High-Flow Nasal Oxygen for Severe Hypoxemia: Oxygenation Response and Outcome in Patients with COVID-19

Rationale: The "Berlin definition" of acute respiratory distress syndrome (ARDS) does not allow inclusion of patients receiving high-flow nasal oxygen (HFNO). However, several articles have proposed that criteria for defining ARDS should be broadened to allow inclusion of patients receiving HFNO. Objectives: To compare the proportion of patients fulfilling ARDS criteria during HFNO and soon after intubation, and 28-day mortality between patients treated exclusively with HFNO and patients transitioned from HFNO to invasive mechanical ventilation (IMV). Methods: From previously published studies, we analyzed patients with coronavirus disease (COVID-19) who had PaO2/FiO2 of ⩽300 while treated with ⩾40 L/min HFNO, or noninvasive ventilation (NIV) with positive end-expiratory pressure of ⩾5 cm H2O (comparator). In patients transitioned from HFNO/NIV to invasive mechanical ventilation (IMV), we compared ARDS severity during HFNO/NIV and soon after IMV. We compared 28-day mortality in patients treated exclusively with HFNO/NIV versus patients transitioned to IMV. Measurements and Main Results: We analyzed 184 and 131 patients receiving HFNO or NIV, respectively. A total of 112 HFNO and 69 NIV patients transitioned to IMV. Of those, 104 (92.9%) patients on HFNO and 66 (95.7%) on NIV continued to have PaO2/FiO2 ⩽300 under IMV. Twenty-eight-day mortality in patients who remained on HFNO was 4.2% (3/72), whereas in patients transitioned from HFNO to IMV, it was 28.6% (32/112) (P < 0.001). Twenty-eight-day mortality in patients who remained on NIV was 1.6% (1/62), whereas in patients who transitioned from NIV to IMV, it was 44.9% (31/69) (P < 0.001). Overall mortality was 19.0% (35/184) and 24.4% (32/131) for HFNO and NIV, respectively (P = 0.2479). Conclusions: Broadening the ARDS definition to include patients on HFNO with PaO2/FiO2 ⩽300 may identify patients at earlier stages of disease but with lower mortality.

Protective ventilation in patients with acute respiratory distress syndrome related to COVID-19: always, sometimes or never?

PURPOSE OF REVIEW

To review current evidence on the pathophysiology of COVID-19-related acute respiratory distress syndrome (ARDS) and on the implementation of lung protective ventilation.

RECENT FINDINGS

Although multiple observations and physiological studies seem to show a different pathophysiological behaviour in COVID-19-ARDS compared with 'classical' ARDS, numerous studies on thousands of patients do not confirm these findings and COVID-19-ARDS indeed shares similar characteristics and interindividual heterogeneity with ARDS from other causes. Although still scarce, present evidence on the application of lung protective ventilation in COVID-19-ARDS shows that it is indeed consistently applied in ICUs worldwide with a possible signal towards better survival at least in one study. The levels of positive end-expiratory pressure (PEEP) usually applied in these patients are higher than in 'classical' ARDS, proposing once again the issue of PEEP personalization in hypoxemic patients. In the absence of robust evidence, careful evaluation of the patient is needed, and empiric settings should be oriented towards lower levels of PEEP.

SUMMARY

According to the present evidence, a lung protective strategy based on low tidal volume and plateau pressures is indicated in COVID-19-ARDS as in ARDS from other causes; however, there are still uncertainties on the appropriate levels of PEEP.

Impact of Maximizing Css/MIC Ratio on Efficacy of Continuous Infusion Meropenem Against Documented Gram-Negative Infections in Critically Ill Patients and Population Pharmacokinetic/Pharmacodynamic Analysis to Support Treatment Optimization

Introduction: optimal treatment of Gram-negative infections in critically ill patients is challenged by changing pathophysiological conditions, reduced antimicrobial susceptibility and limited therapeutic options. The aim of this study was to assess the impact of maximizing Css/MIC ratio on efficacy of continuous infusion (CI) meropenem in treating documented Gram-negative infections in critically ill patients and to perform a population pharmacokinetic/pharmacodynamic analysis to support treatment optimization.

Materials and Methods: Classification and regression tree (CART) analysis was used to identify whether a cutoff of steady-state meropenem concentration (Css)-to-minimum inhibitory concentration (MIC) (Css/MIC) ratio correlated with favorable clinical outcome. A non-parametric approach with Pmetrics was used for pharmacokinetic analysis and covariate evaluation. The probability of target attainment (PTA) of the identified Css/MIC ratio was calculated by means of Monte Carlo simulations. Cumulative fraction of response (CFRs) were calculated against common Enterobacterales, P. aeruginosa and A. baumannii as well.

Results: a total of 74 patients with 183 meropenem Css were included. CART analysis identified a Css/MIC ratio ≥4.63 as cutoff value significantly associated with favorable clinical outcomes. Multivariate regression analysis confirmed the association [OR (95%CI): 20.440 (2.063–202.522); p < 0.01]. Creatinine clearance (CL_CR) was the only covariate associated with meropenem clearance. Monte Carlo simulations showed that, across different classes of renal function, dosages of meropenem ranging between 0.5 and 2 g q6h over 6 h (namely by CI) may grant PTAs of Css/MIC ratios ≥4.63 against susceptible pathogens with an MIC up to the EUCAST clinical breakpoint of 2 mg/L. The CFRs achievable with these dosages were very high (>90%) against Enterobacterales across all the classes of renal function and against P. aeruginosa among patients with CL_CR < 30 ml/min/1.73 m², and quite lower against A. baumannii.

Discussion: our findings suggest that Css/MIC ratio ≥4.63 may be considered the pharmacodynamic target useful at maximizing the efficacy of CI meropenem in the treatment of Gram-negative infections in critically ill patients. Dosages ranging between 0.5 g q6h and 2 g q6h by CI may maximize the probability of favorable clinical outcome against meropenem-susceptible Gram-negative pathogens among critically ill patients having different degrees of renal function.

High-flow nasal therapy vs standard oxygen during breaks off noninvasive ventilation for acute respiratory failure: A pilot randomized controlled trial

PURPOSE

To assess the role of high-flow nasal therapy (HFNT) compared to standard oxygen (SO) as complementary therapy to non-invasive ventilation (NIV).

METHODS

Multicenter trial including patients (n = 54) anticipated to receive NIV for ≥24 h due to acute or acute-on-chronic respiratory failure. Subjects were randomized (1:1) to SO or HFNT during breaks off NIV. Primary outcome was total time on and off NIV. Secondary outcomes were comfort and dyspnea, respiratory rate (RR), oxygen saturation (S_pO₂), tolerance and side effects.

RESULTS

Total time per patient on NIV (1315 vs 1441 min) and breaks (1362 vs 1196 min), and mean duration of each break (520 vs 370 min) were similar in the HFNT and SO arms (p > .05). Comfort score was higher on HFNT than on SO (8.3 ± 2.7 vs 6.9 ± 2.3, p = .001). Dyspnea, RR and S_pO₂ were similar in the two arms, but the increase in RR and dyspnea seen with SO during breaks did not occur with HFNT.

CONCLUSION

Compared to SO, HFNT did not reduce time on NIV. However, it was more comfortable and the increase in RR and dyspnea seen with SO did not occur with HFNT. Therefore, HFNT could be a suitable alternative to SO during breaks off NIV.

Oronasal mask versus helmet in acute hypercapnic respiratory failure

The choice of the interface for noninvasive ventilation (NIV) is a key factor in NIV success. We hypothesised that a new helmet specifically design to improve performance in hypercapnic patients would be clinically equivalent to a standard oronasal mask. In a multicentre, short-term, physiological, randomised trial in chronic obstructive pulmonary disease patients facing an acute hypercapnic respiratory failure episode, we compared the changes in arterial blood gases (ABGs) and tolerance score obtained using the helmet or mask, and, as secondary end-points, dyspnoea, vital signs, early NIV discontinuation and rate of intubation. 80 patients were randomly assigned to receive NIV either with the helmet (n=39) or mask (n=41), using an intensive care unit ventilator. Compared with baseline, in the first 6 h, NIV improved ABGs, dyspnoea and respiratory rate (p<0.05) in both groups. Changes in ABGs and discomfort were similar with the two groups, while dyspnoea decreased more (p<0.005) using the mask. The rate of intubation and the need for interface change during the whole period of NIV were very low and not different between groups. The new helmet may be a valid alternative to a mask in improving ABGs and achieving a good tolerance during an episode of acute hypercapnic respiratory failure.

Different tracheotomy tube diameters influence diaphragmatic effort and indices of weanability in difficult to wean patients

OBJECTIVE

To determine the effects of different tracheotomy tube sizes on diaphragm effort and weanability indices.

METHODS

Ten tracheotomized and difficult to wean subjects were randomized to 2 T-piece trials, with different tracheotomy tube diameters: inner diameters 8 mm and 6.5 mm. Diaphragm pressure-time product per min. (PTP(di/min)), lung compliance and resistance (C(L) and R(L)), breathing pattern, tension-time index of the diaphragm (TT(di)), and the ratio of breathing frequency to tidal volume (f/V(T)) were recorded. In an in vitro model, the flow-pressure relationship was measured using the 2 tracheotomy tubes and 2 endotracheal tubes of the same diameter.

RESULTS

The use of a smaller diameter resulted in an increase of PTP(di) (337.63 ± 194.35 cm H(2)O · s/min vs 263.28 ± 156.23 cm H(2)O · s/min for 6.5 mm and 8 mm, respectively, P = .004) and R(L) (16.74 ± 8.10 cm H(2)O · s/min vs 11.72 ± 7.88 cm H(2)O · s/min, respectively, P = .008). Both weanability indices were also significantly higher using the smaller tube: f/V(T) 93.32 ± 20.91 vs 77.06 ± 19.26 for 6.5 mm and 8 mm, respectively, P < .02; TT(di) 0.09 ± 0.052 vs 0.08 ± 0.04, respectively, P < .02. In vitro measurements confirmed that the resistances were higher with the smaller diameter and similar between the tracheotomy tubes and the endotracheal tubes of the same diameters.

CONCLUSIONS

In tracheotomized difficult to wean subjects the decrease of the tracheotomy tube size was associated with an increased PTP(di), f/V(T), and TT(di), that were otherwise normal, using a higher diameter. The in vitro study showed that the resistances increased similarly for tracheotomy tube and endotracheal tube, decreasing the diameter and increasing the flows.