AUTOMATIC SELECTION OF BREATHING PATTERN USING ADAPTIVE SUPPORT VENTILATION.
Electronic Supplementary material
Jean-Michel ARNAL MD*, Marc WYSOCKI MD†, Cyril NAFATI MD*, Stéphane DONATI MD*, Isabelle GRANIER MD*, Gaëlle CORNO MD*, Jacques DURAND-GASSELIN MD*.
* Service de Réanimation Polyvalente, Hôpital Font Pré, Toulon, France
† Department of Medical Research, Hamilton medical, Bonaduz, Switzerland.
This study was conducted in the Intensive Care Unit of Font Pré Hospital, Toulon, France.
Conflicts of interest: JM Arnal has been supported by Hamilton Medical AG for presenting the results of this study at international congress. C Nafati, S Donati, I Granier, G Corno, and J Durand-Gasselin have no conflict of interest. M Wysocki is an employee of Hamilton Medical AG and, as the Head of Medical Research, was involved in the initial discussions regarding the design of the study and in helping to write the manuscript. He was not involved in collecting and analyzing the data.
Communicating author:
Dr Arnal Jean-Michel, Service de réanimation polyvalente, Hôpital Font Pré, 1208 avenue du colonel Picot, 83100 Toulon, France
Phone: 33 4 94 61 80 95/Fax: 33 4 94 61 80 93
Email:
Word count: 1650
Methods:
Patients:
Patients admitted between January and August 2004 to the 11-bed adult general intensive care unit of Font Pré Hospital (Toulon, France) were included if they were invasively ventilated. Patients for whom leaks were an issue (noninvasive ventilation and broncho-pleural fistula) were not included. All patients were ventilated with a GALILEO Gold ventilator (Hamilton Medical, Bonaduz, Switzerland) using ASV as the primary mode.
Ethical considerations:
This observational and descriptive study has been approved by the ethic committee of the French Society of Critical Care (SRLF). Before starting the study, ASV was used in the unit as a routine clinical care for more than two years and 90% of the patients received ASV as a default mode. Because the study was purely observational and descriptive and involved no intervention, the investigators believed possible to waive the informed consent. Information to the patients and the families regarding the ventilatory treatment was given according to the general practice in the unit.
Definitions:
For any given patient, each ventilation-day was categorized using one of the five following clinical conditions:
Normal lungs defined by no underlying respiratory disease, a normal chest X-ray, and a PaO2/FiO2 ratio ≥ 300 mmHg.
ALI/ARDS as defined by the American–European Consensus Conference [1].
COPD as defined by the GOLD criteria [2].
Chest wall stiffness included patients with cyphoscoliosis, morbid obesity or a neuromuscular disorder [3].
Acute respiratory failure (ARF) defined by a PaO2/FiO2 ratio ≤ 300 mmHg without the ARDS/ALI criteria from the American–European Consensus Conference [1].
For any given patient, each ventilation-day was categorized as a passive or an active ventilation-day, the latter being defined by a patient’s spontaneous RR being > 75% of the total RR.
Daily categorization makes it possible for a given patient to be categorized differently based on his changing condition. As an example, a patient might be categorized as “passive ARDS” on the first days after admission, while he might be categorized as “active normal lungs” on the following days, if he improves.
The categorization was done daily (including public holidays and weekends) between 8 to 9 a.m. based on the medical team’s consensual decision, taking into account the medical history, chest examination, daily chest X-ray, daily arterial blood gases, and any other exams that may have been performed (e.g., CT scan, bacteriologic sample).
ASV description and settings:
ASV has been fully described in previous papers [4, 5, 6]. In short, MV is set by the clinician and controlled through a VT–RR combination based on breath-by-breath estimation of RCexp [7, 8] according to the minimal work of breathing concept developed by Otis [9]. At the onset of mechanical ventilation, the ventilator delivers three test breaths, during which RCexp is determined from the expiratory flow volume curve [7, 8]. Subsequently, and on a breath-by-breath basis, two simultaneous closed-loop controls continuously regulate 1) Pinsp, to achieve the target VT, and 2) inspiratory and expiratory times (TI and TE), to achieve the target RR in passive patients and in active patients when the patient’s rate is below the target RR.
To avoid extreme and potentially dangerous values of VT and RR, ASV applies, on a breath-by-breath basis, a safety window for target VT and RR values. The minimal target VT is defined as twice the anatomical dead space estimated from the predicted body weight (PBW). The maximal target VT is defined as the maximal clinician-set pressure times the dynamic compliance of the total respiratory system. The minimal value for the target RR is 5 breaths/min. The maximal value for the target RR is defined as the ratio 20/RCexp. MV is set by the clinician and expressed as a percentage (%MV) of a physiological MV (0.1 L/kg of PBW/min in the adult patient): 100% MV being equal to 0.1 L/kg PBW/min, i.e., 7 L/min for 70 kg of PBW. The patient PBW is entered through a specific control in the ASV control panel. In the present study PBW was calculated according to height and gender [10]. In active and passive patients, the starting %MV was initially set at 110% (i.e., plus 10% above normal MV to compensate for increased dead space when a heat and moisture exchanger is inserted between the ventilator pneumotachograph and the endotracheal tube). The %MV was later adjusted according to the desired PaCO2 in passive patients and according to the clinical condition in active patients:
In passive patients, the target PaCO2 was 38 – 42 mmHg and 40 – 45 mmHg in normal lungs and ALI/ARDS respectively. In ALI/ARDS patients a permissive respiratory acidosis down to a pH of 7.25 (including maximal reduction of the apparatus dead space) was allowed to keep the total inspiratory pressure below or equal to 35 cmH2O [11]. In passive patients with chronic respiratory disease, the %MV was adjusted to normalize the arterial pH between 7.38 and 7.42. In all passive patients the target PaCO2 was refined to 30 – 35 mmHg in cases of increased intracranial pressure.
In active patients, the %MV was adjusted according to clinical conditions as in pressure support mode. In cases of insufficient support evidenced by a high RR (> 30 breaths/min), accessory muscle activation, etc., the %MV was increased whatever the PaCO2 level. In cases of sufficient support evidenced by a RR below 20 breaths/min without clinical signs of respiratory distress, the %MV was progressively reduced to 110%.
Other settings
All patients were orally intubated and tracheotomized when necessary for weaning. The positive end-expiratory pressure (PEEP) level was set at 5 cmH2O in normal lung patients and passive obstructive pulmonary disease patients (COPD) . In active COPD patients, PEEP was adjusted by clinical inspection of respiratory muscle activation to trigger the breath [12]. In passive ALI/ARDS patients, PEEP was set at 2 cmH2O above the lower inflection point determined from a quasi-static pressure-volume curve of the respiratory system (P/V Tool®, Hamilton Medical, Bonaduz, Switzerland) repeated twice a day. When a lower inflection point was absent, PEEP was set below 10 cmH2O. In active ALI/ARDS patients, the FiO2–PEEP combination applied by the ARDS Network was used [13]. Otherwise the FiO2 was set at the minimal value to reach a PaO2 between 60 and 70 mmHg in ALI/ARDS and COPD patients and between 80 and 100 mmHg in other patients.
Failure of ASV
ASV failure was defined by one of the following conditions:
1) Total inspiratory pressure (Pinsp + PEEP) ≥ 35 cmH2O, allowing user to manually adjust VT or inspiratory pressure.
2) Broncho-pleural fistula onset during ventilation, because of lack of information about the accuracy of RCexp measurement in cases of leak.
3) Abnormal respiratory rhythms, because of the ASV response-time which can be much slower than the patient’s rhythm.
4) Patient-ventilator asynchrony defined clinically and on the pressure-flow traces [14], allowing user to manually adjust inspiratory pressure.
Patients who failed ASV were switched to a volume-controlled or pressure support mode.
Weaning with ASV
The indication for a T-tube trial was screened daily and the test initiated and monitored by the nurse caring for the patient if all the following criteria were met:
Patient actively breathing (defined by a patient’s spontaneous RR > 75% of the total RR).
PaO2/FiO2 ratio 200 mmHg
PEEP ≤ 5 cmH2O
%MV setting ≤ 130%
Pinsp ≤ 20 cmH2O
Adequate coughing during suctioning
No infusion of sedative or vasopressor agents [15]
If the previous criteria were met, the ventilator mode was switched to a CPAP setting of 5 cmH2O to measure the RR/VT ratio over a 5-minute period. A RR/VT ratio < 105 breaths/min per liter was required to start the T-tube trial.
A one-hour (two-hour for patients with previous chronic respiratory disease) T-tube trial was performed by disconnecting the patient from the ventilator, suctioning the patient, deflating the cuff of the endotracheal prosthesis, and providing oxygen supplementation to maintain the arterial oxygen saturation above 95%.
The T-tube trial was defined as a failure and the patient reconnected to the ventilator with the previous settings if at least one of the following criteria was present [15]:
1) RR > 35 breaths/min sustained for at least 5 minutes
2) Arterial oxygen saturation < 90% despite oxygen supplementation
3) Heart rate ≥ 140 beats per minute or a sustained change in the heart rate of 20% in either direction
4) Systolic blood pressure > 180 mmHg or < 90 mmHg
5) Increased anxiety and/or diaphoresis
After three T-tube test failures, the patient was either tracheotomized or extubated and given immediate non-invasive ventilation support. If the T-tube test was successful (i.e., the above criteria were absent), the patient was extubated.
Data collection:
Recording was done at 6 a.m., with arterial blood gas analysis by an independent investigator who did not participate in the categorization. This time of day was chosen to avoid ventilatory perturbation due to nursing procedures. Ventilator settings (PBW, %MV, PEEP, and FiO2), breathing pattern (VT, total and spontaneous RR, TI, TE), and respiratory mechanics parameters were read from the ventilator display along with arterial blood gas results (pH, PaO2, PaCO2). Airway pressure and flow were measured using the proximal pneumotachograph (single-use flow sensor, PN 279331, Hamilton Medical, Bonaduz, Switzerland, linear between -120 and 120 L/min with a ±5% error of measure) inserted between the endotracheal tube and the Y-piece of the ventilator circuit. VT was obtained by integration of the flow signal. In passive patients, static compliance (Cstat) and inspiratory resistance (Rins) were measured using the least square fit method [16]. RCexp was automatically calculated on a breath-to-breath basis as the ratio between the volume and the simultaneous expiratory flow measured at the point corresponding to 75% of the expiratory tidal volume [8, 9].
Statistical analysis:
Values are given as medians with their 25th and 75th quartiles. Comparisons were done using a nonparametric analysis of variance (ANOVA). Differences were considered significant when p < 0.05. When significant differences were observed, two-by-two multiple comparisons were performed using Dunn’s method (SigmaStat, version 3.0, SPSS, Inc., Chicago, IL USA).
References:
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