Intelligent ventilation in the intensive care unit.
The programming is based on a concept of maximal energetic benefit: for any single breath, the ventilator selects the optimal respiratory rate target, and the optimal tidal volume target that corresponds to the minimal work of breathing of the patient-ventilator unit. The automatic selection of these targets is based on algorithms for minimal dead space and optimal expiratory time constant provided by the lung function analyser that is communicating continuously with the ventilator's controller. The lung function analyser calculates compliance, resistance and air trapping (residual end-expiratory flow), to optimise respiratory flow patterns and inspiration/expiration ratio. Target volume and rate are calculated specifically for each patient to achieve the set target minute volume according to the patient's lung mechanics (compliance, resistance, air trapping, dead space and expiratory time constant) and peak airway pressures. At any breath, the controller compares target and actual data for tidal volume and respiratory rate, and programmes the mandatory rate and the inspiratory pressure to be applied in the next breath, to approach the desired targets. (1,10,11)
[FIGURE 1 OMITTED]
Inspired pressures are delivered using pressure control in apnoeic patients, or pressure support in spontaneously breathing patients.
The use of closed-loop ventilation has previously been advocated for intensive care units. (12,13) Wysocki and Brunner consider ASV an underused, safe and cost-effective modality. They call for more extensive application of this ventilation mode in intensive care units. (13)
ASV has been in use as the primary mode of ventilation in the Medical Intensive Care Unit (MICU) at the Hadassah-Hebrew University Medical Center for the past 10 years. We describe our experience with this automated ventilation technology using prospective data collected over a 6-year period.
Materials and methods
The 9-bed MICU at the Hadassah-Hebrew University Medical Center, a 750-bed academic tertiary referral centre, admits critical, non-surgical cases with acute respiratory, infectious, neurological, haematological-oncological, renal, metabolic and other general medical problems. Data on all patients ventilated in the unit were collected prospectively during the period 1 April 2003-30 November 2009. These data included demographics, chronic diseases, diagnoses, severity of illness scoring, indication for ventilation, ventilation modes, interventions (inotropic support, haemodialysis), need for sedation, complications (respiratory, infectious, etc.), length of ventilation, unit and hospital length of stay, tracheostomy insertion, ventilation outcome and unit and hospital mortality outcomes.
Chronic diseases were defined per organ system as a previously known organ failure requiring ongoing treatment. Diagnoses were grouped into diagnostic categories related to each system; e.g. 'respiratory' included pneumonia, acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) exacerbation, pulmonary embolism and interstitial lung disease.
Actual ventilator settings were chosen at the attending physicians' discretion, and the primary ventilator mode recommended for all patients was ASV, delivered by either a Galileo or Raphael Model ventilator (Hamilton Medical AG). Initial settings included ideal body weight (IBW), determined using gender and height tables, and minute ventilation as a percentage of the value of 100 ml/kg of IBW/min. This was started at 100% and subsequently reduced according to arterial partial pressure of carbon dioxide (PaC[O.sub.2]) measurements and the patient's spontaneous efforts. The fractional inspired oxygen (Fi[O.sub.2]) was set targeting an arterial partial pressure of oxygen (Pa[O.sub.2]) of 70 mmHg or more. The level of positive end-expiratory pressure (PEEP) was determined using either the inbuilt volume/pressure (V/P) tool indicating the lower inflection point as minimal PEEP or the optimal PEEP required for adequate oxygenation with Fi[O.sub.2] less than 0.6 if possible. The upper pressure limit was set according to protective lung strategy guidelines, usually below 35 cm[H.sub.2]0. The ASV controller, according to the ventilator programmer, then automatically modified the delivered ventilator parameters. In occasional cases where a Galileo or Raphael ventilator was not available, patients were connected to other available ventilators (Puritan Bennet 7200, CA) and ventilated in conventional modes (synchronised intermittent mandatory ventilation, pressure control, pressure support).
Cases in which the physician determined that ASV was not being tolerated for any reason were documented, and a different mode of ventilation was employed. As soon as the reason for converting to another mode was no longer relevant, patients were usually placed back on ASV immediately or when weaning from mechanical ventilation was desired. Duration of ventilation and total number of ventilation days in each mode were documented. Patients who were ventilated with ASV for most of their ventilation duration (more than 50% of the time) were also documented.
Weaning was performed preferably using ASV. As the patient's spontaneous efforts and respiratory mechanics (compliance, resistance) improved, the percentage of target minute volume was gradually manually reduced to a minimum of 60%. Patients were switched to PSV if ASV failed to decrease pressure support levels below 14 cm[H.sub.2]0 due to impaired pulmonary mechanics. Pressure support levels were subsequently manually decreased according to patient effort and respiratory pattern. Patients were not routinely given a spontaneous breathing trial, as this is not part of our routine weaning policy. Extubation was performed at the discretion of the attending physician when pressure support levels (applied by the ASV or manually adjusted in PSV) were below 10 cm[H.sub.2]O and if an adequate cough, conscious level and a patent airway were demonstrated.
Severity of illness was calculated using the Acute Physiology and Chronic Health Evaluation II (APACHE II) score in the first 24 hours of ICU admission. Documented respiratory complications included ventilator-associated pneumonia (VAP), which was defined as the need for antibiotics or an antibiotic change due to a presumed respiratory infection developing in patients ventilated for 48 hours or more. (14) Extubation success was defined as discharge after being weaned from mechanical ventilation. Weaning or extubation failure was defined as patients discharged from the ICU with an ongoing need for mechanical ventilation.
A waiver for the requirement of informed consent for data collection was obtained from the Institutional Review Board.
Data were collected and analysed with the JMP 8.1 (SAS). Normally distributed variables are presented as means and non-normally distributed variables as means and medians. To better define the characteristics of patients who failed ASV, compared with all other ventilated cases, we performed comparative analysis of categorical variables using the Pearson chi-square test. P-values of 0.05 or less were considered statistically significant.
During the study period 1 985 patients were admitted to the MICU; 1 220 were ventilated (61.5%). Patient characteristics and outcomes of ventilated cases are summarised in Table 1. Mean length of hospital stay before ICU admission was 8.1 days (standard deviation (SD) [+ or -] 17; median 2). The mean APACHE II score was 27 [+ or -] 10 with a calculated predicted mortality of 57%, most patients having underlying chronic disease. Overall hospital mortality was 45.7%, giving a standardised mortality ratio of 0.8. The most frequent causes of ICU death were sepsis and multi-organ failure.
Table 2 summarises the descriptive data of the application of ventilation in our patient group, including modes of ventilation, timing and indications for ventilation.
Table 3 demonstrates the duration of ventilation in the different ventilation modes. Mean length of ventilation (all modes) was more than 10 days with a median of 6 days. Sedation was required in 812 patients (67%) for a median length of 2 days. Nine hundred and forty-eight patients were ventilated with ASV for more than 50% of the time (93%).
Sixty-eight patients (6%) required transition from ASV mode to pressure control mode. The primary indication for switching from ASV to PCV was to satisfy our technical requirement for a stable tidal volume to allow administration of inhaled nitric oxide (NO), which is delivered through a continuous-flow device precisely and conservatively, to avoid excessive wastage. Patients were placed back on ASV when NO was discontinued and/or when weaning from ventilation was required. On rare occasions, patient-ventilator asynchrony (usually rapid shallow breathing); precipitated a change to more heavy sedation, and more rarely muscle relaxation and PCV was introduced to achieve the desired controlled minute ventilation.
Comparison of this patient group, who required a mode of ventilation other than ASV, with all other ventilated patients showed that 47% v. 35% (p=0.05) had sepsis or septic shock, 41% v. 35% had pneumonia (p=0.3), 28% v. 5% (p<0.0001) were diagnosed with ARDS, and 16% v. 2% (p<0.0001) had interstitial fibrosis. The mean APACHE II score in this group was 31.3, with an ICU mortality of 79% and hospital mortality of 87%. Fifteen (22%) developed pneumothorax compared with 2% in other ventilated cases (p<0.001). Patients who do not tolerate ASV therefore represent a group of sicker patients with a higher rate of ARDS and interstitial fibrosis and a poorer prognosis.
Ninety-two patients (7.5%) were ventilated for more than 28 days (mean 42.8 [+ or -] 15 days). Thirty-nine per cent in this group were admitted with pneumonia and 9% with chronic obstructive pulmonary disease (COPD). Twelve of these patients (13%) were chronically ventilated before admission, 68 (74%) required insertion of a tracheostomy in the ICU, and 55 (58%) were discharged ventilated from the ICU.
Complications and ventilation outcomes are summarised in Table 4. Respiratory complications included VAP in 288 patients (23.6%), giving an incidence of 23.1/1 000 ventilated days. Pneumothorax developed in a total of 42 patients (3% of all ventilated patients), of whom only 10 were ventilated with ASV at the time (less than 1% of all patients ventilated with ASV). Twenty-three per cent of patients developed sepsis in the ICU, 55% required inotropic support, and 19% needed haemodialysis.
Weaning from mechanical ventilation was mostly (86%) performed with ASV. In 54 cases (4%), pressure support mode was used after ASV had failed to wean completely. The rate of extubation success for all patients was 81%, and that for patients weaned with pressure support mode was 54%. Tracheostomy was required in 159 (13%) of all ventilated patients (Table 4). Seventy-seven patients were admitted to the MICU chronically ventilated with a tracheostomy in place. Indications for tracheostomy in the ICU included facilitation of chronic ventilation (57%), as part of the weaning process (28%), and for upper airway problems (4%).
Two hundred and thirty-five patients (19%) were discharged ventilated from the ICU to the general ward or to a chronic ventilation care facility (Table 4), of whom 42 had been admitted to the MICU with a tracheostomy in place. The mean duration of ventilation in this patient group was 18.7 [+ or -] 17.5 days, with a median of 14 days.
We have described our experience with intelligent ASV as the preferred mode of ventilatation in the MICU. This is the first such report of a large group of complex medical patients ventilated with ASV for relatively long periods of time. The mean age of our patient population was 63.1 years and the mean APACHE II score was 27, suggesting relatively high severity of illness. The APACHE II score might have been affected by the patients' age and a median Glasgow Coma Score (GCS) of 9, attributed to impaired neurological status and/or sedation. Our complication rates were low, and weaning rates were acceptable for this complicated patient group. Most previous reports examined smaller groups of patients, mostly surgical, who needed ventilation for much shorter periods of time. (15-17)
ASV requires that an adequate and optimal target minute volume is set according to the ideal body weight. (17) Calculations of dead space, peak inspiratory pressures and respiratory function such as compliance, resistance and expiratory time constant are measured, so that optimal target volumes and rates are provided. (18-19) Target volumes are provided by increasing inspiratory pressures as necessary, and these are decreased as patient respiratory function and effort improve. (20) Few manipulations of the ventilator are therefore required, (21) and the automated controller provides rapid adaptation to changing ventilator needs of ventilated patients. (8,22) Our unit does not employ respiratory therapists trained in setting ventilators. Such changes are therefore left to the ICU medical staff, who are not always available to respond quickly to changing ventilation requirements. The ASV mode therefore reduces the need for manipulation of the ventilator settings, as it adjusts automatically to altered lung mechanics and patient effort, compensating for reduced staffing levels.
Previous studies have tested the efficiency, safety and adaptability of ASV in various lung diseases, in patients undergoing general anaesthesia, and during position changes and transition between two- and one-lung ventilation. (11,23,24) Tassaux and colleagues demonstrated improvement in patient-ventilator interaction and reduction in signs of asynchrony with ASV compared with synchronised intermittent mandatory ventilation (SIMV) and pressure-support ventilation (PS) in patients during early weaning with partial ventilator support. (6) In their study reporting the use of ASV as the primary mode of ventilation in a mixed ICU (322 patients), Arnal and colleagues found that ASV was used in 98% of invasive ventilation days, and appropriately selected different rate/volume combinations for patients with different types of underlying lung disease, including ARDS and COPD. (25)
ASV has been shown to hasten weaning from ventilation compared with other modes. (26) It can appropriately decrease ventilator support in patients with chronic respiratory failure who tolerated a conventional weaning trial, suggesting that this mode may facilitate respiratory weaning. (20) ASV is practical as a respiratory weaning protocol in post-surgical patients, and it may accelerate tracheal extubation and simplify ventilatory management in patients after cardiac surgery. (21,27) It has also been shown to be a safe weaning modality, as patient demands are adequately met during weaning from ventilation. (20,28) In our patient population, which included complicated medical patients with chronic diseases, 6% of whom were chronically ventilated before admission to the ICU, our weaning failure rate (patients discharged ventilated form the ICU) was 19%. This may be viewed as high, but it must be stressed that usual practice in Israel does not include 'terminal weaning' and withdrawal of ventilation, so patients who are ventilator dependent usually undergo tracheostomy and remain fully or partially ventilated indefinitely. In our experience, ASV is highly suitable for patients with COPD and for weaning most patients from ventilatory support. There was only minimal need to convert any patient from ASV to other modalities during the weaning phase, with only partial success. Changes to modes other than ASV were only required in a small percentage of patients.
ASV has been shown to be safe in a model of ARDS, by limiting peak pressures and reducing tidal volumes. (29) We found that most patients in our database with ARDS tolerated ASV well throughout the required ventilation of their lung disease. However, a minority of patients (6%) required transition to PCV, due to patient-ventilator asynchrony, severe hypoxia necessitating inhaled NO or a desire by the attending clinicians to provide more inverse ratio ventilation than the ASV controller allowed. This patient group was sicker and required a more sophisticated ventilation approach, such as the administration of muscle relaxants, deeper sedation, induced hypothermia and inhaled NO. Other potential problems with ASV include that fact that ASV guarantees a minimum preset minute volume but not a constant tidal volume. However, we found that when targeting optimal ideal body weight and thus target tidal volumes, ASV will provide adequate pressures to achieve these volumes optimally.
Our overall pneumothorax rate was 3%, which is comparable to that reported in the literature. (30) Less than 1% of patients who were ventilated using only ASV developed pneumothorax. Most patients who developed a pneumothorax were either ventilated with other modes or the pneumothorax was related to procedures and central line insertion (Table 4). A subgroup of patients with severe respiratory dysfunction and hypoxaemia, requiring PCV, had a higher rate of pneumothorax compared with other ventilated patients (22% v. 2%, p<0.001). Although this may represent selection bias, it raises the need for comparative studies looking at the safety of PCV versus closed-loop ventilation in such high-risk patients.
ASV has been the sole mode of ventilation in some chronic care facilities in Israel for several years (31) and has been shown to be cost-effective, safe and efficient in ventilating and weaning patients with chronic respiratory failure.
Limitations to our study include the fact that we chose not to randomise patients to different modes of ventilation, but rather to describe our experience with the ASV modality of ventilation as the preferred mode in our ICU. Although we found that some patients required change to a different mode of ventilation, mostly those with difficult oxygenation and more severe disease, we did not randomise these patients to receive ASV or another mode. Our data suggest, however, that further studies are required to assess the precise limitations of ASV in patients with severe pulmonary restriction and hypoxia who require more inverse ratio ventilation and a stable tidal volume to facilitate NO inhalation. We also did not use a weaning protocol, which might have standardised our practices with different ventilation modes. ASV, however, automatically weans most patients and therefore requires less manipulation of the ventilator during the weaning process. The diagnosis of VAP was defined as the need for antibiotics or an antibiotic change due to a presumed respiratory infection developing in patients ventilated for 48 hours or more. This definition may cause an overestimation of the actual VAP rate, as it may include patients with a false-positive diagnosis of VAP. Our patient population consists of complicated medical patients. The implications of our findings to surgical patients may require further research.
The recent introduction of automatic Fi[O.sub.2] and PEEP adjustment to the ASV design, which uses feedback from on-line monitored Sa[O.sub.2] and end-tidal C[O.sub.2] as well as heart-lung interaction parameters, will in theory greatly enhance the intelligent ventilation capability of these microprocessor-controlled ventilators. (32) We are currently studying this advanced S1 version of IntelliVent (Hamilton Medical AG) in our MICU during the mechanical ventilation of patients with critical ARDS who also require NO administration. Our initial experience with this has been most positive, avoiding the need for any mode changes to the fully automated closed-loop system.
ASV is an acceptable mode of ventilation for complicated medical patients in the MICU, with a good weaning success rate and low complication rate. In critical ARDS and other forms of severe restrictive lung disease, e.g. pneumonitis, interstitial fibrosis and chest stiffness, the temporary use of PCV is preferred to the basic ASV mode in more heavily sedated and paralysed patients when inhaled NO is used, to optimise patient-ventilator synchrony and oxygenation.
In the future, use of the IntelliVent may avoid this need to change from ASV to PCV in critical ARDS patients.
Acknowledgements. We would like to thank Professor P V van Heerden, who worked for 6 months in our MICU on sabbatical, for his assistance in reviewing the manuscript. A part of this work was presented as a poster at the Annual Congress of the European Society of Intensive Care Medicine, Berlin, Germany, in October 2007, by P D Levin.
Conflict of interest. Drs S Sviri, A Bayya, P D Levin, R Khalaila and D M Linton have no conflicts of interest in this study. Mrs I Stav was paid from the internal unit funds for preparation of the database.
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Medical Intensive Care Unit, Hadassah-Hebrew University Medical Center, Jerusalem, Israel
Sigal Sviri, MD
Abed Bayya, MD
General Intensive Care Unit, Hadassah-Hebrew University Medical Center, Jerusalem, Israel
Phillip D Levin, MD
Medical Intensive Care Unit, Hadassah-Hebrew University Medical Center, Jerusalem, Israel
Rabia Khalaila, PhD
David M Linton, MD
Corresponding author: S Sviri (firstname.lastname@example.org)
Table 1. Patient profiles and outcomes Ventilated patients (N = 1 220) Age (years), mean (SD) 63.1 (18.8) Gender 60% male, 40% female Source of admission, n (%) Emergency room 382 (32) Ward 652 (53) Other ICU 75 (6) Other hospital 109 (9) Chronic disease profile, n (%) Respiratory disease 410 (34) Cardiac disease 470 (39) Renal failure 283 (23) Liver failure 125 (10) Diagnostic categories, n (%) Respiratory 663 (54) Infectious 480 (39) Cardiac 234 (19) Renal 238 (20) GI and hepatic 137 (11) Neurological 176 (14) Haematological 96 (8) Metabolic 47 (4) Other 174 (14) APACHE II score, mean (SD) 27 (10) Predicted mortality (%) 57 (28) GCS (median) 9 Outcomes Length of ICU stay (days), 12.8 (13.5), median 9 mean (SD) Total length of hospital stay (days), 30.4 (30.2), median 21 mean (SD) Died in ICU, n (%) 406 (33.3) Died in hospital, n (%) 558 (45.7) Cause of death in ICU (N = 406), n (%) Sepsis and MOF 288 (71) Respiratory 38 (9) Cardiac 32 (8) Anoxic brain damage 18 (4) GI = gastrointestinal; APACHE = Acute Physiological and Chronic Health Evaluation; GCS = Glasgow Coma Scale; MOF = multi-organ failure. Table 2. Descriptive data of ventilation Time of ventilation (N = 1 220), n (%) At admission 931 (76) After admission 289 (24) Primary indication for ventilation (N = 784), n (%) Respiratory failure 474 (60.5) Shock 111 (14) CPR 48 (6) Neurological 100 (13) Procedure 11 (1.5) Chronic 14 (2) Other 26 (3) Modes of ventilation (N = 1 214), n (%) ASV 1 016 (84) SIMV 258 (21) Pressure control 152 (13) Assist control 34 (3) Pressure support 141 (12) CPR = cardiopulmonary resuscitation; ASV = adaptive support ventilation; SIMV = synchronised intermittent mandatory ventilation. Table 3. Length of ventilation (days) Mode Total ventilation Median All modes (N = 1 217) * 12 467 6 ASV (n = 1 212) 9 220 6 SIMV (n = 176) 965 3 Pressure control (n = 124) 628 3 Pressure support (n = 131) 662 3 Relative ventilation days with ASV 9 220/12 467 (74%) Mode Mean (SD) All modes (N = 1 217) * 10.2 (12) ASV (n = 1 212) 9.1 (10.5) SIMV (n = 176) 5.5 (5.9) Pressure control (n = 124) 5.1 (6.2) Pressure support (n = 131) 5.05 (6.1) Relative ventilation days with ASV * Data missing for 3 patients. ASV = adaptive support ventilation; SIMV = synchronised intermittent mandatory ventilation. Table 4. Complications and ventilation outcomes Complications, n (%) Pneumothorax--all causes (N = 42) 42 (3) Ventilation with ASV 10 (24) Ventilation with other modes 16 (38) Central line 6 (14) Intubation and procedures 10 (24) Ventilator-associated pneumonia * 288 (24) Sepsis in the ICU 284 (23) Ventilation outcomes, n (%) Tracheostomy in the ICU 159 (13) Chronic ventilation with tracheostomy 77 (6) Failed extubation ([dagger]) 97/784 (12) Ventilated on discharge 235/1 220 (19) Data available for 784/1 220 patients. * Defined as the need for antibiotics or an antibiotic change due to a presumed respiratory infection developing in patients ventilated for 48 hours or more. ([dagger]) Extubation requiring re-intubation within 48 hours, including self-extubation.
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|Author:||Sviri, Sigal; Bayya, Abed; Levin, Phillip D.; Khalaila, Rabia; Stav, Ilana; Linton, David M.|
|Publication:||Southern African Journal of Critical Care|
|Date:||Jul 1, 2012|
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