2.1 Study Design

This observational prospective cohort study included invasively mechanically ventilated patients with COVID-19-associated ARDS admitted to the intensive care unit (ICU) during the first COVID-19 pandemic in April 2020.

2.2 Setting

The study was conducted at VieCuri Medical Center, a teaching hospital in southern Netherlands. The ICU has 16 beds for both medical and surgical patients, expanded to 32 beds during the first COVID-19 pandemic. The study was conducted from April 9th to 20th, 2020.

2.3 Medical Ethical Considerations

The Medical Ethical Review Board of azM/UM (METC 2020-1339) confirmed that the Medical Research Involving Human Subjects Act (WMO) did not apply to our study, and official approval was not required. Local approval was obtained from the VieCuri MC Ethics Review Board. The Ethics Review Board waived the requirement for written informed consent from patients or their legal representatives. Only routine, de-identified data retrieved from the patient data management system were used.

Patients were eligible for inclusion in the study if they were 18 years or older and required adequate sedation. Patients undergoing acute dialysis were excluded. For patients with multiple ICU admissions, only the first admission was considered.

2.4 Clinical Policies Relating to Sedation and Analgesia

Upon ICU admission, patients with COVID-19 and respiratory failure were intubated and mechanically ventilated, often in the prone position [10]. Analgesics used included fentanyl and nonopioids (acetaminophen and S-ketamine). Targeted sedation (Richmond Agitation Sedation Scale [RASS] of −4 to −5) was achieved using intravenous propofol, midazolam, and dexmedetomidine [11, 12].

The ICU pharmacists reviewed medication daily. Additional sedatives were used at the discretion of the attending physician. Enteral lormetazepam (Noctamid®, Schering Nederland B.V.) 2 mg twice daily (8:00 a.m. and 20:00 h) was administered via a nasogastric tube. The choice of lormetazepam as a sedative-hypnotic drug was based on local experience with psychotropic drugs, ease of administration, tolerance, and low cost. In the Netherlands, prescribing and administering enteral lormetazepam for sedation is considered off-label.

Prokinetic drugs (metoclopramide and erythromycin) were administered to prevent poor bioavailability due to gastric dysmotility [13]. The IV midazolam dosage was adjusted to achieve the desired sedation level. The dosages of fentanyl and other sedatives were unchanged. Sedation levels and ventilation status were assessed six times daily and documented in the electronic health record. Observed versus planned RASS scores were evaluated, and sedation was titrated.

All included patients had severe ARDS with low PaO2/FiO2 ratios (< 200). Prone positioning and neuromuscular blockade were used to improve ventilation and oxygenation.

2.5 Clinical Parameters

Patient clinical characteristics (weight, date of birth, sex) and severity of illness at ICU admission (Acute Physiologic And Chronic Health Evaluation [APACHE] IV) were obtained from the electronic patient data management system. The most recent pre-ICU body weights were used for drug dosing and PK analyses. In addition, the following data were retrieved that were considered potentially influential PK parameters: the exact dose and time of lormetazepam administration and laboratory parameters (serum creatinine, albumin, and C-reactive protein [CRP] level).

2.6 Blood Sampling

Residual arterial blood samples were collected for PK analysis from the Laboratory of Clinical Chemistry and Hematology unless the patient objected to its use for clinical research. This was performed concomitantly with routine blood sampling. The routine sampling times in our ICU were 6:30, 16:00, and 22:00. Blood samples were centrifuged within 30 min and stored at 2–8 °C. Residual samples were collected and the supernatant plasma was stored at −80 °C for up to 2 months until batch-wise analysis.

2.7 Laboratory Analysis of Lormetazepam and Lormetazepam-Glucuronide

Lormetazepam and its glucuronide were analyzed using a validated ultra-high performance liquid chromatography-tendem mass spectrometry (UHPLC-MS/MS) method in the ISO-15189 certified clinical pharmaceutical laboratory of VieCuri MC. The method was validated for specificity, linearity, repeatability, intermediate precision, recovery, and lower limit of quantitation (LOQ) according to the International Conference on Harmonization (ICH) guidelines [14]. In summary, the test results were within the acceptable limits for clinical use. The linearity of the method was confirmed in the range of 1–20 µg/L for both lormetazepam and lormetazepam glucuronide. Samples with concentrations above the upper LOQ were diluted and reanalyzed according to the developed dilution method.

2.8 The Lormetazepam Metabolic Ratio

The lormetazepam metabolite-to-parent drug ratio (MR) was calculated as the ratio of the main metabolite of lormetazepam (lormetazepam glucuronide) to the parent drug, lormetazepam, and expressed as a range [15]. The MR was corrected for the molecular weights of lormetazepam (335.2 g/mol) and lormetazepam glucuronide (511.31 g/mol) in a manner similar to that described for the midazolam metabolic ratio [15, 16].

2.9 Pharmacokinetic Model Development

Individual and population analyses were conducted using a one-compartment model with the KinPop plugin of Edsim++ (version 2.4 Mediware, Prague, Czech Republic), utilizing the iterative two-stage Bayesian (ITSB) population modeling method [17,18,19]. The coefficient of variation (CV) was used for interindividual variability (IIV), and the accuracy of the parameter estimate was denoted with relative standard error (RSE). Here, IIV was obtained by a statistical procedure and is therefore not a model parameter estimate. This is in contrast to NONMEM, where the IIV is a model parameter that is estimated. Therefore, NONMEM IIV parameters also have a residual RSE value. The ITSB method is currently the only method supported by KinPop and has been proven to perform well in both rich and sparse data sets [17, 18].

For the bootstrap runs, descriptive statistics for each parameter estimate were summarized to evaluate validity of the final parameter estimates including the absorption constant (Ka), volume of distribution (Vd/F), clearance of total lormetazepam (CL/F), and their 95% confidence intervals (CIs) calculated based on normal approximation under the final model.

2.10 Development of Alternative Lormetazepam Dosing Recommendation

PK analysis was conducted using PK/PD software (MwPharm++ version 2.40; Mediware, Prague, Czech Republic). Target trough levels above 6 µg/L were used based on previous literature data [20].

2.11 Statistical Analysis

Of the patients included in the present study, only a fixed number of samples were available for analysis owing to the use of residual blood samples. They were deemed sufficiently large for PK analysis. All the statistical analyses were performed using SPSS (version 25, IBM Corporation, Armonk, NY, USA). Normality was assessed using the Shapiro-Wilk and Kolmogorov-Smirnov tests. Normally distributed data are presented as mean ± SD or median (25–75%, interquartile range [IQR]), categorical variables as absolute numbers, and population frequencies in percentages.