Adaptive Support Ventilation (ASV).

Adaptive Support Ventilation is a novel ventilation mode, a closed-loop control mode that may switch automatically from a PCV-like behaviour to an SIMV-like or PSV-like behaviour, according to the patient status. The operating principles are based on pressure-controlled SIMV with pressure levels and SIMV rate automatically adjusted according to measured lung mechanics at each breath. ASV provided a safe and effective ventilation in patients with normal lungs, restrective or obstructive diseases. In cardiac surgery tracheal extrubation was faster in ASV patients then in controls. In the early weaning phase of acute ventilatory insufficiency the need of resetting ventilator parameters was decreased, suggesting potential benefit for patient care.

Principles and history of closed-loop controlled ventilation.

Respiratory management of intubated patients is a complex problem, even if airway management, sedation, and infection control are excluded and only the limited problem of "how to set a ventilator" is considered. Four dimensions shape the overall strategy for setting a ventilator: time, physiologic task, primary lung disease, and general therapeutic approach. Initiation (start-up), maintenance, and weaning are the principal dimensions in time. This article discusses the closed-loop control method.

Assessment of pulmonary mechanics in mechanical ventilation: effects of imprecise breath detection, phase shift and noise.

OBJECTIVE: In mechanical ventilation, the assessment of pulmonary mechanics, mainly of total compliance (Crs), total resistance (Rrs), and intrinsic positive end-expiratory pressure (PEEPint), is clinically important. By using airway pressure (Paw) and flow (V'aw), the least squares fit (LSF) method allows the continuous calculation of these parameters. The objective of this work was to study the influence of imprecise breath detection, phase shift between airway pressure and flow signals, and noise on the determination of Crs, Rrs, and PEEPint. METHODS: Paw and V'aw were mathematically simulated as well as recorded in mechanically ventilated patients. Crs, Rrs, and PEEPint were computed off-line using the LSF method. The boundaries of the breath data window and the phase relationship between Paw and V'aw signals were manipulated and noise was superimposed. RESULTS: Both simulated and patient data gave similar results. Crs and Rrs were not sensitive to imprecise breath detection. If the first portion of the breath was missed, the mean relative error on PEEPint was 20% or 53% when the exact beginning of inspiration was missed by 0.1 or 0.3 sec, respectively. Paw lag of 66 ms with respect to V'aw yielded a relative error of -15 +/- 4% (mean +/- SD) for Rrs, -5 +/- 2% for Crs, and +13 +/- 16% for PEEPint. Paw lead of 66 ms with respect to V'aw yielded a relative error of +5 +/- 4% for Rrs, +7 +/- 3% for Crs, and +14 +/- 18% for PEEPint. Noise had very little impact on the accuracy of Crs, Rrs, and PEEPint. CONCLUSIONS: We conclude that the LSF method allows the assessment of Crs, Rrs, and PEEPint even with high levels of noise in patients with normal lungs provided that Paw and V'aw signals are precisely synchronised and a reliable breath detection algorithm is used.

Changes in pulmonary mechanics after fiberoptic bronchoalveolar lavage in mechanically ventilated patients.

OBJECTIVE: We prospectively assessed the impact of bronchoalveolar lavage (BAL) on respiratory mechanics in critically ill, mechanically ventilated patients. STUDY DESIGN: Mechanically ventilated patients underwent BAL of one lung segment using 5 x 20 ml of sterile, physiologic saline with a temperature of 25-28 degrees C. The fractional inspired oxygen was increased to 1.0, but ventilator settings were otherwise left unchanged. Static pulmonary compliance, pulmonary resistance, alveolar ventilation, and serial dead space were measured 60 min and 2 min before and 8, 60, and 180 min after BAL to assess the consequences of the procedure. In addition, blood gases [partial pressure of carbon dioxide in arterial blood (PaCO2) and arterial oxygen tension (PaO2)], hemodynamic variables (heart rate, systolic and diastolic blood pressure), and body temperature were recorded at the same time points. SETTING: Intensive care unit of a university hospital. PATIENTS: 18 consecutive critically ill, mechanically ventilated patients. RESULTS: Pulmonary compliance decreased by 23% (p < 0.05) and pulmonary resistance increased by 22% (p < 0.05) shortly after BAL. The changes in pulmonary compliance and resistance were more than 30% in one third of the patient population. One hour after the procedure, PaO2 was significantly lower and PaCO2 significantly higher than before the procedure. Three hours after the procedure, pulmonary resistance returned to pre-BAL values but compliance remained 10% below baseline values (p < 0.05). CONCLUSION: BAL in mechanically ventilated patients is associated with deterioration of pulmonary mechanics and function.

Unfavorable mechanical effects of heat and moisture exchangers in ventilated patients.

OBJECTIVE: To investigate the mechanical effects of artificial noses. SETTING: A general intensive care unit of a university hospital. PATIENTS: 10 patients in pressure support ventilation for acute respiratory failure. INTERVENTIONS: The following three conditions were randomly tested on each patient: the use of a heated humidifier (control condition), the use of a heat and moisture exchanger without filtering function (HME), and the use of a combined heat and moisture exchanger and mechanical filter (HMEF). The pressure support level was automatically adapted by means of a closed-loop control in order to obtain constancy, throughout the study, of patient inspiratory effort as evaluated from airway occlusion pressure at 0.1 s (P0.1). Patient's ventilatory pattern, P0.1, work of breathing, and blood gases were recorded. MEASUREMENTS AND MAIN RESULTS: The artificial noses increased different components of the inspiratory load: inspiratory resistance, ventilation requirements (due to increased dead space ventilation), and dynamic intrinsic positive end-expiratory pressure (PEEP). The additional load imposed by the artificial noses was entirely undertaken by the ventilator, being the closed-loop control of P0.1 effective to maintain constancy of patient inspiratory work by means of adequate increases in pressure support level. CONCLUSIONS: The artificial noses cause unfavorable mechanical effects by increasing inspiratory resistance, ventilation requirements, and dynamic intrinsic PEEP. Clinicians should consider these effects when setting mechanical ventilation and when assessing patients' ability to breathe spontaneously.

New aspects of pulmonary mechanics: "slowly" distensible compartments of the respiratory system, identified by a PEEP step maneuver.

OBJECTIVE: The aims of the present study were 1) to evaluate a method for identification of "slowly" distensible compartments of the respiratory system (rs), which are characterized by long mechanical time constants (RC) and 2) to identify "slowly" distensible rs-compartments in mechanically ventilated patients. DESIGN: Prospective study on a physical lung model. SETTING: Intensive Care Unit, University Hospital, Tübingen. PATIENTS AND PARTICIPANTS: 19 patients with severe lung injury (acute respiratory distress syndrome, ARDS) and on 10 patients with mild lung injury. MEASUREMENTS AND RESULTS: Positive end-expiratory pressure (PEEP)-increasing and -decreasing steps of about 5 cmH2O were applied and the breath-by-breath differences of inspiratory and expiratory volumes (delta V) were measured. The sequence of delta Vs were analyzed in terms of volume change in the "fast" compartment (Vfast), the "slow" compartment (Vslow), total change in lung volume (delta VL) and mechanical time constant of the slow compartment (RCslow). Thirty-eight measurements in a lung model revealed a good correlation between the preset Vslow/delta VL and Vslow/delta VL measured: r2 = 0.91. The Vslow/delta VL measured amounted to 0.94 +/- 0.15 of Vslow/delta VL in the lung model. RCslow measured was 0.92 +/- 0.43 of the RCslow reference. Starting from a PEEP level of 11 cmH2O PEEP-increasing and PEEP-decreasing steps were applied to the mechanically ventilated patients. Three out of ten patients with mild lung injury (30%) and 7/19 patients with ARDS (36.8%) revealed "slowly" distensible rs-compartments in a PEEP-increasing step, whereas 15/19 ARDS patients and 1/10 patients with mild lung injury showed "slowly" distensible rs-compartments in a PEEP-decreasing step (78.9% vs 10%, P < 0.002, chi-square test). CONCLUSIONS: The gas distribution properties of the respiratory system can be easily studied by a PEEP-step maneuver. The relative contribution of the "slow" units to the total increase of lung volume following a PEEP step could be adequately assessed. "Slowly" distensible rs-compartments could be detected in patients with severe and mild lung injury, however significantly more ARDS patients revealed "slow" rs-compartments in PEEP-decreasing steps. The influence of "slowly" distensible rs-compartments on pulmonary gas exchange is unknown and has yet to be studied.

Closed-loop control of airway occlusion pressure at 0.1 second (P0.1) applied to pressure-support ventilation: algorithm and application in intubated patients.

OBJECTIVE: Airway occlusion pressure at 0.1 sec (P0.1) is an index of respiratory center output. During pressure-support ventilation, P0.1 correlates with the mechanical output of the inspiratory muscles and has an inverse relationship with the amount of pressure-support ventilation. Based on these observations, we designed a closed-loop control which, by automatically adjusting pressure-support ventilation, stabilizes P0.1, and hence patient inspiratory activity, at a desired target. The purpose of the study was to demonstrate the feasibility of the method, rather than its efficacy or even its influence on patient outcome. DESIGN: Prospective, randomized trial. SETTING: A general intensive care unit of a university hospital in Italy. PATIENTS: Eight stable patients intubated and ventilated with pressure-support ventilation for acute respiratory failure. INTERVENTIONS: Patients were transiently connected to a computer-controlled ventilator on which the algorithm for closed-loop control was implemented. The closed-loop control was based on breath by breath measurement of P0.1, and on comparison with a target set by the user. When actual P0.1 proved to be higher than the target value, the P0.1 controller automatically increased pressure-support ventilation, and decreased it when P0.1 proved to be lower than the target value. For safety, a volume controller was also implemented. Four P0.1 targets (1.5, 2.5, 3.5, and 4.5 cm H2O) were applied at random for 15 mins each. MEASUREMENTS AND MAIN RESULTS: The closed-loop algorithm was able to control P0.1, with a difference from the set targets of 0.59 +/- 0.27 (SD) cm H2O. CONCLUSIONS: The study shows that P0.1 can be automatically controlled by pressure-support ventilation adjustments with a computer. Inspiratory activity can thus be stabilized at a level prescribed by the physician.

The automatic selection of ventilation parameters during the initial phase of mechanical ventilation.

OBJECTIVE: To test a method that allows automatic set-up of the ventilator controls at the onset of ventilation. DESIGN: Prospective randomized crossover study. SETTING: ICUs in one adult and one children's hospital in Switzerland. PATIENTS: Thirty intubated stable, critically ill patients (20 adults and 10 children). INTERVENTIONS: The patients were ventilated during two 20-min periods using a modified Hamilton AMADEUS ventilator. During the control period the ventilator settings were chosen immediately prior to the study. During the other period individual settings were automatically determined by the ventilatior (AutoInit). MEASUREMENTS AND RESULTS: Pressure, flow, and instantaneous CO2 concentration were measured at the airway opening. From these measurements, series dead space (V(DS)), expiratory time constant (RC), tidal volume (VT, total respiratory frequency (f(tot), minute ventilation (MV), and maximal and mean airway pressure (Paw, max and Paw, mean) were calculated. Arterial blood gases were analyzed at the end of each period. Paw, max was significantly less with the AutoInit ventilator settings while f(tot) was significantly greater (P < 0.05). The other values were not statistically significant. CONCLUSIONS: The AutoInit ventilator settings, which were automatically derived, were acceptable for all patients for a period of 20 min and were not found to be inferior to the control ventilator settings. This makes the AutoInit method potentially useful as an automatic start-up procedure for mechanical ventilation.

Noninvasive evaluation of instantaneous total mechanical activity of the respiratory muscles during pressure support ventilation.

OBJECTIVE: The measurement of esophageal pressure (Pes) is the conventional method for the evaluation of the forces applied to the respiratory system by the respiratory muscles. As an alternative to Pes measurement, we propose the calculation of the instantaneous net pressure applied by the respiratory muscles [Pmusc(t)]. DESIGN: Prospective, randomized study. SETTING: A general ICU of a university hospital. PATIENTS: Eight intubated patients submitted to pressure support ventilation for acute respiratory failure. INTERVENTIONS: Four different levels of pressure support were used to unload progressively the respiratory muscles. Pmusc(t) was calculated at all levels of pressure support and compared with Pes corrected for chest wall load as a reference. Pmusc(t) was further used to calculate inspiratory work of breathing, which in turn was compared with data obtained with the conventional method. MEASUREMENTS AND RESULTS: Airway pressure, airflow, and Pes were measured. Both for amplitude and for timing, Pmusc(t) showed good agreement with reference measurements. Work of breathing as calculated from Pmusc(t) agreed well with the measurement obtained with the conventional method (mean difference, 0.057 +/- 0.157 J). CONCLUSIONS: Noninvasive evaluation of Pmusc(t) allows extended monitoring of mechanical ventilation, which is particularly interesting for pressure preset ventilation modes.

Simple method to measure total expiratory time constant based on the passive expiratory flow-volume curve.

OBJECTIVE: In intubated, mechanically ventilated patients, inspiration is forced by externally applied positive pressure. In contrast, exhalation is passive and depends on the time constant of the total respiratory system. The expiratory time constant is thus an important determinant of mechanical ventilation. The aim of this study was to evaluate a simple method for measuring the expiratory time constant in ventilated subjects. DESIGN: Prospective study using a lung simulator and ten dogs. SETTING: University hospital. SUBJECTS: Commercially available lung simulator and ten greyhound dogs. INTERVENTIONS: Different expiratory time constants were set on the lung simulator. In the dogs, the endotracheal tube was clamped to increase airways resistance by 22.5 cm H2O/(L/sec) and the lungs were injured with hydrochloric acid to decrease total respiratory compliance by 16 mL/cm H2O. This procedure resulted in a wide range of expiratory time constants. MEASUREMENTS AND MAIN RESULTS: Pneumotachography was used to measure flow and volume. The ratio of exhaled volume and peak flow was calculated from these signals, corrected for the limited exhalation time yielding the "calculated expiratory time constant" and compared with the actual expiratory time constant. The typical error was +/- 0.19 sec for the lung simulator and +/- 0.15 sec for the dogs. CONCLUSIONS: The volume and peak flow corrected for limited exhalation time is a good estimate of the total expiratory time constant in passive subjects and may be useful for the titration of mechanical ventilation.

Respiratory mechanics by least squares fitting in mechanically ventilated patients: applications during paralysis and during pressure support ventilation.

OBJECTIVE: To evaluate a least squares fitting technique for the purpose of measuring total respiratory compliance (Crs) and resistance (Rrs) in patients submitted to partial ventilatory support, without the need for esophageal pressure measurement. DESIGN: Prospective, randomized study. SETTING: A general ICU of a University Hospital. PATIENTS: 11 patients in acute respiratory failure, intubated and assisted by pressure support ventilation (PSV). INTERVENTIONS: Patients were ventilated at 4 different levels of pressure support. At the end of the study, they were paralyzed for diagnostic reasons and submitted to volume controlled ventilation (CMV). MEASUREMENTS AND RESULTS: A least squares fitting (LSF) method was applied to measure Crs and Rrs at different levels of pressure support as well as in CMV. Crs and Rrs calculated by the LSF method were compared to reference values which were obtained in PSV by measurement of esophageal pressure, and in CMV by the application of the constant flow, end-inspiratory occlusion method. Inspiratory activity was measured by P0.1. In CMV, Crs and Rrs measured by the LSF method are close to quasistatic compliance (-1.5 +/- 1.5 ml/cmH2O) and to the mean value of minimum and maximum end-inspiratory resistance (+0.9 +/- 2.5 cmH2O/(l/s)). Applied during PSV, the LSF method leads to gross underestimation of Rrs (-10.4 +/- 2.3 cmH2O/(l/s)) and overestimation of Crs (+35.2 +/- 33 ml/cmH2O) whenever the set pressure support level is low and the activity of the respiratory muscles is high (P0.1 was 4.6 +/- 3.1 cmH2O). However, satisfactory estimations of Crs and Rrs by the LSF method were obtained at increased pressure support levels, resulting in a mean error of -0.4 +/- 6 ml/cmH2O and -2.8 +/- 1.5 cmH2O/(l/s), respectively. This condition was coincident with a P0.1 of 1.6 +/- 0.7 cmH2O. CONCLUSION: The LSF method allows non-invasive evaluation of respiratory mechanics during PSV, provided that a near-relaxation condition is obtained by means of an adequately increased pressure support level. The measurement of P0.1 may be helpful for titrating the pressure support in order to obtain the condition of near-relaxation.

Automatic weaning from mechanical ventilation using an adaptive lung ventilation controller.

STUDY OBJECTIVE: To evaluate a new method of closed-loop mechanical ventilation using an adaptive lung ventilation (ALV) controller in patients with different pathologic causes of respiratory failure at a time when they first met standard weaning criteria. STUDY DESIGN: Prospective, open, selected case study. SETTING: The 10-bed, multidisciplinary respiratory intensive care unit at Groote Schuur hospital, which is a teaching unit of the University of Cape Town. PATIENTS: Twenty-seven patients (9 patients in each of 3 groups: normal lungs, parenchymal lung disease, and COPD) who required prolonged mechanical ventilation and who met standard weaning criteria were included. Our institutional committee for ethical research approved the study and informed consent was obtained. INTERVENTIONS: The patients were mechanically ventilated and had daily measurements of vital capacity, respiratory rate, and arterial blood gas analysis until they met standard weaning criteria. On the day that each patient met the weaning criteria, a closed loop control algorithm providing ALV was implemented on a modified ventilator (Hamilton AMADEUS) with a PC-based lung function analyzer. After measuring gross alveolar ventilation, patients were placed in ALV and ventilatory and hemodynamic parameters were measured at baseline, 5 min, 30 min, and 2 h. Pertinent parameters measured included airway pressures, pressure support levels, respiratory rates, rapid shallow breathing indices, airway resistance indices, and patient respiratory drive and work indices. MEASUREMENTS AND RESULTS: In 22 patients, ALV reduced pressure support to 5 cm H2O and an intermittent mandatory ventilation rate of 4 breaths/min within 30 min, and all but 1 of these patients were successfully extubated within 24 h. In four patients, pressure support was maintained by ALV at a mean level of 14.6 cm H2O +/- for 2 h and these patients were recorded as having failed to wean. There was a measurable difference in an index of airway resistance relative to muscular activity between the successfully weaned and failed wean patients with COPD during the attempted wean by the ALV controller. CONCLUSIONS: ALV will provide a safe, efficient wean and will respond immediately to inadequate ventilation in patients when standard weaning criteria are met.

Automatic selection of tidal volume, respiratory frequency and minute ventilation in intubated ICU patients as start up procedure for closed-loop controlled ventilation.

OBJECTIVE: Before a patient can be connected to a mechanical ventilator, the controls of the apparatus need to be set up appropriately. Today, this is done by the intensive care professional. With the advent of closed loop controlled mechanical ventilation, methods will be needed to select appropriate start up settings automatically. The objective of our study was to test such a computerized method which could eventually be used as a start-up procedure (first 5-10 minutes of ventilation) for closed-loop controlled ventilation. DESIGN: Prospective Study. SETTINGS: ICU's in two adult and one children's hospital. PATIENTS: 25 critically ill adult patients (age > or = 15 y) and 17 critically ill children selected at random were studied. INTERVENTIONS: To stimulate 'initial connection', the patients were disconnected from their ventilator and transiently connected to a modified Hamilton AMADEUS ventilator for maximally one minute. During that time they were ventilated with a fixed and standardized breath pattern (Test Breaths) based on pressure controlled synchronized intermittent mandatory ventilation (PCSIMV). MEASUREMENTS AND MAIN RESULTS: Measurements of airway flow, airway pressure and instantaneous CO2 concentration using a mainstream CO2 analyzer were made at the mouth during application of the Test-Breaths. Test-Breaths were analyzed in terms of tidal volume, expiratory time constant and series dead space. Using this data an initial ventilation pattern consisting of respiratory frequency and tidal volume was calculated. This ventilation pattern was compared to the one measured prior to the onset of the study using a two-tailed paired t-test. Additionally, it was compared to a conventional method for setting up ventilators. The computer-proposed ventilation pattern did not differ significantly from the actual pattern (p > 0.05), while the conventional method did. However the scatter was large and in 6 cases deviations in the minute ventilation of more than 50% were observed. CONCLUSIONS: The analysis of standardized Test Breaths allows automatic determination of an initial ventilation pattern for intubated ICU patients. While this pattern does not seem to be superior to the one chosen by the conventional method, it is derived fully automatically and without need for manual patient data entry such as weight or height. This makes the method potentially useful as a start up procedure for closed-loop controlled ventilation.

An adaptive lung ventilation controller.

Closed loop control of ventilation is traditionally based on end-tidal or mean expired CO2. The controlled variables are the respiratory rate RR and the tidal volume VT. Neither patient size or lung mechanics were considered in previous approaches. Also the modes were not suitable for spontaneously breathing subjects. This report presents a new approach to closed loop controlled ventilation, called Adaptive Lung Ventilation (ALV). ALV is based on a pressure controlled ventilation mode suitable for paralyzed, as well as spontaneously breathing, subjects. The clinician enters a desired gross alveolar ventilation (V'gA in l/min), and the ALV controller tries to achieve this goal by automatic adjustment of mechanical rate and inspiratory pressure level. The adjustments are based on measurements of the patient's lung mechanics and series dead space. The ALV controller was tested on a physical lung model with adjustable mechanical properties. Three different lung pathologies were simulated on the lung model to test the controller for rise time (T90), overshoot (Ym), and steady state performance (delta max). The pathologies corresponded to restrictive lung disease (similar to ARDS), a "normal" lung, and obstructive lung disease (such as asthma). Furthermore, feasibility tests were done in 6 patients undergoing surgical procedures in total intravenous anesthesia. In the model studies, the controller responded to step changes between 48 seconds and 81 seconds. It did exhibit an overshoot between 5.5% and 7.9% of the setpoint after the step change.(ABSTRACT TRUNCATED AT 250 WORDS)

A simple method to estimate functional residual capacity in mechanically ventilated patients.

OBJECTIVE: The aim of the present study was to evaluate a simplified method for FRC measurement. DESIGN: Accuracy and precision of the method were assessed in a physical lung model; reproducibility was tested in 10 mechanically ventilated patients. In each patient FRC was measured at three PEEP levels. SETTING: Post-operative intensive care unit in a university hospital. MEASUREMENTS AND RESULTS: Gas flow, CO2 concentration, and O2 concentration were measured during in- and expiration by pneumotachography, a mainstream capnometer and a sidestream O2-analyser. For FRC-measurement inspiratory O2 concentration was changed by 30%. FRC was determined as mean value of a N2 washout and N2 washin procedure. Evaluation of this method in a lung model shows a good correlation between FRC set in the lung model and FRC measured (FRC measured = 1.028*FRG model + 22.92 ml; r2 = 0.957; n = 30). The mean difference was 4.4% of FRC-reference (range -8.4% to +21.7%). Duplicate determinations in 10 mechanically ventilated patients differed by an average of -2.7% (range -30.1% to +27.3%). CONCLUSION: Our results suggest that the proposed method can be used in daily clinical work.

Computerized blood gas interpretation as tool for classroom and ICU.

OBJECTIVE: To describe structure and function of a PC based blood gas interpretation program (ABG-consultant) developed for nurses and physicians, and to test educational impact and user acceptance. DESIGN: Prospective, blinded study SETTING: Interdisciplinary ICU of a county hospital in Switzerland PARTICIPANTS: Nurses specialized in intensive care INTERVENTIONS: Exposure to the ABG-consultant program MEASUREMENT AND RESULTS: A first group of nurses was subjected to a written examination, then the ABG-consultant was made available for them for 2 months, and finally the same examination was taken again. Additionally, they completed a questionnaire related to the performance of the ABG-consultant. A second group of nurses took the same sequence of examinations but had no access to the ABG-consultant. The score of the examinations increased by 4.8 points in the first group (p < 0.0001) and by 1.3 points (p < 0.16) in the second group. More than 400 consultations were conducted over a period of 2 months and the users themselves stated that the system was of help and easy to use. CONCLUSION: The results have shown that exposure to the ABG-consultant has increased the blood gas knowledge of the ICU nurses. It therefore appears desirable and worthwhile to address other areas of clinical medicine by a similar teaching-consulting approach.