Characteristics of the ventilator pressure- and flow-trigger variables

Patient-Ventilator Dyssynchrony

In mechanically ventilated patients, assisted mechanical ventilation (MV) is employed early, following the acute phase of critical illness, in order to eliminate the detrimental effects of controlled MV, most notably the development of ventilator-induced diaphragmatic dysfunction. Nevertheless, the benefits of assisted MV are often counteracted by the development of patient-ventilator dyssynchrony. Patient-ventilator dyssynchrony occurs when either the initiation and/or termination of mechanical breath is not in time agreement with the initiation and termination of neural inspiration, respectively, or if the magnitude of mechanical assist does not respond to the patient's respiratory demand. As patient-ventilator dyssynchrony has been associated with several adverse effects and can adversely influence patient outcome, every effort should be made to recognize and correct this occurrence at bedside. To detect patient-ventilator dyssynchronies, the physician should assess patient comfort and carefully inspect the pressure- and flow-time waveforms, available on the ventilator screen of all modern ventilators. Modern ventilators offer several modifiable settings to improve patient-ventilator interaction. New proportional modes of ventilation are also very helpful in improving patient-ventilator interaction.

Comparison of different inspiratory triggering settings in automated ventilators during cardiopulmonary resuscitation in a porcine model

BACKGROUND

Mechanical ventilation via automated in-hospital ventilators is quite common during cardiopulmonary resuscitation. It is not known whether different inspiratory triggering sensitivity settings of ordinary ventilators have different effects on actual ventilation, gas exchange and hemodynamics during resuscitation.

METHODS

18 pigs enrolled in this study were anaesthetized and intubated. Continuous chest compressions and mechanical ventilation (volume-controlled mode, 100% O2, respiratory rate 10/min, and tidal volumes 10ml/kg) were performed after 3 minutes of ventricular fibrillation. Group trig-4, trig-10 and trig-20 (six pigs each) were characterized by triggering sensitivities of 4, 10 and 20 (cmH2O for pressure-triggering and L/min for flow-triggering), respectively. Additionally, each pig in each group was mechanically ventilated using three types of inspiratory triggering (pressure-triggering, flow-triggering and turned-off triggering) of 5 minutes duration each, and each animal matched with one of six random assortments of the three different triggering settings. Blood gas samples, respiratory and hemodynamic parameters for each period were all collected and analyzed.

RESULTS

In each group, significantly lower actual respiratory rate, minute ventilation volume, mean airway pressure, arterial pH, PaO2, and higher end-tidal carbon dioxide, aortic blood pressure, coronary perfusion pressure, PaCO2 and venous oxygen saturation were observed in the ventilation periods with a turned-off triggering setting compared to those with pressure- or flow- triggering (all P<0.05), except when compared with pressure-triggering of 20 cmH2O (respiratory rate 10.5[10/11.3]/min vs 12.5[10.8/13.3]/min, P = 0.07; coronary perfusion pressure 30.3[24.5/31.6] mmHg vs 27.4[23.7/29] mmHg, P = 0.173; venous oxygen saturation 46.5[32/56.8]% vs 41.5[33.5/48.5]%, P = 0.575).

CONCLUSIONS

Ventilation with pressure- or flow-triggering tends to induce hyperventilation and deteriorating gas exchange and hemodynamics during CPR. A turned-off patient triggering or a pressure-triggering of 20 cmH2O is preferred for ventilation when an ordinary inpatient hospital ventilator is used during resuscitation.

Patient-Ventilator Interactions

Ventilatory muscle fatigue is a reversible loss of the ability to generate force or velocity of contraction in response to increased elastic and resistive loads. Mechanical ventilation should provide support without imposing additional loads from the ventilator (dys-synchrony). Interactive breaths optimize this relationship but require that patient effort and the ventilator response be synchronous during breath initiation, flow delivery, and termination. Proper delivery considers all 3 phases and uses clinical data, ventilator graphics, and sometimes a trial-and-error approach to optimize patient-ventilator interactions. Newer modes optimize interactions but await good clinical outcome data before routine use.

A mathematical model approach quantifying patients' response to changes in mechanical ventilation: Evaluation in pressure support

PURPOSE

This article evaluates how mathematical models of gas exchange, blood acid-base status, chemical respiratory drive, and muscle function can describe the respiratory response of spontaneously breathing patients to different levels of pressure support.

METHODS

The models were evaluated with data from 12 patients ventilated in pressure support ventilation. Models were tuned with clinical data (arterial blood gas measurement, ventilation, and respiratory gas fractions of O2 and CO2) to describe each patient at the clinical level of pressure support. Patients were ventilated up to 5 different pressure support levels, for 15 minutes at each level to achieve steady-state conditions. Model-simulated values of respiratory frequency (fR), arterial pH (pHa), and end-tidal CO2 (FeCO2) were compared to measured values at each pressure support level.

RESULTS

Model simulations compared well to measured data with Bland-Altman bias and limits of agreement of fR of 0.7 ± 2.2 per minute, pHa of -0.0007 ± 0.019, and FeCO2 of -0.001 ± 0.003.

CONCLUSION

The models describe patients' fR, pHa, and FeCO2 response to changes in pressure support with low bias and narrow limits of agreement.

Patient-ventilator interactions. Implications for clinical management

Assisted/supported modes of mechanical ventilation offer significant advantages over controlled modes in terms of ventilator muscle function/recovery and patient comfort (and sedation needs). However, assisted/supported breaths must interact with patient demands during all three phases of breath delivery: trigger, target, and cycle. Synchronous interactions match ventilator support with patient demands; dyssynchronous interactions do not. Dyssynchrony imposes high pressure loads on ventilator muscles, promoting muscle overload/fatigue and increasing sedation needs. On current modes of ventilation there are a number of features that can monitor and enhance synchrony. These include adjustments of the trigger variable, the use of pressure versus fixed flow targeted breaths, and a number of manipulations of the cycle variable. Clinicians need to know how to use these modalities and monitor them properly, especially understanding airway pressure and flow graphics. Future strategies are emerging that have theoretical appeal but they await good clinical outcome studies before they become commonplace.

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

Control of breathing during mechanical ventilation: who is the boss?

Over the past decade, concepts of control of breathing have increasingly moved from being theoretical concepts to "real world" applied science. The purpose of this review is to examine the basics of control of breathing, discuss the bidirectional relationship between control of breathing and mechanical ventilation, and critically assess the application of this knowledge at the patient's bedside. The principles of control of breathing remain under-represented in the training curriculum of respiratory therapists and pulmonologists, whereas the day-to-day bedside application of the principles of control of breathing continues to suffer from a lack of outcomes-based research in the intensive care unit. In contrast, the bedside application of the principles of control of breathing to ambulatory subjects with sleep-disordered breathing has out-stripped that in critically ill patients. The evolution of newer technologies, faster real-time computing abilities, and miniaturization of ventilator technology can bring the concepts of control of breathing to the bedside and benefit the critically ill patient. However, market forces, lack of scientific data, lack of research funding, and regulatory obstacles need to be surmounted.

A bench study of intensive-care-unit ventilators: new versus old and turbine-based versus compressed gas-based ventilators

OBJECTIVE

To compare 13 commercially available, new-generation, intensive-care-unit (ICU) ventilators in terms of trigger function, pressurization capacity during pressure-support ventilation (PSV), accuracy of pressure measurements, and expiratory resistance.

DESIGN AND SETTING

Bench study at a research laboratory in a university hospital.

METHODS

Four turbine-based ventilators and nine conventional servo-valve compressed-gas ventilators were tested using a two-compartment lung model. Three levels of effort were simulated. Each ventilator was evaluated at four PSV levels (5, 10, 15, and 20 cm H2O), with and without positive end-expiratory pressure (5 cm H2O). Trigger function was assessed as the time from effort onset to detectable pressurization. Pressurization capacity was evaluated using the airway pressure-time product computed as the net area under the pressure-time curve over the first 0.3 s after inspiratory effort onset. Expiratory resistance was evaluated by measuring trapped volume in controlled ventilation.

RESULTS

Significant differences were found across the ventilators, with a range of triggering delays from 42 to 88 ms for all conditions averaged (P < 0.001). Under difficult conditions, the triggering delay was longer than 100 ms and the pressurization was poor for five ventilators at PSV5 and three at PSV10, suggesting an inability to unload patient's effort. On average, turbine-based ventilators performed better than conventional ventilators, which showed no improvement compared to a bench comparison in 2000.

CONCLUSION

Technical performance of trigger function, pressurization capacity, and expiratory resistance differs considerably across new-generation ICU ventilators. ICU ventilators seem to have reached a technical ceiling in recent years, and some ventilators still perform inadequately.

Patient-ventilator synchrony during pressure-targeted versus flow-targeted small tidal volume assisted ventilation

PURPOSE

Low tidal volume (V(T)) delivered by flow-targeted breaths reduces ventilator-induced lung injury but may increase patient breathing effort because of limited flow. We hypothesized that a variable-flow, pressure-targeted breath would improve breathing effort versus a fixed flow-targeted breath.

MATERIALS AND METHODS

We compared pressure assist-control ventilation and volume assist-control ventilation (VACV) in 12 patients with acute respiratory failure receiving 6 to 8 mL/kg V(T). Backup frequency, V(T), inspiratory time, applied positive end-expiratory pressure and fraction of inspired oxygen were held constant. Patient breathing effort was assessed by airway pressure (Paw) drop below baseline 0.1 second after the breath initiation (P(0.1)), the maximal Paw drop during the triggering phase (Ptr), the magnitude of ventilator work during flow delivery, and the presence of an active expiratory effort during cycling and air trapping judged by the magnitude of residual flow at end-expiration.

RESULTS

Compared with VACV, pressure assist-control ventilation decreased P(0.1), Ptr (by 25% and 16%, respectively), and evidence for trapped gas but not ventilator work during flow delivery or cycle dys-synchrony. Peak inspiratory flow was comparable between the 2 modes.

CONCLUSIONS

In patients receiving small V(T) VACV with increased breathing effort, variable-flow, pressure-targeted ventilation may provide more comfort by decreasing respiratory drive during the triggering phase.

Modulation and treatment of patient–ventilator dyssynchrony

PURPOSE OF REVIEW

The coupling between ventilator delivered inspiratory flow and patient's demands both in terms of timing and drive is a challenging task that has become largely feasible in recent years. This review addresses the new advances to modulate and treat patient-ventilator dyssynchrony.

RECENT FINDINGS

Patient-ventilator dyssynchrony is a common phenomenon with conventional modes of mechanical ventilation which influence the duration of mechanical ventilation. Inspection of pressure, volume and flow waveforms represents a valuable tool for the physician to recognize and take the appropriate action to improve patient-ventilator synchrony. New developments have been introduced aiming to improve patient ventilator synchrony by modulating the triggering function and the variables that control the flow delivery and the cycling off.

SUMMARY

Patient-ventilator dyssynchrony may affect patients' outcome. New modes of assisted mechanical ventilation have been introduced and represent a major step forward in modulating patient-ventilator dyssynchrony.

Bedside waveforms interpretation as a tool to identify patient-ventilator asynchronies

OBJECTIVE

During assisted modes of ventilatory support the ventilatory output is the final expression of the interaction between the ventilator and the patient's controller of breathing. This interaction may lead to patient-ventilator asynchrony, preventing the ventilator from achieving its goals, and may cause patient harm. Flow, volume, and airway pressure signals are significantly affected by patient-ventilator interaction and may serve as a tool to guide the physician to take the appropriate action to improve the synchrony between patient and ventilator. This review discusses the basic waveforms during assisted mechanical ventilation and how their interpretation may influence the management of ventilated patients. The discussion is limited on waveform eye interpretation of the signals without using any intervention which may interrupt the process of mechanical ventilation.

DISCUSSION

Flow, volume, and airway pressure may be used to (a) identify the mode of ventilator assistance, triggering delay, ineffective efforts, and autotriggering, (b) estimate qualitatively patient's respiratory efforts, and (c) recognize delayed and premature opening of exhalation valve. These signals may also serve as a tool for gross estimation of respiratory system mechanics and monitor the effects of disease progression and various therapeutic interventions.

CONCLUSIONS

Flow, volume, and airway pressure waveforms are valuable real-time tools in identifying various aspects of patient-ventilator interaction.

How to choose a mechanical ventilator

PURPOSE OF REVIEW

To provide some practical and clinical considerations that may guide users through the decision process when choosing mechanical ventilators

RECENT FINDINGS

Although the complexity of mechanical ventilators is steadily increasing, the importance of many devices developed over the course of the technical evolution is still a matter of discussion. Recent data demonstrate that the technical performance of equivalent ventilators (ie, machines of the same generation and category) is pretty similar, suggesting that the different manufacturers keep in step with new developments. Thus, other factors than technical limitations will probably influence the choice of ventilators. Among them the ability of the staff to understand the rationale of the different devices and controls as well as deal with the complexity of the ventilator may be particularly important.

SUMMARY

Choosing mechanical ventilators should begin by defining the algorithms of how to ventilate a patient. Once this is done, a ventilator should allow the transformation of specific strategies into practice and the adaptation of the mechanical support to the needs of the individual patient. This procedure is crucially important, because ventilator therapy should always be determined by the physician and based on solid physiologic rationales rather than by the technical features of the machine.

Effects of the flow waveform method of triggering and cycling on patient-ventilator interaction during pressure support

OBJECTIVE

To examine patient-ventilator interaction during pressure support ventilation in critically ill patients when they were ventilated: (1) by a new system (Vision, Respironics) which uses the flow waveform as a method of triggering and cycling; and (2) by a new generation ventilator (Evita 4, Drager) which uses the traditional flow triggering (2 l/min) and cycling criterion (25% of peak flow).

DESIGN

Prospective clinical and bench study.

METHODS

Twelve mechanically ventilated patients were studied at three levels of pressure support, applied randomly with both ventilators. The two systems of triggering were further studied at controlled levels of dynamic hyperinflation and respiratory drive using an active lung model.

RESULTS

Patients' breathing patterns, respiratory effort, and arterial blood gases were not affected by the type of ventilator. The flow waveform method of triggering was more sensitive to patient effort than the flow triggering, resulting in less ineffective effort but a greater number of auto-triggerings. At controlled levels of dynamic hyperinflation and inspiratory effort the simulated patient effort needed to trigger the ventilator was considerably less with the flow waveform method of triggering than that with the flow triggering. The flow waveform method of cycling resulted in mechanical breaths with similar characteristics to those that used the traditional flow criterion of breath termination.

CONCLUSIONS

We conclude that the flow waveform method of triggering improves the ventilator function and decreases the patient effort during the triggering phase. This system is highly sensitive, but under certain circumstances may be unstable.

Positive pressure mechanical ventilation

There have been numerous advances in the application of positive pressure mechanical ventilation in the last two decades. As knowledge of pulmonary physiology expands, the application of modes and parameters to maximize the efficacy and minimize the complications of ventilatory support continues to advance. As the use of noninvasive ventilation becomes more widespread, its usefulness in certain clinical entities such as COPD exacerbations and acute cardiogenic pulmonary edema will become more prominent. The role of specific modes and parameters of these devices likely will be further refined to maximize outcomes.

Progress in mechanical ventilation

Mechanical ventilation is a life-supporting process employed in the management of respiratory failure. Over the years, our understanding of the pathophysiology of lung injury has greatly improved, and has aided the technological development of ventilatory modes that are more patient 'sensitive' and less traumatizing to the lungs. This review will discuss the fundamental modes of mechanical ventilation, and present current concepts regarding patient-ventilator interaction that either promote lung healing and weaning from positive pressure ventilation or delay recovery because of the injudicious use of ventilatory modalities that are incapable of meeting the ventilatory demands of the patient on a breath-by-breath basis. In addition, the current strategy for mechanical ventilation in acute lung injury and acute respiratory distress syndrome will be summarized.

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.

Work of breathing characteristics of seven portable ventilators

Portable ventilators (PVs) are used for patient transport with increasingly frequency. Due to design differences it would not be unexpected to find differences among these ventilators in the imposed work of breathing (WOBI) during spontaneous respiratory efforts. The purpose of this investigation was to compare the WOBI characteristics during spontaneous breathing of seven PVs; Bird Avian, Bio-Med Crossvent 4, Pulmonetics LTV 1000, Hamilton Max, Drägerwerk Oxylog 2000, Impact Uni-Vent 750, and Impact Uni-Vent 754 using a model of spontaneous breathing. Differences between the PVs in regards to the measured parameters increased with increases in simulated breathing demand. WOBI, peak inspiratory pressure, and pressure-time product were consistently less with the LTV 1000 over the range of simulated breathing conditions. During pressure support ventilation these parameters were significantly less with the LTV 1000 compared with the Crossvent 4. Only the WOBI produced by the LTV was consistently lower than the physiologic work of breathing across the simulated spontaneous breathing conditions. Based on these results it is predicted PVs with flow triggering and positive end-expiratory pressure compensation will consistently offer the least WOBI. Clinicians should be aware of these characteristics when using PVs with spontaneous breathing patients.

Mechanical ventilation in chronic obstructive lung disease

Exacerbations of COPD are a leading indication for MV in the intensive care unit. A thorough understanding of the pathophysiology of AVF in COPD is critical for physicians caring for these patients. In particular, physicians should understand DHI and use the ventilator and ancillary techniques to minimize its impact. Noninvasive positive-pressure ventilation should be considered strongly in relatively stable patients with an adequate mental status and manageable secretions. Once AVF resolves, patients should be removed from the ventilator as soon as is safe to do so to minimize the adverse effects of prolonged MV. An organized approach to weaning and identifying patients capable of independent breathing is crucial. Most patients with COPD and AVF benefit from MV and generally return to or approach their premorbid functional status. A significant subset, however, will not benefit from, or choose not to undergo, MV. Deciding upon appropriate therapeutic options for these patients relies heavily on effective communication between physician and patient. Comprehensive discussions before the development of AVF can assist decision-making after respiratory failure develops.

Physiological effects of alveolar, tracheal, and "standard" pressure supports

Pressure support (PS) is characterized by a pressure plateau, which is usually generated at the ventilator level (PS(vent)). We have built a PS device in which the pressure plateau can be obtained at the upper airway level (PS(aw)) or at the alveolar level (PS(A)). The effect of these different PS modes was evaluated in seven healthy men during air breathing and 5% CO(2) breathing. Minute ventilation during air breathing was higher with PS(A) than with PS(aw) and lower with PS(vent) (16 +/- 3, 14 +/- 3, and 11 +/- 2 l/min, respectively). By contrast, there were no significant differences in minute ventilation during 5% CO(2) breathing (25 +/- 5, 27 +/- 7, and 23 +/- 5 l/min, respectively). The esophageal pressure-time product per minute was lower with PS(A) than with PS(aw) and PS(vent) during air breathing (29 +/- 26, 44 +/- 44, and 48 +/- 30 cmH(2)O. s, respectively) and 5% CO(2) breathing (97 +/- 40, 145 +/- 62, and 220 +/- 41 cmH(2)O. s, respectively). In conclusion, during PS, moving the inspiratory pressure plateau from the ventilator to the alveolar level reduces pressure output, particularly at high ventilation levels.

Esophageal-directed pressure support ventilation in normal volunteers

STUDY OBJECTIVES

To ascertain whether inspiratory pressure support (IPS) can be triggered reliably from and targeted at esophageal pressures (Pes), and to compare the work of breathing and time delay to initiation of inspiratory flow between conventional pressure support and esophageal-directed pressure support (EDPS).

DESIGN

Prospective laboratory study.

SETTING

University medical school.

PATIENTS OR PARTICIPANTS

Five normal volunteers.

INTERVENTIONS

IPS at a level to achieve tidal volume of 10 mL/kg, and EDPS with a target Pes of 0 cm H2O via full facemask.

MEASUREMENTS AND RESULTS

Pes, airway pressure, and inspiratory flow were measured during spontaneous breathing. Peak Pes and pressure time product (PTP) of Pes were calculated during spontaneous breathing and through linear resistances. Measurements were repeated during IPS and EDPS ventilation. At rest, PTP was 7.56 (+/- 3.6) and peak Pes was -5.8 cm H2O (+/- 1.44). When subjects were breathing through the resistors, PTP increased to 12.4 (+/- 8.1) and 30.3 (+/- 8.9) and peak Pes decreased to -7.2 and -15.3 cm H2O respectively. With facemask IPS, unloaded PTP fell to 1.7 (+/- 1.3) and peak Pes fell to -3.3 cm H2O (+/- 1.3). When ventilated through the highest resistance with IPS, mean PTP increased to 21.9 and peak Pes increased to -11.9 (+/- 4.2) cm H2O relative to baseline. During EDPS with the resistor, PTP fell to 1.5+/-1.1 (p < 0.007) and peak Pes fell to -1.9+/-1.1 cm H2O (p < 0.0001).

CONCLUSIONS

It was possible to initiate supported breathing from Pes values. The work performed, as measured by PTP, was lower during EDPS than during either unsupported breathing or conventional IPS.

Work of breathing, inspiratory flow response, and expiratory resistance during continuous positive airway pressure with the ventilators EVITA-2, EVITA-4 and SV 300

OBJECTIVE

To analyze work of breathing (WOB) imposed by the respirators EVITA-2, EVITA-4 (Drägerwerk, Lübeck, Germany) and SV 300 (Siemens-Elema, Sweden) as well as inspiratory flow response and expiratory flow resistance during continuous positive airway pressure (CPAP).

DESIGN

Five study conditions on a lung model (CPAP at 0, 5, and 10 mbar, CPAP 5 mbar plus pressure support 2 mbar with both EVITA models, and CPAP 5 mbar with decreasing levels of flow and pressure trigger sensitivity with the SV 300) and three randomized study conditions in nine patients recovering from open heart surgery (condition A: EVITA-2, CPAP 5 mbar; condition B: SV 300, CPAP 5 mbar, flow trigger; condition C: SV 300, pressure trigger-4 mbar).

SETTING

University hospital intensive care unit and laboratory of pulmonary physiology.

MEASUREMENTS AND RESULTS

At each study condition we measured WOB, pressure-time product (PTP), WOB and PTP imposed (WOBimposed and PTPimposed), tidal volume, minute ventilation, respiratory rate, inspiratory trigger time, trigger pressure, trigger PTP, duration of inspiration, mean and peak inspiratory flow, and the delay from the onset of inspiration to peak inspiratory flow. Since the SV 300 automatically generates an additional pressure support of 2 cm H2O PTP, WOB, WOBimposed, and PTPimposed were higher with the EVITA-2 and EVITA-4 regardless of the trigger sensitivity set on the SV 300. The difference was neutralized with both types of EVITA ventilator by adding 2 mbar of pressure support during CPAP in order to achieve comparable conditions. Inspiratory flow response was faster with both EVITA models, expiratory flow resistance was higher with the SV 300. Decrements of trigger sensitivity with the SV 300 accelerated the flow response.

CONCLUSIONS

Under similar conditions, no difference in WOBimposed was observed, although inspiratory flow response and expiratory flow resistance differed substantially between the three ventilators tested. Trigger sensitivity plays a minor role in determining PTP and WOB but has major influence on flow.

Liberation from mechanical ventilation: a decade of progress

Multiple complications associated with mechanical ventilation mandate that clinicians expeditiously define and reverse the pathophysiologic processes that precipitate respiratory failure and then, detect the earliest point that a patient can breathe without the ventilator. Over the past decade, numerous laboratory and clinical studies have been reported that may inform transformation of the "art of weaning" to the science of liberation. We review these studies and use them to formulate a systematic approach to assure early, safe, and successful liberation of patients from mechanical ventilation.

Respiratory muscle dysfunction in mechanically-ventilated patients

The interaction between a patient and a ventilator is the major determinant of the amount of respiratory muscle rest achieved by the machine. We are beginning to acquire a better understanding of the mechanisms that underlie this complex interaction, but this information has yet to be integrated into the routine clinical management of ventilator-supported patients. To achieve that goal, we need better techniques of detecting and monitoring patient-ventilation asynchrony, and the development of simple algorithms that can minimize its occurrence. Finally, research is needed to determine the occurrence and importance of respiratory muscle fatigue during failed weaning attempts so as to better guide the timing and pace of the weaning process in problematic patients.

Effects of flow triggering on breathing effort during partial ventilatory support

The effects of flow triggering (FT) as compared with pressure triggering (PT) on breathing effort have been the focus of several studies, and discrepant results have been reported. In the initial part of our study, a lung model was used to quantify triggering effort (airway pressure-time product, PTPaw) for a range of sensitivity settings in nine new-generation ventilators. A ventilator providing both FT and PT was then used to compare these systems during pressure-support (PSV) and volume-targeted assist-control ventilation (ACV) in eight ventilator-dependent patients, using sensitivity settings (2 L/min for FT and -2 cm H2O for PT) that had proven significantly different in the initial bench study. Indexes of effort included the esophageal and transdiaphragmatic pressure-time products and inspiratory work of breathing per minute (PTPes/min, PTPdi/min, and Wi/min, respectively). The experimental study revealed significant differences between ventilators in PTPaw at commonly used settings. In two of three ventilators featuring both systems, PTPaw was significantly lower with FT than PT (p < 0.001). In the clinical study, FT as compared with PT, was associated with reductions in all indexes of breathing effort during PSV: 16 +/- 6% (p < 0.001), 13 +/- 10% (p < 0.01), and 14 +/- 12% (p < 0.05) for PTPdi/min, PTPes/min, and Wi/min, respectively. By contrast, no differences were found when FT was used during ACV. Although FT reduced triggering effort in both modes (p < 0.001), the effects observed during the post-trigger phase differed, and explained the discrepant results between the two modes. We conclude that FT more effectively reduces breathing effort when used in conjunction with a pressure-targeted mode than with a volume-targeted mode, especially when flow delivery is close to or below demand.

Pressure vs flow triggering during pressure support ventilation

BACKGROUND

Adult mechanical ventilators have traditionally been pressure- or time-triggered. More recently, flow triggering has become available and some adult ventilators allow the choice between pressure or flow triggering. Prior studies have supported the superiority of flow triggering during continuous positive airway pressure, but few have compared pressure and flow triggering during pressure support ventilation (PSV). The purpose of this study was to compare pressure and flow triggering during PSV in adult mechanically ventilated patients.

METHODS

The study population consisted of 10 adult patients ventilated with a mechanical ventilator (Nellcor-Puritan-Bennett 7200ae) in the PSV mode. In random order, we compared pressure triggering of -0.5 H2O, pressure triggering -1 cm H2O, flow triggering of 5/2 L/min, and flow triggering 10/3 L/min. Pressure was measured for 5 min at the proximal endotracheal tube using a data acquisition rate of 100 Hz. From the airway pressure signal, trigger pressure (deltaP) was defined as the difference between positive end-expiratory pressure (PEEP) and the maximum negative deflection prior to onset of the triggered breath. Pressure-time product (PTP) was defined as the area produced by the pressure waveform below PEEP during onset of the triggered breath. Trigger time (deltaT) was defined as the time interval below PEEP during onset of the triggered breath.

RESULTS

A pressure trigger of -0.5 cm H2O was significantly more sensitive than the other trigger methods for deltaP, PTP, and deltaT (p<0.001). There was also a significant difference between patients for deltaP, deltaT, and PTP for each trigger method (p<0.001).

CONCLUSIONS

For this group of patients, flow triggering was not superior to pressure triggering at -0.5 cm H2O during PSV.

Comparison of pressure- and flow-triggered pressure-support ventilation on weaning parameters in patients recovering from acute respiratory failure

OBJECTIVE

To compare the effects of pressure- and flow-triggered pressure-support ventilation on weaning parameters during recovery from acute respiratory failure.

DESIGN

Prospective, randomized, clinical trial.

SETTING

Intensive care unit in a university hospital.

PATIENTS

Sixteen orotracheally intubated adult patients recovering from acute respiratory failure of various etiologies, without chronic obstructive pulmonary disease.

INTERVENTIONS

Randomized application of pressure- and flow-triggered pressure-support ventilation at 100% and 75% ventilatory support levels in each triggering system. A total of four conditions were applied for 30 mins each in all patients.

MEASUREMENTS AND MAIN RESULTS

Ventilatory, respiratory, and hemodynamic data were measured. For the measurement of weaning parameters, pressure and volume signals were directed to a computerized respiratory monitor by means of an esophageal probe and a flow sensor between the "Y" piece of the ventilatory circuit and the endotracheal tube. During both pressure-triggered (trigger sensitivity of -1 cm H2O) and flow-triggered (trigger sensitivity of 0.7 to 2.0 L/min) pressure-support ventilation with a ventilator, peak airway pressures were applied so as to decrease the work of breathing performed by the patient to zero (full ventilatory support). Partial ventilatory support was applied at 75% of the peak airway pressures achieved during full ventilatory support with each triggering system. A total of four experimental conditions were evaluated at identical FiO2 and positive and-expiratory pressure levels during pressure-support ventilation in each patient. Total ventilation volumes, arterial blood gas data, and hemodynamics did not differ among the four experimental conditions. During partial ventilatory support, the work of breathing, rapid shallow breathing index, and esophageal pressure increased significantly with both triggering systems when compared with data obtained at full ventilatory support. The mean data for the weaning parameters during the condition of partial ventilatory support were comparable between pressure- and flow-triggered pressure-support ventilation (i.e., 0.38 +/- 0.24 vs. 0.42 +/- 0.26 joule/L for work of breathing, 2.6 +/- 1.6 vs. 3.3 +/- 1.7 cm H2O for tracheal occlusion pressure, and 40.2 +/- 12.9 vs. 50.4 +/- 18.3 breaths/min/L for rapid shallow breathing index, respectively).

CONCLUSIONS

The application of either a pressure- or flow-triggered system during pressure-support ventilation with the ventilator did not significantly affect short-term changes in gas exchange, respiratory mechanics, and inspiratory workload in patients recovering from acute respiratory failure of various etiologies without chronic obstructive pulmonary disease.

Patient-ventilator interactions

Patient-ventilator synchrony is important in the management of the ventilator-dependent patient. Factors inherent to the patient and the ventilator influence patient-ventilator synchrony. Detection of patient-ventilator synchrony may require monitoring of airway pressure and flow waveforms.

Effect of tracheal gas insufflation on demand valve triggering and total work during continuous positive airway pressure ventilation

Tracheal gas insufflation (TGI) improves CO2 clearance and may reduce work of breathing by lowering the required minute ventilation (VE). However, TGI might also impair the ability to trigger the ventilator, because to lower external circuit pressures, inspiratory effort must outstrip catheter flow rate (Vc) and overcome the dynamic hyperinflation caused by TGI. We studied these effects using a two-chamber lung model of the respiratory muscles (RM) and lungs (L). The RM-chamber was ventilated using a sinusoidal flow pattern with a tidal volume (VT) of 0.5 L at various peak inspiratory flow rates (Vpk) to simulate differences in effort intensity. The L-chamber was connected to a 60-L/min continuous flow circuit with a 10 cm H2O positive end-expiratory pressure valve and to 3 different ventilatory demand valve circuits, each set at continuous positive airway pressure (CPAP) of 10 cm H2O. We used continuous TGI at 0, 2.5, 5, 10, and 15 L/min. The work of triggering (W-trig) increased with increasing Vc and decreased with increasing Vpk. The L-ventilator failed to trigger when Vc was 15 L/min and Vpk was 20 L/min. At a fixed VE, the effect of TGI on total mechanical inspiratory work (W-tot) was relatively small and varied among the different CPAP systems used. We conclude that weak patients may fail to open the demand valve of the CPAP system during TGI at high catheter flow rates. The net effect of TGI on the effort made by ventilated patients would depend not only on the interactions between TGI and the ventilator, but also on the efficiency of TGI in decreasing dead-space and lowering the VE requirement.