Intermittent mandatory ventilation: a new approach to weaning patients from mechanical ventilators

Complex Heart-Lung Ventilator Emergencies in the CICU

This review aims to enhance the comprehension and management of cardiopulmonary interactions in critically ill patients with cardiovascular disease undergoing mechanical ventilation. Highlighting the significance of maintaining a delicate balance, this article emphasizes the crucial role of adjusting ventilation parameters based on both invasive and noninvasive monitoring. It provides recommendations for the induction and liberation from mechanical ventilation. Special attention is given to the identification of auto-PEEP (positive end-expiratory pressure) and other situations that may impact hemodynamics and patients' outcomes.

“The role of a negative pressure ventilator coupled with oxygen helmet against COVID-19: a review”

Background

The coronavirus (SARS-COV-2) pandemic has provoked the global healthcare industry by potentially affecting more than 20 14 million people across the globe, causing lasting damage to the lungs, notably pneumonia, ARDS (acute respiratory distress 15 syndrome), and sepsis with the rapid spread of infection. To aid the functioning of the lungs and to maintain the blood oxygen 16 saturation (SpO₂) in coronavirus patients, ventilator assistance is required.

Materials and methods

The main purpose of this article is to outline the need 17 for the introduction of a non-invasive negative pressure ventilator (NINPV) as a promising alternative to positive pressure 18 ventilator (PPV) by elucidating the cons of non-invasive ventilators in clinical conditions like ARDS. Another motive is to 19 profoundly diminish the rate of infection spread by the employment of oxygen helmets, instead of endotracheal intubation in 20 invasive positive pressure ventilator (IPPV) or non-invasive positive pressure ventilator (NIPPV) like face masks and high-flow 21 nasal cannula (HFNC).

Result and conclusion

The integration of oxygen helmet with NPV would result in a number of notable facets including the 22 degree of comfort delivered to patients who are exposed to various ventilator-induced lung injuries (VILI) in the forms of 23 atelectasis, barotrauma, etc. Likewise, preventing the aerosol-generating procedures (AGP) diminishes the rate of nosocomial 24 infections and providing a better environment to both the patients and the healthcare professionals.

[A critical carol : Being an essay on anemia, suffocation, starvation, and other forms of intensive care, after the manner of Dickens]

Irgendwo in den USA – Einige Tage vor Heiligabend wird der angetrunkene Charlie Cratchit bei dem Versuch, eine Straße zu überqueren, von einem Bus angefahren und schwer verletzt: Rippenserienfraktur, Femur- und Fibulafraktur, Milz- und Pankreaslazeration, Darmrisse. In einem US-amerikanischen Krankenhaus der Maximalversorgung wird er operativ versorgt und anschließend auf die Intensivtherapiestation verlegt und dort kontinuierlich von einem namenlosen, sehr erfahrenen Arzt betreut. Vier Tage vor Heiligabend, erscheint am Patientenbett der Geist des berühmten britischen Physiologen Ernest Henry Starling. Er tritt in einen Dialog mit dem namenlosen Arzt, interessiert sich sehr für den Swan-Ganz-Katheter und verschwindet dann wieder. Die Besuche wiederholen sich in den kommenden 3 Nächten. Einmal kritisiert er Cratchits niedrigen Hämatokrit, beim nächsten Mal zeigt er sich unzufrieden mit der Respiratoreinstellung, und beim letzten Besuch ist er äußerst besorgt über den Ernährungszustand des Patienten. Der namenlose Arzt ist zunächst indigniert über des Geistes Kritik und Belehrungen, erkennt aber, dass darin der Schlüssel zu Cratchits Genesung liegt und handelt letztlich nach seinen Vorschlägen. Mit Erfolg: Nach der vom Geist Starlings angeregten Umstellung der maschinellen Ventilation, Gabe von 3 Erythrozytenkonzentraten und Aufnahme einer parenteralen Ernährung kann Charlie Cratchit am Weihnachtsabend extubiert und am Neujahrstag von der Intensivtherapiestation entlassen werden. In diesem Essay hat Robert Bartlett Charles Dickens’ „Weihnachtsgeschichte“ in die Welt der Intensivmedizin verlegt. Sie soll den Intensivmediziner anregen, therapeutische Interventionen wie maschinelle Ventilation, hämodynamische Interventionen und Gabe von Blutprodukten kritisch zu hinterfragen. Hintergrundinformationen und Kommentare zu den angesprochenen aktuellen Problemen der modernen Intensivmedizin ergänzen den Essay.

Trends in mechanical ventilation: are we ventilating our patients in the best possible way?

This review addresses how the combination of physiology, medicine and engineering principles contributed to the development and advancement of mechanical ventilation, emphasising the most urgent needs for improvement and the most promising directions of future development. Several aspects of mechanical ventilation are introduced, highlighting on one side the importance of interdisciplinary research for further development and, on the other, the importance of training physicians sufficiently on the technological aspects of modern devices to exploit properly the great complexity and potentials of this treatment.

EDUCATIONAL AIMS

To learn how mechanical ventilation developed in recent decades and to provide a better understanding of the actual technology and practice.To learn how and why interdisciplinary research and competences are necessary for providing the best ventilation treatment to patients.To understand which are the most relevant technical limitations in modern mechanical ventilators that can affect their performance in delivery of the treatment.To better understand and classify ventilation modes.To learn the classification, benefits, drawbacks and future perspectives of automatic ventilation tailoring algorithms.

Analytic Reviews : Initiating Mechanical Ventilation

Mechanical ventilation is a central facet of intensive care medicine. Technological progress has led to major advances in this field. Initiation of mechanical ventila tion remains standard; however, many options are now available to continue mechanical ventilation. These mechanical options include assist-control ventilation, intermittent mandatory ventilation, pressure support ventilation, positive end-expiratory pressure, and high- frequency ventilation. The sophistication of ventilators has also allowed more procedures to be performed dur ing mechanical ventilation, including bronchoscopy and transbronchial biopsy. Complications do occur during mechanical ventilation, but can be minimized by vigi lance and use of proper procedures. The recent increase in home care will lead to increased use of alternatives to conventional ventilatory modes. Finally, more well- designed studies are needed as newer techniques are promoted.

Weaning from Mechanical Ventilation

For most patients who require mechanical ventilation weaning and extubation is simple. In these patients a variety of strategies can be successful. In addition, sim ple criteria may predict when the patient is ready for extubation. For the small group of patients who require prolonged mechanical ventilation, however, contro versy exists about how best to remove ventilator sup port by weaning, and available data are sparse. Much of the controversy has centered on T-piece weaning ver sus intermittent mandatory ventilation. To date no con trolled study has demonstrated the superiority of either intermittent mandatory ventilation or T-piece weaning in difficult-to-wean patients. In the evolution of this con troversy, concern has developed over the potential for increased inspiratory work and expiratory resistance that may be associated with certain intermittent manda tory ventilation systems. The possibility that significant inspiratory work may occur during assist-control venti lation has also been demonstrated. Respiratory muscle weakness and fatigue is likely important in failure to wean. Other possible causes are failure of the cardiovas cular system and impaired ability of the lung to carry out gas exchange. In this article we first examine criteria and techniques for weaning short-term ventilator pa tients. We then examine criteria to begin the weaning process in prolonged ventilation patients, potential causes of failure to wean, and techniques that can be used to remove ventilator support from patients who are difficult to wean. Much literature has been devoted to techniques and criteria for weaning and extubation of patients from mechanical ventilation. For most patients who require ventilatory support, weaning and extuba tion can be easily accomplished by a variety of tech niques [1-4]. At one referral center 77.2% of all surviving patients were weaned from the ventilator within 72 hours of the onset of mechanical ventila tion, and 91% were weaned within 7 days [1]. Less than 10% of ventilated patients potentially posed problems in weaning from mechanical ventilation. Similarly, at a community hospital, few surviving patients required prolonged ventilatory support [2]. In easy-to-wean patients, Sahn and Lakshminarayan [5] described simple criteria that are predictive of successful discontinuation of ventilator support. For the small group of patients who require pro longed mechanical ventilation, however, minimal data are available. In these patients criteria to deter mine weaning ability or which measurements to follow are not clearly defined. Furthermore, no controlled trials are available to compare the differ ent weaning techniques proposed. In this article we first address routine weaning of the patient who has not required prolonged ventilator support. We then examine the difficult-to-wean patient and dis cuss criteria to begin the weaning process, poten tial causes of failure to wean, and available weaning techniques.

Real-time pulmonary graphics

Real-time pulmonary graphics now enable clinicians to view lung mechanics and patient-ventilator interactions on a breath-to-breath basis. Displays of pressure, volume, and flow waveforms, pressure-volume and flow-volume loops, and trend screens enable clinicians to customize ventilator settings based on the underlying pathophysiology and responses of the individual patient. This article reviews the basic concepts of pulmonary graphics and demonstrates how they contribute to our understanding of respiratory physiology and the management of neonatal respiratory failure.

Breaking the glass ceiling: an interview with Dr. Shirley Graves, a pioneering woman in medicine

Shirley Graves M.D., D.Sc. (honorary) (1936), Professor Emeritus of Anesthesiology and Pediatrics at the University of Florida, was one of the most influential women in medicine in the 1960 and 1970s, a time when the medical profession was overwhelmingly male-dominated. In today's society, it is hard to believe that only 50 years ago, women were scarce in the field of medicine. Yet Dr. Graves was a pioneer in the fields of pediatric anesthesia and pediatric critical care medicine. She identifies her development of the pediatric intensive care unit and her leadership in the Division of Pediatric Anesthesia at the University of Florida as her defining contributions. Through her journal articles, book chapters, national and international lectures, and leadership in the American Society of Anesthesiology and the Florida Society of Anesthesiology, she inspired a generation of men and women physicians to conquer the unthinkable and break through the glass ceiling.

Pilot Study of Two Nurse-Led Weaning Protocols in Patients with Tracheostomies

This is a pilot study, comparing two commonly-used weaning techniques in patients with a tracheostomy to establish if one technique resulted in shorter time to successful weaning. In a prospective, single-centre randomised, controlled trial, conducted in a 15-bed multidisciplinary intensive care unit, fifty patients mechanically ventilated for at least 48 hours and who had a tracheostomy inserted primarily for weaning purposes, were randomised to one of two weaning techniques: increasing periods of spontaneous ventilation, or reducing pressure support ventilation. Each technique was protocolised for implementation by the nursing staff and consisted of two stages: a weaning and a verification stage. This pilot study did not find a statistically significant difference in the length of time spent weaning when two nurse-led protocolised weaning techniques of increasing periods of spontaneous ventilation or reducing pressure support ventilation were compared in patients with a tracheostomy inserted primarily for weaning purposes. No safety issues were identified in either protocol.

Mechanical ventilation: past lessons and the near future

The ability to compensate for life-threatening failure of respiratory function is perhaps the signature technology of intensive care medicine. Unchanging needs for providing effective life-support with minimized risk and optimized comfort have been, are now, and will be the principal objectives of providing mechanical ventilation. Important lessons acquired over nearly half-a-century of ICU care have brought us closer to meeting them, as technological advances in instrumentation now effectively put this hard-won knowledge into action. Rising demand in the face of economic constraints is likely to drive future innovations focused on reducing the need for user input, automating multi-element protocols, and carefully monitoring the patient for progress and complications.

On the pulmonary toxicity of oxygen: III. The induction of oxygen dependency by oxygen use

Oxygen is central to the development of neonatal lung injury. The increase in oxygen exposure of the neonatal lung during the onset of extrauterine air breathing is an order of magnitude, from a range of 10-12 to 110-120Torr. The contributions of oxygen and the volume and pressure relationships of ventilatory support to lung injury are not easily distinguished in the clinical setting. Sequential changes in inspired air or 100% oxygen were studied in 536 newborn rabbits without ventilatory support. Bilateral cervical vagotomies (BCV) were performed at 24h post natal to induce ventilatory distress which eventuates in hyaline membrane disease. The sequences applied yielded evidence for an induced state of oxygen dependency from oxygen use which was reflected in differences in survival and the extent of pulmonary injury. The median survival for animals kept in air throughout was 3h. Oxygen before vagotomy or during the first 3h afterwards extended the survival significantly but produced more extensive, more severe, and more rapid lung lesions. Returning animals to air after prior oxygen exposure reduced the number of survivors past 10h and shortened the maximum survival in those groups. These features indicate the development of a dependency of the defense mechanisms on the availability of oxygen at the higher level for metabolic and possibly other aspects of the pulmonary and systemic response to injury, beyond the usual physiological need. Subset analysis revealed additive and latent effects of oxygen and demonstrated a remarkable rapidity in onset of severe lesions under some circumstances, illustrating the toxicity of oxygen per se.

Outcomes of patients ventilated with synchronized intermittent mandatory ventilation with pressure support: a comparative propensity score study

BACKGROUND

Few data are available regarding the benefits of one mode over another for ventilatory support. We set out to compare clinical outcomes of patients receiving synchronized intermittent mandatory ventilation with pressure support (SIMV-PS) compared with assist-control (A/C) ventilation as their primary mode of ventilatory support.

METHODS

This was a secondary analysis of an observational study conducted in 349 ICUs from 23 countries. A propensity score stratified analysis was used to compare 350 patients ventilated with SIMV-PS with 1,228 patients ventilated with A/C ventilation. The primary outcome was in-hospital mortality.

RESULTS

In a logistic regression model, patients were more likely to receive SIMV-PS if they were from North America, had lower severity of illness, or were ventilated postoperatively or for trauma. SIMV-PS was less likely to be selected if patients were ventilated because of asthma or coma, or if they developed complications such as sepsis or cardiovascular failure during mechanical ventilation. In the stratified analysis according to propensity score, we did not find significant differences in the in-hospital mortality. After adjustment for propensity score, overall effect of SIMV-PS on in-hospital mortality was not significant (odds ratio, 1.04; 95% CI, 0.77-1.42; P = .78).

CONCLUSIONS

In our cohort of ventilated patients, ventilation with SIMV-PS compared with A/C did not offer any advantage in terms of clinical outcomes, despite treatment-allocation bias that would have favored SIMV-PS.

History of technology in the intensive care unit

Critical care medicine is a young specialty and since its inception has been heavily reliant upon technology. Invasive monitoring has its humble beginnings in the continuous monitoring of heart rate and rhythm. From the development of right heart catheterization to the adaption of the echocardiogram for use in shock, intensivists have used technology to monitor hemodynamics. The care of the critically ill has been buoyed by investigators who sought to offer renal replacement therapy to unstable patients and worked to improve the monitoring of oxygen saturation. The evolution of mechanical ventilation for the critically ill embodies innumerable technological advances. More recently, critical care has insisted upon rigorous testing and cost-benefit analysis of technological advances.

Respiratory management of inhalation injury

Advances in the care of patients with major burns have led to a reduction in mortality and a change in the cause of their death. Burn shock, which accounted for almost 20 percent of burn deaths in the 1930s and 1940s, is now treated with early, vigorous fluid resuscitation and is only rarely a cause of death. Burn wound sepsis, which emerged as the primary cause of mortality once burn shock decreased in importance, has been brought under control with the use of topical antibiotics and aggressive surgical debridement. Inhalation injury has now become the most frequent cause of death in burn patients. Although mortality from smoke inhalation alone is low (0-11 percent), smoke inhalation in combination with cutaneous burns is fatal in 30 to 90 percent of patients. It has been recently reported that the presence of inhalation injury increases burn mortality by 20 percent and that inhalation injury predisposes to pneumonia. Pneumonia has been shown to independently increase burn mortality by 40 percent, and the combination of inhalation injury and pneumonia leads to a 60 percent increase in deaths. Children and the elderly are especially prone to pneumonia due to a limited physiologic reserve. It is imperative that a well organized, protocol driven approach to respiratory care of inhalation injury be utilized so that improvements can be made and the morbidity and mortality associated with inhalation injury be reduced.

Remembrance of weaning past: the seminal papers

The approach to ventilator weaning has changed considerably over the past 30 years. Change has resulted from research in three areas: pathophysiology, weaning-predictor testing, and weaning techniques. Physiology research illuminated the mechanisms of weaning failure. It also uncovered markers of weaning success. Through more reliable prediction, patients whose weaning would have been tedious in the 1970s are now weaned more rapidly. The weaning story offers several lessons in metascience: importance of creativity, the asking of heretical questions, serendipity, mental-set psychology, cross-fertilization, and the hazards of precocity. Weaning research also illustrates how Kuhnian normal (me-too) science dominates any field. Making the next quantum leap in weaning will depend on spending less time on normal science and more on the raising (and testing) of maverick ideas.

Clinical review: biphasic positive airway pressure and airway pressure release ventilation

This review focuses on mechanical ventilation strategies that allow unsupported spontaneous breathing activity in any phase of the ventilatory cycle. By allowing patients with the acute respiratory distress syndrome to breathe spontaneously, one can expect improvements in gas exchange and systemic blood flow, based on findings from both experimental and clinical trials. In addition, by increasing end-expiratory lung volume, as occurs when using biphasic positive airway pressure or airway pressure release ventilation, recruitment of collapsed or consolidated lung is likely to occur, especially in juxtadiaphragmatic lung legions. Traditional approaches to mechanical ventilatory support of patients with acute respiratory distress syndrome require adaptation of the patient to the mechanical ventilator using heavy sedation and even muscle relaxation. Recent investigations have questioned the utility of sedation, muscle paralysis and mechanical control of ventilation. Furthermore, evidence exists that lowering sedation levels will decrease the duration of mechanical ventilatory support, length of stay in the intensive care unit, and overall costs of hospitalization. Based on currently available data, we suggest considering the use of techniques of mechanical ventilatory support that maintain, rather than suppress, spontaneous ventilatory effort, especially in patients with severe pulmonary dysfunction.

The neurologic assessment and treatment of the “difficult to extubate” patient

Neurologic issues are involved in the patient who is difficult to wean. Assessing the patient and performing a complete neurologic examination are important when developing a successful weaning strategy. The neurologist contributes to this process by providing expertise in the various neurologic conditions and skill in performing a thorough neurologic examination.

Controlled versus assisted mechanical ventilation

On the basis of currently available data, it can be suggested that maintained spontaneous breathing during mechanical ventilation should not be suppressed even in patients with severe pulmonary dysfunction if no contraindications, such as increased intracranial pressure, are present. Improvements in pulmonary gas exchange, systemic blood flow, and oxygen supply to tissues, which have been observed when spontaneous breathing was allowed during ventilatory support, are reflected in the clinical improvement in the patient's condition, as indicated by significantly fewer days with ventilation, earlier extubation, and shorter stays in the intensive care unit. The positive effects of spontaneous breathing have been documented only for some of the available partial ventilatory support modalities. If ventilatory modalities are limited to those whose positive effects have been documented, then partial ventilatory support can be used as a primary modality even in patients with severe pulmonary dysfunction. Whereas controlled mechanical ventilation followed by weaning with partial ventilatory support modalities has been the earlier standard in ventilation therapy, this approach should be reconsidered in view of the available data.

Effects of different continuous positive airway pressure devices and periodic hyperinflations on respiratory function

OBJECTIVE

To compare the effect on respiratory function of different continuous positive airway pressure systems and periodic hyperinflations in patients with respiratory failure.

DESIGN

Prospective

SETTING

Hospital intensive care unit.

PATIENTS

Sixteen intubated patients (eight men and eight women, age 54 +/- 18 yrs, PaO2/FiO2 277 +/- 58 torr, positive end-expiratory pressure 6.2 +/- 2.0 cm H2O).

INTERVENTIONS

We evaluated continuous flow positive airway pressure systems with high or low flow plus a reservoir bag equipped with spring-loaded mechanical or underwater seal positive end-expiratory pressure valve and a continuous positive airway pressure by a Servo 300 C ventilator with or without periodic hyperinflations (three assisted breaths per minute with constant inspiratory pressure of 30 cm H2O over positive end-expiratory pressure).

MEASUREMENTS AND MAIN RESULTS

We measured the respiratory pattern, work of breathing, dyspnea sensation, end-expiratory lung volume, and gas exchange. We found the following: a) Work of breathing and gas exchange were comparable between continuous flow systems; b) the ventilator continuous positive airway pressure was not different compared with continuous flow systems; and c) continuous positive airway pressure with periodic hyperinflations reduced work of breathing (10.7 +/- 9.5 vs. 6.3 +/- 5.7 J/min, p <.05) and dyspnea sensation (1.6 +/- 1.2 vs. 1.1 +/- 0.8 cm, p <.05) increased end-expiratory lung volume (1.6 +/- 0.8 vs. 2.0 +/- 0.9 L, p <.05) and PaO2 (100 +/- 21 vs. 120 +/- 25 torr, p <.05) compared with ventilator continuous positive airway pressure.

CONCLUSIONS

The continuous flow positive airway pressure systems tested are equally efficient; a ventilator can provide satisfactory continuous positive airway pressure; and the use of periodic hyperinflations during continuous positive airway pressure can improve respiratory function and reduce the work of breathing.

The art and science of predicting patient readiness for weaning from mechanical ventilation

Weaning from mechanical ventilation is attempted when the patient's underlying condition has resolved and when the patient is able to maintain cardiovascular and respiratory stability within normal parameters. From a medical perspective, when to wean is based on patient readiness determined by objective, physiological criteria. Psychological readiness is equally important, yet criteria determining psychological readiness is generally omitted from the list of ready to wean parameters. Reasons for this may be that psychological readiness is difficult to measure and is based upon subjective opinions. Nursing research exploring critical care nurses' and patients' experiences of weaning has extended knowledge concerning patients' psychological readiness to wean. From examination of this research, three important criteria emerge. It is recommended that the addition of these criteria to the list of physiological criteria will not only achieve a holistic assessment of patients' readiness to wean, but will also acknowledge the important and complementary role of the nurse in the weaning process.

Mechanical ventilation of the newborn. An overview

Mechanical ventilation of the newborn infant is an ever-changing area. Its evolution has been hampered and stimulated by problems of small size, inadequate technology, unexpected complications, and changing expectations. With synchronized ventilation, a new technique in the neonatal ICU, clinicians again are reassessing their assumptions. HFV, a "new" technique for 15 years, has found a niche in the treatment of infants failing CV. Its use as an initial therapy for RDS, advocated by some, remains controversial. Monitoring gas flow patterns, tidal and minute volumes, and lung mechanics has become a part of the CV, but complications still occur. The only thing certain is that change will continue.

Weaning from ventilatory support

Resumption of spontaneous unassisted breathing after an episode of acute respiratory failure often is achieved without major difficulty. In a significant number of patients however, weaning from mechanical ventilation is a long and difficult process that markedly increases the duration of mechanical ventilation and consumes a significant fraction of critical care resources. Some criteria have been suggested to predict early and more accurately the moment the patient is ready to be separated from the ventilator. At the present time, the f/VT ratio (rapid shallow breathing index) appears to yield the best predictive power. None of these indices, however, is powerful enough to be relied on solely, and their use should be limited to that of aids to the critical care physician. The inability to sustain spontaneous ventilation usually is the consequence of an imbalance between respiratory demand and respiratory muscle capacity. Increased elastic workload, increased resistive workload, and increased VE are the main causes of excessive demand imposed on the respiratory system. Respiratory muscle pump failure usually relates to peripheral nerve dysfunction or muscular dysfunction. Left ventricular dysfunction also is an important cause of weaning failure. The usual methods of weaning from mechanical ventilation are T-piece trials with abrupt definitive discontinuation of mechanical ventilation if tolerated or with progressive intermittent trials, IMV, and PSV. All have their advantages and disadvantages, and the method of weaning per sé is not the only critical factor. Although their conclusions were different regarding the best method of weaning, however, two recent clinical trials suggest that ventilatory management has a major influence on the outcome of weaning from mechanical ventilation in difficult-to-wean patients. The global management of such patients requires a systematic approach with consideration of all factors involved in the process of separation from the ventilator. New computer-assisted systems already are tested and, in the future, may provide a significant advantage in the management of weaning from mechanical ventilation.

The effects of positive end-expiratory pressure of intrapulmonary shunt and ventilatory deadspace in nonhypoxic trauma patients

Controversy exists regarding the routine use of positive end-expiratory pressure (PEEP) in mechanically ventilated patients. We hypothesized that nonhypoxic patients receiving 5-cm H2O PEEP would have improved shunt and PaO2/F10(2) ratios (P/F), without an increased dead space to tidal volume ratio (VD/VT) versus patients receiving no PEEP. Forty-four trauma patients were randomized to receive 5-cm H2O PEEP (PEEP) or 0-cm H2O PEEP (ZEEP). Shunt VD/VT and P/F were measured at 0, 12, 24, 36, and 48 hours after intubation and after extubation. PEEP and ZEEP comparisons used Student's t test and the General Linear Models procedure. Shunt was significantly increased at t = 0 and at extubation in the PEEP group. At extubation, the PEEP group demonstrated significantly higher VD/VT and poorer P/F ratios. After correction for baseline values, no statistically significant differences were noted in spite of a trend toward worsening pulmonary function in all measured parameters. These results suggest that routine use of 5-cm H2O PEEP in mechanical ventilated trauma patients is not necessary.

A comparison of four methods of weaning patients from mechanical ventilation. Spanish Lung Failure Collaborative Group

BACKGROUND

Weaning patients from mechanical ventilation is an important problem in intensive care units. Weaning is usually conducted in an empirical manner, and a standardized approach has not been developed.

METHODS

We carried out a prospective, randomized, multicenter study involving 546 patients who had received mechanical ventilation for a mean (+/- SD) of 7.5 +/- 6.1 days and who were considered by their physicians to be ready for weaning. One hundred thirty patients had respiratory distress during a two-hour trial of spontaneous breathing. These patients were randomly assigned to undergo one of four weaning techniques: intermittent mandatory ventilation, in which the ventilator rate was initially set at a mean (+/- SD) of 10.0 +/- 2.2 breaths per minute and then decreased, if possible, at least twice a day, usually by 2 to 4 breaths per minute (29 patients); pressure-support ventilation, in which pressure support was initially set at 18.0 +/- 6.1 cm of water and then reduced, if possible, by 2 to 4 cm of water at least twice a day (37 patients); intermittent trials of spontaneous breathing, conducted two or more times a day if possible (33 patients); or a once-daily trail of spontaneous breathing (31 patients). Standardized protocols were followed for each technique.

RESULTS

The median duration of weaning was 5 days for intermittent mandatory ventilation (first quartile, 3 days; third quartile, 11 days), 4 days for pressure-support ventilation (2 and 12 days, respectively), 3 days for intermittent (multiple) trials of spontaneous breathing (2 and 6 days, respectively), and 3 days for a once-daily trial of spontaneous breathing (1 and 6 days, respectively). After adjustment for other covariates, the rate of successful weaning was higher with a once-daily trial of spontaneous breathing than with intermittent mandatory ventilation (rate ratio, 2.83; 95 percent confidence interval, 1.36 to 5.89; P < 0.006) or pressure-support ventilation (rate ratio, 2.05; 95 percent confidence interval, 1.04 to 4.04; P < 0.04). There was no significant difference in the rate of success between once-daily trials and multiple trials of spontaneous breathing.

CONCLUSIONS

A once-daily trial of spontaneous breathing led to extubation about three times more quickly than intermittent mandatory ventilation and about twice as quickly as pressure-support ventilation. Multiple daily trials of spontaneous breathing were equally successful.

Optimisation of respiratory function

OBJECTIVE

To apply pathophysiological principles in the management of patients with acute and chronic respiratory impairment.

CONCLUSIONS

Knowledge of the pathophysiology of respiratory disorders is a key element in treatment of patients with impending or established respiratory failure. Therapies to improve and support abnormal gas exchange will minimise the complications of hypoxia and interventions to optimise ventilation and work of breathing may obviate the need for mechanical ventilation.

Extracorporeal carbon dioxide removal performed with surface-heparinized equipment in patients with ARDS

To avoid the drawbacks of systemic anticoagulation during prolonged extracorporeal circulation in patients with adult respiratory distress syndrome (ARDS) a heparinization technique has been developed by which partially degraded heparin can be covalently end-point attached to the surface of the equipment constituting the extracorporeal circuit (Carmeda Bio-Active Surface, CBAS) thereby localizing the anticoagulatory effect. Since 1986 we have used extracorporeal circuits and membrane lungs coated with the CBAS for extracorporeal lung assistance (ECLA) in 14 patients suffering from ARDS. The patients were on ECLA for 3 to 55 days with a survival rate of 43%. Our experience so far is that by using equipment coated with CBAS it is possible to perform long-term extracorporeal circulation with a minimum of intravenously administered heparin, thus avoiding the risk of major coagulation defects.

Distribution of ventilation and perfusion with different modes of mechanical ventilation

We compared pulmonary gas exchange during synchronized intermittent mandatory ventilation (SIMV), pressure support ventilation (PSV), and airway pressure release ventilation (APRV). Nine subjects aged 56 to 75 yr were studied from 4 to 19 h after cardiac operations. When subjects were ready to be weaned from mechanical ventilation their ventilation-perfusion distribution was estimated using the multiple inert gas elimination technique during SIMV. The subjects then received PSV and APRV during alternating periods on a randomized basis, and the gas-exchange measurements were repeated. Vasoactive infusions and inspired oxygen fraction were held constant throughout the investigation. The results indicated that the major characteristics of the main mode of the VA/Q distributions (mean, standard deviation, and skew) were similar during all three modes. Dead space was lower during APRV (30.1 +/- 1.7% [SEM]) than during SIMV (36.2 +/- 1.5%) and PSV (37.1 +/- 2.7%) (p less than 0.05). Right-to-left shunt was significantly greater during APRV (19.9 +/- 2.3%) than during SIMV (15.4 +/- 1.7%) (p less than 0.05). Peak airway pressure (Paw) was higher during SIMV (32.8 +/- 1.3 cm H2O) than both PSV (19.4 +/- 2.1 cm H2O) and APRV (14.3 +/- 1.0 cm H2O) (p less than 0.05). Minute ventilation was lower during APRV (7.5 +/- 0.07 L/min) than during SIMV (9.4 +/- 0.6 L/min) and PSV (9.0 +/- 0.5 L/min) (p less than 0.05). Hemodynamic variables were similar during all three modes. We conclude that all three modes provide acceptable oxygenation and ventilatory support.

Influence of SIMV plus inspiratory pressure support on VA/Q distributions during postoperative weaning

Since the introduction of synchronized intermittent mandatory ventilation (SIMV) several advantages have been attributed to this ventilatory mode, one of them being a more homogeneous distribution of ventilation and perfusion than during controlled mechanical ventilation (CMV). Up to now no data are available to confirm whether this is true when SIMV is used in combination with inspiratory pressure support (IPS). Therefore, we compared the influence of CMV and SIMV + IPS on the distributions of ventilation and perfusion in 9 patients undergoing weaning from postoperative mechanical ventilation. Continuous distributions of ventilation and perfusion were assessed using the multiple inert gas elimination technique (MIGET). SIMV + IPS did not induce any change in the hemodynamic or oxygenation parameters, in particular CI and PaO2 remained constant. Physiological dead space (VD/VT) increased, but PaCO2 remained unchanged due to increased minute ventilation (from 9.5 +/- 0.9 l.min-1 to 11.3 #/- 1.2 l.min-1). The perfusion distributions remained unaltered; there was no change in QS/QT nor in the perfusion of the low VA/Q lung regions. This result was underscored by the unchanged dispersion of the perfusion distribution (log SDQ). The increased VD/VT was caused by increased inert gas dead space (from 22.0 +/- 9.6 to 26.8 +/- 8.7%) which was accompanied by increased ventilation of lung regions with high VA/Q ratios (10 less than VA/Q less than 100) in 3 patients. These results show that in our group of patients partial removal of CMV together with pressure support assistance of spontaneous ventilation did not induce a clinically significant loss of the efficiency of the breathing pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical ventilation

Among the many advances made in intensive care therapeutics in recent years, few have rivaled the impact of mechanical ventilators. Their expanded use affects all who practice in the critical care setting. This article reviews the physiologic basis for mechanical ventilation, how ventilators are classified, the various modes, and specific indications. A basic introduction is made into ventilator set up, weaning techniques, adjunctive drug therapy, and complications. The pharmacotherapy specialist who understands interactions between patients and ventilators, and the effects of mechanical ventilation on cardiopulmonary function will be best equipped to individualize drug therapy.

The reduction of weaning time from mechanical ventilation using tidal volume and relaxation biofeedback

We sought to determine if biofeedback could reduce weaning time for the hard-to-wean patient by improving important weaning factors that are not effectively dealt with by present weaning methods. These include respiratory muscle electromyograph (EMG) efficiency, respiratory drive, and the anxiety of the ventilator-dependent patient. After the patient had received mechanical ventilation for 7 days and the day weaning began (start), the patient was randomly assigned to biofeedback or to the control group. There were 20 patients assigned to each group, with mean ages of 60.2 (biofeedback) and 59.3 (control) yr. The patients assigned to the biofeedback group received daily, until extubation or being placed on no resuscitation status (termination), frontalis electromyographic (EMG) relaxation feedback for anxiety reduction and improved respiratory muscle EMG efficiency, tidal volume/diaphragm EMG (VT/DAP), and VT feedback for increasing VT and respiratory drive defined as tidal volume/inspiratory time (VT/TI). The control group was visited daily to control for attention and reassurance. The results showed a significant (p less than 0.01) reduction in mean ventilator days for the biofeedback group of 20.6 +/- 8.9 SD compared with 32.6 +/- 17.6 SD mean days for the control group. From start to termination, there was a significant (p less than 0.01) increase in baseline VT, from 295 +/- 41 to 415 +/- 45 ml, and a significant (p less than 0.02) increase in VT/DAP, from 0.33 +/- 0.09 to 0.94 +/- 0.22 L/mV for the biofeedback group but no significant change in these parameters for the control group.(ABSTRACT TRUNCATED AT 250 WORDS)