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PubMed2006

Linear model and algorithm to automatically estimate the pressure limit of pressure controlled ventilation for delivering a target tidal volume.

OBJECTIVE: To theoretically assess the viability of an automatic procedure to support the anesthesiologist in properly setting mechanical ventilators when the operating conditions are switched from volume controlled to pressure controlled ventilation whilst maintaining the preset tidal volume. The procedure is based on a simple linear model of the ventilator breathing system with constant parameters and utilizes the signals gathered by the ventilator without the need to add further equipment. After a short period of stable volume controlled ventilation with the desired tidal volume, the herewith described algorithm allows the calculation of the value of pressure limit to set in pressure controlled mode which assures the previously settled tidal volume with the same breathing frequency and inspiratory-expiratory time ratio. METHODS: The algorithm allows the online identification of the four parameters necessary for the mathematical model that are obtained by means of a direct comparison between the pressure, flow and volume waveforms generated by the model and the analog signals provided by the ventilator. The theoretical approach was validated by two different ventilators, various settings, two breathing circuits, endotracheal tubes of various sizes and two mechanical simulators of the respiratory system operating in various conditions. RESULTS: Errors usually less than 5% (p < 0.05) on the target tidal volume were obtained for various settings typically used for adult ventilation in less than 10 s. The theoretical approach shows its limitations (errors of 10+/- 5%, p < 0.05) at high breathing frequencies (30-40 bpm) and low tidal volumes (200-300 ml). CONCLUSIONS: The proposed theoretical approach shows the viability, for adult settings, of one of the simplest mathematical model for mechanical ventilation in order to quickly and safely switch from volume controlled to pressure controlled ventilation. The algorithm could easily be in perspective implemented in the software of the ventilator providing the anesthesiologist with an indication on the value of pressure limit to set in order to safely switch ventilation mode.

Algorithms↗
PubMed2005

Effect of inspiratory time on tidal volume delivery in anesthesia and intensive care unit ventilators operating in pressure control mode.

STUDY OBJECTIVE: To compare the effect of inspiratory time and lung compliance on tidal volume (Vt) delivery in anesthesia and intensive care unit (ICU) ventilators operating in pressure control mode. SETTING: Respiratory research laboratory of a tertiary care medical center. DESIGN: Two anesthesia ventilators with pressure control capability (Narkomed 6000, Drager Medical, Inc, Telford, Pa, and the Datex-Ohmeda Aestiva 5, Datex-Ohmeda, Inc, Madison, Wis) and one critical care ventilator (Puritan Bennett 7200, Puritan-Bennett, Pleasanton, Calif) were studied under varying inspiratory time and lung compliance conditions using a mechanical lung model. INTERVENTION: Each ventilator was set to pressure control mode at a fixed inspiratory/expiratory (I/E) ratio. The respiratory rate (RR) was varied between 6 and 28 breaths per minute. Lung compliance and inspiratory time settings were set to simulate clinical conditions known to affect anesthesia ventilator performance. MEASUREMENTS: Inspiratory flow, Vts, and peak airway pressures were measured using the on-board monitor for each ventilator, and confirmed with the Bicore CP-100 pulmonary mechanics monitor (Bicore Monitoring Systems, Inc, Irvine, Calif). To assess differences in inspiratory flow between ventilators, airway pressures were continuously monitored during inspiration. MAIN RESULTS: Increasing RRs caused delivered Vts to decrease for all ventilators. However, decreases in Vts were significantly larger for anesthesia than for ICU ventilators. At a lung compliance of 0.02 L/cm H(2)O and set Vt of 700 mL, Vt delivery for the Puritan Bennett 7200 ventilator remained at 88% of baseline, but decreased to 76% for the Aestiva 5 when RRs were increased from 6 to 28 breaths per minute (P < .0025). Airway pressure tracings demonstrated a slower increase in inspiratory airway pressure for the Aestiva 5 than for the other ventilators. CONCLUSION: Differences in inspiratory flow delivery between ICU and anesthesia ventilators can cause differences in Vt delivery when the pressure control mode is used at high RRs. These differences can significantly impact the perioperative care of critically ill patients requiring ventilatory support.

Air Pressure↗
PubMed1998

Ventilation heterogeneity is increased in hypocapnic dogs but not pigs.

Hypocapnia increases ventilation/perfusion (VA/Q) heterogeneity in dogs, possibly by adversely affecting distribution of ventilation through its effects on collateral ventilation. Because pigs lack collateral ventilation, we compared the effects of hypocapnia on ventilation heterogeneity in pentobarbital-anesthetized, mechanically-ventilated dogs and pigs. Simultaneous multiple breath washouts of helium and nitrogen were used to assess the uniformity of the ventilation distribution by the phase III (SnIII) method. Ventilation heterogeneity was partitioned into two components, e.g. convective-dependent inhomogeneity (cdi) and diffusive-convective-dependent inhomogeneity (dcdi). Pulmonary gas exchange was also measured in pigs by the multiple inert gas elimination technique. Ventilation heterogeneity was increased (P < 0.01) in hypocapnic dogs. Inspiration of CO2 decreased ventilation heterogeneity by decreasing dcdi (P < 0.01). In contrast, ventilation heterogeneity was not increased in hypocapnic pigs. However, hypocapnia increased VA/Q heterogeneity by 18% (P < 0.05) in pigs. We conclude that hypocapnia increases ventilation heterogeneity in dogs but not in pigs, most likely related to an interspecies difference in collateral ventilation.

Analysis of Variance↗
PubMed2002

[Mechanical ventilation in a respiratory ward. Evolution from 1994 to 2000].

OBJECTIVE: To investigate the absolute and relative frequency of mechanical ventilation in the management of patients on a respiratory medicine ward between 1994 and 2000. To describe reasons for admission, mean hospital stay and outcomes. SETTING: A tertiary-care university hospital. METHODS: Observational, descriptive study of a case series. RESULTS: During the study period, 257 admissions involved mechanical ventilation of 132 patients. During that time, there was a progressive increase in the total number of ventilated patients as well as in the relative frequency, such that ventilated patients eventually accounted for 6.1% of all admissions for respiratory care in 2000. Nearly 80% of admissions were related to the service's home mechanical ventilation program, either to initiate and adapt ventilation for new patients or to treat exacerbations or diagnose and treat other medical or surgical problems in already-ventilated patients. Patients transferred from the intensive care unit (ICU) because of weaning difficulties (median ventilation, 31 days) had the highest mean stay. Nine of the 132 patients had to be transferred to the ICU and 18 died while hospitalized (7% of admissions and 13.6% of patients). The patients who died were those who were more acutely and severely ill (acute exacerbation in home-ventilated patients, patients with acute respiratory failure treated initially with non-invasive ventilation and patients transferred from the ICU due to weaning difficulties). CONCLUSIONS: Admissions requiring mechanical ventilation have increased and most are related to the home mechanical ventilation program. The mean stay and the mortality rate were related to the reason for admission.

Adolescent↗
PubMed2001

Pressure-controlled inverse ratio ventilation after cardiac surgery.

BACKGROUND AND OBJECTIVE: Pressure-controlled inverse ratio ventilation was compared with controlled mechanical ventilation in patients after cardiac surgery. METHODS: Ten patients were ventilated after sternal closure using a Siemens Servo 900C ventilator to a target end-tidal PCO2 of 4.0 kPa. They were randomized to receive controlled mechanical ventilation or pressure-controlled inverse ratio ventilation. CO2-based data were recorded on a laptop personal computer, which together with arterial PCO2 permitted measurement of the respiratory dead space. Once measurements were complete the ventilator was switched to the other mode and new measurements taken. RESULTS: PaCO2 and VCO2 were virtually the same in both modes. Peak airway pressure (17.2 +/- 2.7 vs. 20.8 +/- 2.5 cmH2O, P < 0.01) and minute ventilation (4.9 +/- 1.1 vs. 5.3 +/- 1.1 cmH2O, P < 0.01) were less during pressure-controlled inverse ratio ventilation. Physiological dead space fraction (0.39 +/- 0.06 vs. 0.51 +/- 0.05, P < 0.001), airway dead space (56 +/- 15 vs. 81 +/- 15 mL, P < 0.001) and alveolar dead space fraction (0.25 +/- 0.07 vs. 0.31 +/- 0.09, P < 0.01) were all less during pressure-controlled inverse ratio ventilation. There were no differences in heart rate or mean arterial pressure. CONCLUSIONS: The prolonged inspiratory period and pressure-controlled flow pattern of pressure-controlled inverse ratio ventilation reduce the alveolar and airway dead spaces, and give lower peak airway pressures, compared with conventional ventilation, in cardiac surgical patients.

Adult↗
PubMed1999

Minor redistribution of ventilation and perfusion within the lung during exercise in sheep.

Considerable heterogeneity unrelated to the effect of gravity has been demonstrated for both local ventilation (V) and perfusion (Q) in the lung. Local ventilation and perfusion are well matched, so that the heterogeneity of the V/Q ratio is less than for ventilation or perfusion alone (Melsom et aL 1997). We are searching for the mechanisms responsible for the coordinate heterogeneity of ventilation and perfusion. Here, we ask how and to what extent physical exercise induces changes in the distribution of ventilation and perfusion. We measured local (approximately 1.5 cm3 tissue volume) pulmonary ventilation and perfusion simultaneously in six sheep before, during and after running on a treadmill. Local ventilation was determined from the deposition of labelled aerosol particles and local perfusion from trapping of radioactive microspheres. Cardiac output increased approximately 2.5-fold during exercise. V/Q-ratios were not normally distributed and we therefore present the heterogeneity as the interquartile range. At rest, the average interquartile ranges for local ventilation, perfusion and V/Q-ratio were 0.48, 0.51 and 0.39, respectively. During exercise, the corresponding values were 0.44, 0.40 and 0.32. Thus, the distribution of local V/Q-ratio was narrower than for ventilation and perfusion also during exercise. We found a moderate redistribution of relative flow towards the dorsal parts of the lungs when perfusion increased, but the increase in total perfusion and ventilation was for the most part throughout the lung. The results indicate that the coupling between local ventilation and perfusion is at least as potent during exercise as at rest. The correlation (r) between paired values in the two resting periods was 0.93 for ventilation and 0.91 for perfusion and thus indicates time stability for the two variables.

Animals↗
PubMed1997

Capnography during jet ventilation for laryngoscopy.

Jet ventilation is often used during laryngoscopy to permit improved visualization of the larynx and to eliminate a potentially flammable endotracheal tube when laser surgery of the airway is performed. Observation of chest wall movement and blood gas analysis are the usual standards for assessing the adequacy of ventilation during jet ventilation. It is reasonable to hypothesize that measurement of end-tidal CO2 concentrations during jet ventilation can be used to assess the adequacy of ventilation during jet ventilation. To test this hypothesis, end-tidal CO2 concentrations were determined during mechanical ventilation through an endotracheal tube and during jet ventilation. At the time that each end-tidal measurement was obtained, a sample of arterial blood was also obtained for later blood gas analysis. For both mechanical ventilation and jet ventilation, well defined relationships between end-tidal CO2 and arterial CO2 tensions were obtained. However, the relationships are distinct: the difference in arterial to end-tidal CO2 tension during supraglottic jet ventilation at a conventional respiratory rate was found to be 13.4 +/- 6.8 mm Hg (mean +/- SD) compared with 5.7 +/- 5.2 mm Hg obtained during conventional ventilation through an endotracheal tube.

Capnography↗
PubMed1997

Inaccuracies of nitric oxide delivery systems during adult mechanical ventilation.

BACKGROUND: Various systems to administer inhaled nitric oxide (NO) have been used in patients and experimental animals. We used a lung model to evaluate five NO delivery systems during mechanical ventilation with various ventilatory patterns. METHODS: An adult mechanical ventilator was attached to a test lung configured to separate inspired and expired gases. Four injection systems were evaluated with NO injected either into the inspiratory circuit 90 cm proximal to the Y piece or directly at the Y piece and delivered either continuously or only during the inspiratory phase. Alternatively, NO was mixed with air using a blender and delivered to the high-pressure air inlet of the ventilator. Nitric oxide concentration was measured from the inspiratory limb of the ventilator circuit and the tracheal level using rapid- and slow-response chemiluminescence analyzers. The ventilator was set for constant-flow volume control ventilation, pressure control ventilation, pressure support ventilation, or synchronized intermittent mandatory ventilation. Tidal volumes of 0.5 l and 1 l were evaluated with inspiratory times of 1 s and 2 s. RESULTS: The system that premixed NO proximal to the ventilator was the only one that maintained constant NO delivery regardless of ventilatory pattern. The other systems delivered variable NO concentration during pressure control ventilation and spontaneous breathing modes. Systems that injected a continuous flow of NO delivered peak NO concentrations greater than the calculated dose. These variations were not apparent when a slow-response chemiluminescence analyzer was used. CONCLUSIONS: NO delivery systems that inject NO at a constant rate, either continuously or during inspiration only, into the inspiratory limb of the ventilator circuit produce highly variable and unpredictable NO delivery when inspiratory flow is not constant. Such systems may deliver a very high NO concentration to the lungs, which is not accurately reflected by measurements performed with slow-response analyzers.

Adult↗
PubMed2006

The impact of spontaneous breathing during mechanical ventilation.

PURPOSE OF REVIEW: In patients with acute respiratory distress syndrome, controlled mechanical ventilation is generally used in the initial phase to ensure adequate alveolar ventilation, arterial oxygenation, and to reduce work of breathing without causing further damage to the lungs. Although introduced as weaning techniques, partial ventilator support modes have become standard techniques for primary mechanical ventilator support. This review evaluates the physiological and clinical effects of persisting spontaneous breathing during ventilator support in patients with acute respiratory distress syndrome. RECENT FINDINGS: The improvements in pulmonary gas exchange, systemic blood flow and oxygen supply to the tissue which have been observed when spontaneous breathing has been maintained during mechanical ventilation are reflected in the clinical improvement in the patient's condition. Computer tomography observations demonstrated that spontaneous breathing improves gas exchange by redistribution of ventilation and end-expiratory gas to dependent, juxtadiaphragmatic lung regions and thereby promotes alveolar recruitment. Thus, spontaneous breathing during ventilator support counters the undesirable cyclic alveolar collapse in dependent lung regions. In addition, spontaneous breathing during ventilator support may prevent increase in sedation beyond a level of comfort to adapt the patient to mechanical ventilation which decreases duration of mechanical ventilator support, length of stay in the intensive care unit, and overall costs of care giving. SUMMARY: In view of the recently available data, it can be concluded that maintained spontaneous breathing during mechanical ventilation should not be suppressed even in patients with severe pulmonary functional disorders.

Analgesia↗
PubMed1983

Phase locking of the respiratory rhythm in cats to a mechanical ventilator.

Mechanical ventilation of paralyzed, pentobarbital-anesthetized adult cats was performed while recording phrenic nerve activity. The periodic changes in lung volume owing to mechanical ventilation affected the rhythm of central respiratory activity, resulting in a variety of regular and irregular patterns of coupling between respiratory system output, monitored by phrenic activity, and the mechanical ventilator. Phase-locked patterns, in which phrenic burst onset occurred at specific and repetitive phase(s) of the mechanical ventilator, with ratios of ventilator frequency: phrenic burst frequency of 1:2, 1:1, 3:2, 2:1, and 3:1 were observed. Regular and irregular patterns occurred over specific ranges of frequency and volume of the mechanical ventilator. A careful study was made of the 1:1 phase locking as the frequency and inflation volume of the mechanical ventilator were changed. The inspiratory time (TI) was defined as the interval between the time when phrenic activity began to rise and the onset of its rapid decline, and the expiratory time (TE) as the time between inspirations. In the 1:1 phase-locking region, as the frequency of the ventilator was increased both TI and TE decreased, and the phase of phrenic onset in the ventilator cycle changed. During ventilation with frequencies higher than the intrinsic phrenic frequency (initial burst frequency of phrenic activity with the ventilator turned off) inspiratory activity was prematurely terminated by lung inflation (Hering-Breuer inspiratory inhibitory reflex). During ventilation with frequencies lower than the intrinsic phrenic frequency, the onset of phrenic activity was delayed (TE was prolonged) by lung inflation (Hering-Breuer expiratory promoting reflex).

Animals↗
PubMed2001

Distribution of pulmonary ventilation using Xe-enhanced computed tomography in prone and supine dogs.

Xe-enhanced computed tomography (CT; Xe-CT) is a method for the noninvasive measurement of regional pulmonary ventilation in intact subjects, determined from the washin and washout rates of the radiodense, nonradioactive gas Xe, as measured in serial CT scans. We used the Xe-CT ventilation method, along with other quantitative CT measurements, to investigate the distribution of regional lung ventilation and air content in healthy, anesthetized, mechanically ventilated dogs in the prone and supine postures. Vertical gradients in regional ventilation and air content were measured in five mongrel dogs in both prone and supine postures at four axial lung locations. In the supine position, ventilation increased with dependent location, with a mean slope of 7.3%/cm lung height, whereas no ventilation gradients were found at any location in the prone position. These results agree quantitatively with other published studies. In addition, six different animals were studied (3 supine, 3 prone) to examine the longitudinal distribution of ventilation and air content. The prone lungs were more uniformly inflated compared with the supine, which were less well expanded at the base than apex. Ventilation index, a measure of regional ventilation relative to whole lung ventilation, increased steeply from apex to base in the supine animals, whereas it was again more uniform in the prone condition. We conclude that the Xe-CT method provides a reasonable, quantitative measurement of regional ventilation and promises to be a valuable tool for the noninvasive determination of regional lung function.

Animals↗
PubMed2006

Systematic review of determinants of mortality in high frequency oscillatory ventilation in acute respiratory distress syndrome.

INTRODUCTION: Mechanical ventilation has been shown to cause lung injury and to have a significant impact on mortality in acute respiratory distress syndrome. Theoretically, high frequency oscillatory ventilation seems an ideal lung protective ventilation mode. This review evaluates determinants of mortality during use of high frequency oscillatory ventilation. METHODS: PubMed was searched for literature reporting randomized trials and cohort studies of high frequency ventilation in adult patients with acute respiratory distress syndrome. Data on mortality and determinants were extracted for patients treated with high frequency oscillatory ventilation. Linear regression analyses were conducted to produce graphical representations of adjusted effects of determinants of mortality. RESULTS: Cohorts of patients treated with high frequency oscillatory ventilation from two randomized trials and seven observational studies were included. Data from cohorts comparing survivors with non-survivors showed differences in age (42.3 versus 51.2 years), prior time on conventional mechanical ventilation (4.0 versus 6.2 days), APACHE II score (22.4 versus 26.1), pH (7.33 versus 7.26) and oxygenation index (26 versus 34). Each extra day on conventional ventilation was associated with a 20% higher mortality adjusted for age and APACHE II score (relative risk (RR) 1.20, 95% confidence interval (CI) 1.15-1.25). However, this association was confounded by differences in pH (pH adjusted RR 1.03, 95% CI 0.73-1.46). Oxygenation index seemed to have an independent effect on mortality (RR 1.10, 95% CI 0.95-1.28). CONCLUSION: Prolonged ventilation on conventional mechanical ventilation prior to high frequency oscillatory ventilation was not related to mortality. Oxygenation index was a determinant of mortality independent of other disease severity markers.

Cohort Studies↗
PubMed1997

Performance characteristics of bilevel pressure ventilators: a lung model study.

Bilevel pressure ventilators are being used increasingly to provide noninvasive ventilatory support in the management of obstructive sleep apnea, chronic ventilatory failure, and acute respiratory failure. However, the ability of these ventilators to respond to inspiratory demand without imposing expiratory loads has not been evaluated extensively. We evaluated the performance of nine bilevel pressure ventilators in a lung model, as compared with the Nellcor Puritan-Bennett 7200ae adult critical care ventilator. All ventilators were set to provide pressure support ventilation (PSV) and positive end-expiratory pressure (PEEP) at a rate of 10 breaths/min with an inspiratory time of 1.0 s. Simulated pleural pressure, airway pressure, and flow at airway opening were continuously monitored. We studied the effects of three PSV levels (5, 10, and 15 cm H2O) with 5 cm H2O PEEP at two lung compliances (50 and 80 mL/cm H2O) and four peak inspiratory flow demands (20, 40, 60, and 80 L/min) on seven dependent variables: inspiratory delay time (D-I), inspiratory trigger pressure (P-I), inspiratory area percent (Area I%), expiratory delay time (D-E), supraplateau expiratory pressure change (P-E), expiratory area (Area E), and ventilator peak flow (VPF). Most ventilators performed as well as or significantly (p<0.05) better than the 7200ae in all studied variables. Compliance did not significantly affect ventilator performance. Increasing inspiratory flow demand significantly (p<0.05) increased D-I, P-I, P-E, and VPF and decreased Area I% with most ventilators. As ventilatory demand increased, D-E and Area E significantly (p<0.05) changed. With some units, D-E and Area E increased, while with others they decreased. Most bilevel pressure ventilators evaluated were able to respond to high ventilatory demands and outperformed the Nellcor Puritan-Bennett 7200ae ventilator.

Calibration↗
PubMed1989

Safe intrahospital transport of critically ill ventilator-dependent patients.

STUDY OBJECTIVE: To determine whether manual ventilation during intrahospital transport of mechanically ventilated critically ill patients results in blood gas and/or hemodynamic abnormalities. DESIGN: A single-blind prospective study evaluated arterial blood gas, blood pressure, heart rate, and arrhythmia changes during mechanical ventilation and manual transport ventilation. SETTING: University hospital ICUs and various diagnostic or treatment areas. PATIENTS: Twenty mechanically ventilated critically ill patients during intrahospital transport. INTERVENTION: Each patient received mechanical ventilation (MECH) with a volume ventilator while in the ICU and at the study/treatment area. They were manually ventilated (MAN) by a respiratory therapist during transport between areas. MEASUREMENTS AND MAIN RESULTS: The MECH settings were: VT = 0.75 +/- 0.17 L; f = 16 +/- 4; VE = 12.6 +/- 4.3 L/min; FIO2 = 0.46 +/- 0.2. Mean peak Paw = 31 +/- 12 cm H2O and mean effective Cst = 44 +/- 15 ml/cm H2O. No hemodynamic abnormalities were observed. Arterial blood gas values did not vary to any clinically significant degree, except in two patients: one patient had a reduced PaO2 and increased PaCO2 associated with an accidental O2 disconnection and clamped chest tube; another patient had an increased pH by 0.13 units with only a 9 mm Hg fall in PaCO2. CONCLUSIONS: Manual ventilation during intrahospital transport of critically ill mechanically ventilated patients is safe provided the person performing manual ventilation knows the inspired oxygen fraction and minute ventilation required before transport and is trained to approximate them during transport.

Critical Care↗
PubMed1992

Digital electronic communication between ICU ventilators and computers and printers.

UNLABELLED: Although many modern ICU ventilators offer the option of electronic communication, most of these systems are not used because there is a huge communication gap between the ventilator and the computer it might be connected to. When such systems are now used, a large part of what is communicated is artifactual and misleading. We need to overcome both legal and knowledge barriers in the effort to provide seamless communication between ventilators and computers. With regard to the specific issues raised in this paper, here are our answers. Issue #1: Is it essential to have a digital electronic communication port on an ICU ventilator? ANSWER: No, it is not essential. The purpose of the mechanical ventilator is to support pulmonary ventilation by supplying gas and pressure. There is no vital role for digital communication in the gas-delivery function of the ventilator; however, in the future it will be essential to have effective electronic communication in order to guarantee accurate and timely charting. Issue #2: What impact does electronic communication between a ventilator and a computer have on patient outcome? ANSWER: Our preliminary data show that electronic communication can reduce the number of charting errors and can improve the timeliness of data entry. However, there is little evidence, other than anecdotal, that this has any impact on patient outcome. Automated charting has been shown to reduce the time spent on charting. This time-savings could be used to increase time spent in direct patient care, but there is no conclusive evidence that this occurs. In fact, one report on computerized charting systems indicates that the result is less time spent in direct patient care. Issue #3: If electronic communication is to be effective in the future, how should these interfaces be configured for mechanical ventilation? ANSWER: We recommend an optimal algorithm for automated respiratory care charting that has been suggested. Sampling frequency: Sample data from the ventilator every 10 seconds. Ventilator-setting changes: Report every new setting if change lasts more than 3 minutes. Measured respiratory care data: Filter raw MIB-collected data with a 3-minute moving-median filter. Report one filtered value every hour for each variable. In addition, use a threshold table (Table 3) to define significant events. Report changes that remain above threshold more than 3 minutes. Report all measured respiratory-care data 1 minute following any ventilator-mode changes.

Computer Communication Networks↗
PubMed2002

The effects of tidal volume demand on work of breathing during simulated lung-protective ventilation.

BACKGROUND: Lung-protective ventilation (LPV) can result in a ventilator tidal volume (V(T)) below patient V(T) demand, which may elevate work of breathing (WOB). Increasing the ventilator inspiratory flow may not sufficiently reduce WOB, because the patient's flow-time requirements may exceed the ventilator's flow-time delivery pattern. We investigated (1) the effects of V(T) demand on WOB during LPV and (2) which ventilator pattern best reduced WOB while achieving LPV goals. METHODS: A standard WOB lung model simulated assisted breathing. Using 3 ventilators (Hamilton Veolar, Hamilton Galileo, and Dräger Evita 2 dura), we tested volume-control ventilation with a constant flow pattern (VCV-CF), volume-control ventilation with a decelerating flow (VCV-DF), and pressure-control ventilation (PCV). Simulated V(T) demand was increased from 50-125% of the ventilator-delivered V(T) (400 mL) as ventilator inspiratory time (T(I)) was decreased (0.95, 0.80, 0.65, and 0.45 s) relative to simulated T(I) (0.8 s). WOB was measured with a pulmonary mechanics monitor. RESULTS: During VCV-CF and VCV-DF, a V(T) demand of > or = 100% drastically increased WOB, attributable to imposed WOB from the inspiratory valve. Increasing inspiratory flow by using the decelerating flow pattern and/or decreasing T(I) reduced WOB, but generally not to normal levels. "Double-triggered" breaths, with excessive V(T) delivery, often occurred when ventilator T(I) was well below simulated T(I). PCV was most effective in reducing WOB, but V(T) delivery exceeded the LPV target unless T(I) was reduced. CONCLUSIONS: Given our dual goals of reducing both WOB and V(T) during LPV, VCV-DF with relatively brief T(I) appeared to be the best option, followed by PCV with a relatively brief T(I).

Computer Simulation↗
PubMed1990

Tracheobronchial and pulmonary histopathology following conventional and high-frequency jet ventilation.

The advent of high-frequency jet ventilation (HFJV) as an alternative method of respiratory support for newborns has been implicated as a causative agent of necrotizing tracheobronchitis (NTB). We conducted a controlled trial of prolonged HFJV and conventional mechanical ventilation (CMV) in adult cats to determine effects on airway injury related to mode of ventilation and placement of the jet injector (proximal vs distal trachea). Fifteen cats were randomly assigned to either high-frequency positive pressure ventilation, proximal injection jet ventilation, or distal injection jet ventilation. The animals were ventilated for more than 33 hours. Post mortem, the trachea and respiratory tree were removed en bloc and fixed in formalin. A pathologist, unaware of the mode of ventilation, examined tracheobronchial histology and assigned scores using a technique previously described. Lung parenchymal tissue was also assessed using a similar grading system. Statistical analysis (Kruskal-Wallis analysis of variance) demonstrated no significant differences between tracheobronchial or lung parenchymal histopathology regardless of the mode of ventilation. We conclude that (1) the adult cat serves as a useful model for evaluating histopathologic effects of prolonged ventilation, (2) the etiology and pathogenesis of airway injury appears to be multifactorial, and the mode of ventilation is only one of many contributing variables, and (3) previously demonstrated differences between CMV and HFJV may be related to a much shorter duration of ventilation.

Animals↗
PubMed1990

[Current modalities of mechanical ventilation for acute respiratory failure in chronic respiratory insufficiency].

Acute respiratory failure in patients with chronic obstructive lung disease is a frequent and serious complication, with a mortality rate of 20 p. 100 and 57 p. 100 of the patients kept under mechanical ventilation for more than two weeks. The main problem with mechanical ventilation is an unavoidable intrinsic positive expiratory pressure and a hyperinflation that can be increased by the different modes of ventilation. All the classical modes of mechanical ventilation may be used, including control-mode ventilation, assist/control-mode ventilation, intermittent mandatory ventilation and ventilation with inspiratory assistance. Another major problem is weaning from mechanical ventilation as it may take a long time or even be impossible, so that the patients stay in intensive care units for ages or require long-term home ventilation. At the moment, there is no way of predicting the outcome of weaning in each individual subjects, and none of the various ventilation procedures has proved superior to the others. Nasal ventilation has recently been introduced in intensive care where it constitutes a major step forward being less invasive; it avoids intubation in 60 p. 100 of the patients but is more exacting for the physicians and nursing staff; finally, it makes it possible to treat acute respiratory failure at an earlier stage than previously.

Acute Disease↗