-SILI is generated by intense inspiratory effort yielding: (A) swings in transpulmonary pressure (i.e. lung stress) causing the inflation of big volumes in an aerated compartment markedly reduced by the disease-induced aeration loss; (B) abnormal increases in transvascular pressure, favouring negative-pressure pulmonary edema; (C) an intra-tidal shift of gas between different lung zones, generated by different transmission of muscular force (i.e. pendelluft); (D) diaphragm injury. Experimental data suggest that not all subjects are exposed to the development of P-SILI: patients with a PaO2/FiO2 ratio below 200 mmHg may represent the most at risk population. For them, current evidence indicates that high-flow nasal cannula alone may be superior to intermittent sessions of low-PEEP NIV delivered through face mask, while continuous high-PEEP helmet NIV likely promotes treatment success and may mitigate lung injury. The optimal initial noninvasive treatment of hypoxemic respiratory failure/ARDS remains however uncertain; high-flow nasal cannula and high-PEEP helmet NIV are promising tools to enhance success of the approach, but the best balance between these techniques has yet to be identified. During noninvasive support, careful clinical monitoring remains mandatory for prompt detection of treatment failure, in order not to delay intubation and protective ventilation.

Lung volumes, respiratory mechanics and dynamic strain during general anaesthesia

BACKGROUND

Driving pressure (ΔP) represents tidal volume normalised to respiratory system compliance (C_RS) and is a novel parameter to target ventilator settings. We conducted a study to determine whether C_RS and ΔP reflect aerated lung volume and dynamic strain during general anaesthesia.

METHODS

Twenty non-obese patients undergoing open abdominal surgery received three PEEP levels (2, 7, or 12 cm H₂O) in random order with constant tidal volume ventilation. Respiratory mechanics, lung volumes, and alveolar recruitment were measured to assess end-expiratory aerated volume, which was compared with the patient's individual predicted functional residual capacity in supine position (FRCp).

RESULTS

C_RS was linearly related to aerated volume and ΔP to dynamic strain at PEEP of 2 cm H₂O (intraoperative FRC) (r=0.72 and r=0.73, both P<0.001). These relationships were maintained with higher PEEP only when aerated volume did not overcome FRCp (r=0.73, P<0.001; r=0.54, P=0.004), with 100 ml lung volume increases accompanied by 1.8 ml cm H₂O^-1 (95% confidence interval [1.1-2.5]) increases in C_RS. When aerated volume was greater or equal to FRCp (35% of patients at PEEP 2 cm H₂O, 55% at PEEP 7 cm H₂O, and 75% at PEEP 12 cm H₂O), C_RS and ΔP were independent from aerated volume and dynamic strain, with C_RS weakly but significantly inversely related to alveolar dead space fraction (r=-0.47, P=0.001). PEEP-induced alveolar recruitment yielded higher C_RS and reduced ΔP only at aerated volumes below FRCp (P=0.015 and 0.008, respectively).

CONCLUSIONS

During general anaesthesia, respiratory system compliance and driving pressure reflect aerated lung volume and dynamic strain, respectively, only if aerated volume does not exceed functional residual capacity in supine position, which is a frequent event when PEEP is used in this setting.