Inspiratory limb carbon dioxide entrainment during high-frequency oscillatory ventilation: characterization in a mechanical test lung and swine model.

BACKGROUND: High-frequency oscillatory ventilation (HFOV) has been utilized as a rescue oxygenation therapy in adults with ARDS over the last decade. The HFOV oscillating piston can generate negative pressure during the exhalation cycle, which has been termed active exhalation. We hypothesized that this characteristic of HFOV entrains CO(2) into the inspiratory limb of the circuit and increases the total dead space. The purpose of this study was to determine if retrograde CO(2) entrainment occurs and how it is altered by HFOV parameter settings. METHODS: An HFOV was interfaced to a cuffed endotracheal tube and connected to a mechanical test lung. Negative pressure changes within the circuit's inspiratory limb were measured while HFOV settings were manipulated. Retrograde CO(2) entrainment was evaluated by insufflating CO(2) into the test lung to achieve 40 mm Hg at the carina. Inspiratory limb CO(2) entrainment was measured at incremental distances from the Y-piece. HFOV settings and cuff leak were varied to assess their effect on CO(2) entrainment. Control experiments were conducted using a conventional ventilator. Test lung results were validated on a large hypercapnic swine. RESULTS: Negative pressure was detectable within the inspiratory limb of the HFOV circuit and varied inversely with mean airway pressure (P(-)(aw)) and directly with oscillatory pressure amplitude (ΔP). CO(2) was readily detectable within the inspiratory limb and was proportional to the negative pressure that was generated. Factors that decreased CO(2) entrainment in both the test lung and swine included low ΔP, high mean airway pressure, high oscillatory frequency (Hz), high bias flow, and endotracheal tube cuff leak placement. CO(2) entrainment was also reduced by utilizing a higher bias flow strategy at any targeted mean airway pressure. CONCLUSIONS: Retrograde CO(2) entrainment occurs during HFOV use and can be manipulated with the ventilator settings. This phenomenon may have clinical implications on the development or persistence of hypercapnia.

Critical Care in the Military Health System: A 24-h Point Prevalence Study.

Background: Healthcare expenditures are a significant economic cost with critical care services constituting one of its largest components. The Military Health System (MHS) is the largest, global healthcare system of its kind. In this project, we sought to describe critical care services and the patients who receive them in the MHS. Methods: We surveyed 26 military treatment facilities (MTFs) representing 38 critical care services or intensive care units (ICUs). MTFs with multiple ICUs and critical care services responded to the survey as services (e.g., surgical or medical ICU service), whereas MTFs with only one ICU responded as a unit and gave information about all types of patients (i.e., medical and surgical). Our survey was divided into an administrative portion and a 24-h point prevalence survey of patients and patient care. The administrative portion is reported separately in this journal. The 24-h point prevalence survey collected information about all patients present in, admitted to, or discharged from participating services/units during the same 24-h period in December 2014. The survey was anonymous and protected health information was not collected. Findings: Sixteen MTFs (69%) and 27 ICU services/units (71%) returned the point prevalence survey. MTFs with >200 beds (n = 3, 22%) were categorized as "high capacity centers" (HCCs) whereas those with ≤200 beds (n = 13, 78%) were characterized as low capacity centers (LCCs). Two MTFs (one HCC and one LCC) returned only administrative data. The remaining 16 MTFs reported data about 151 patients. In all, 100 (67%) of the patients were at three HCCs during this study period. One HCC accounted for 39% (59 patients) of all patient care during this study. Most patients were cared for in mixed medical/surgical ICUs (34.4%), followed by medical (21.2%), surgical (18.5%), trauma (11.9%), cardiac (7.9%), and burn (6.0%) ICUs. The most common medical indication for admission was cardiac followed by general medical. The most common surgical indications for admission were trauma, other, and cardiothoracic surgery. The average APACHE II score of all patients across both LCCs and HCCs was 11 ± 8.1 (8 ± 7.8 vs. 13 ± 7.7 p = 0.008). The lower acuity of patients in this study is reflected in a high turnover rate, low rate of arterial and central line placements (33%), and low rates of life support (all types, 30%; mechanical ventilation only, 21.2%; noninvasive mechanic ventilation only, 7.9%; and vasoactive medications, 6.6%). Thirty-five (23.2%) patients within the study were affected by a total of 57 complications. The three most common complications experienced were acute kidney injury, bleeding, and sepsis. Discussion: This is the first detailed report about MHS critical care services and the patients receiving care. It describes a low acuity ICU patient population, concentrated at larger MTFs. This study highlights the need for the establishment of a system that allows for the continuous collection of high priority information about clinical care in the MHS in order to facilitate implementation of standardized protocols and process improvements.

Critical Care in the Military Health System: A Survey-Based Summary of Critical Care Services.

Introduction: Critical care is an important component of in-patient and combat casualty care, and it is a major contributor to U.S. healthcare costs. Regular exposure to critically ill and injured patients may directly contribute to wartime skills retention for military caregivers. Data describing critical care services in the Military Health System (MHS), however, is lacking. This study was undertaken to describe MHS critical care services, their resource utilization, and differences in care practices amongst military treatment facilities (MTFs). Materials and Methods: Twenty-six MTFs representing 38 adult critical care services or intensive care units (ICUs) were surveyed. The survey collected information about organizational structure, resourcing, and unit characteristics at the time of a concurrent 24-h point-prevalence survey designed to describe patient characteristics and staffing in these facilities. The survey was anonymous and protected health information was not collected. We analyzed the data according to high capacity centers (HCCs) (≥200 beds) and low capacity centers (LCCs) (<200 beds). Differences between HCCs and LCCs were compared using Fisher's exact test. Results: Seventeen MTFs (7 HCCs and 10 LCCs), representing 27 ICUs, responded to the survey. This was a 65% response rate for MTFs and a 71% response rate for services/ICUs. HCCs reported more closed vs. open ICUs; more dedicated critical care services (i.e., medical and surgical ICUs vs. mixed ICUs); fewer respiratory therapists available, but more with certification; more total nursing staff and more critical care certified nurses; the use of subjectively more effective protocols (10.5 vs. 6.7 protocols/unit or service); higher utilization of an ICU daily rounds checklist (65% vs. 0%); and less consistency of clinician type participation during multidisciplinary rounds. ICU leadership structure was similar among the institutions. The majority of respondents were unable to provide summary APACHE II scores, but HCCs were more likely to submit this information than LCCs. Most centers perform multidisciplinary rounds daily, but they are more likely to be run by a physician credentialed in critical care at HCCs (85% vs. 59%, p < 0.05). 67% of respondents reported mortality rates <5%. The two services that reported mortality rates greater than 10% were both LCCs. Conclusion: This is the first comprehensive report about MHS critical care services. Despite notable variability in data reporting, an important finding itself, this study highlights notable differences in organizational structure and resourcing between HCCs and LCCs within the MHS. The clinical implication of these differences (i.e., impact on patient outcomes) of these differences require further study. Better understanding of MHS critical care services may improve enterprise decision-making about these services which could ultimately improve care of combat casualties.