Patient work of breathing during pressure support and volume-cycled mechanical ventilation

Respiratory monitoring in adult intensive care unit

INTRODUCTION

The mortality of patients with respiratory failure has steadily decreased with the advancements in protective ventilation and treatment options. Although respiratory monitoring per se has not been proven to affect the mortality of critically ill patients, it plays a crucial role in patients' care, as it helps to titrate the ventilatory support. Several new monitoring techniques have recently been made available at the bedside. The goals of monitoring comprise alerting physicians to detect the change in the patients' conditions, to improve the understanding of pathophysiology to guide the diagnosis and provide cost-effective clinical management. Areas covered: We performed a review of the recent scientific literature to provide an overview of the different methods used for respiratory monitoring in adult intensive care units, including bedside imaging techniques such as ultrasound and electrical impedance tomography. Expert commentary: Appropriate respiratory monitoring plays an important role in patients with and without respiratory failure as a guiding tool for the optimization of ventilation support, avoiding further complications and decreasing morbidity and mortality. The physician should tailor the monitoring strategy for each individual patient and know how to correctly interpret the data.

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.

[Pressure support ventilation and proportional assist ventilation during weaning from mechanical ventilation]

OBJECTIVES

To compare tolerance, duration of mechanical ventilation (MV) and clinical outcomes during weaning from MV in patients subjected to either pressure support ventilation (PSV) or proportional assist ventilation (PAV).

DESIGN

A prospective, observational study was carried out.

SETTING

Intensive Care Unit.

PATIENTS

A total of 40 consecutive subjects were allocated to either the PSV or the PAV group until each group contained 20 patients. Patients were included in the study when they met the criteria to begin weaning and the attending physician decided to initiate the weaning process. The physician selected the modality and set the ventilatory parameters.

INTERVENTIONS

None.

VARIABLES OF INTEREST

Demographic data, respiratory mechanics, ventilatory parameters, duration of MV, and clinical outcomes (reintubation, tracheostomy, mortality).

RESULTS

Baseline characteristics were similar in both groups. No significant differences were observed between the PSV and PAV groups in terms of the total duration of MV (10 [5-18] vs. 9 [7-19] days; P=.85), reintubation (5 [31%] vs. 3 [19%]; P=.69), or mortality (4 [20%] vs. 5 [25%] deaths; P=1). Eight patients (40%) in the PSV group and 6 patients (30%) in the PAV group (P=.74) required a return to volume assist-control ventilation due to clinical deterioration.

CONCLUSIONS

Tolerance, duration of MV and clinical outcomes during weaning from mechanical ventilation were similar in PSV and PAV.

A long-term clinical evaluation of autoflow during assist-controlled ventilation: a randomized controlled trial

BACKGROUND

Many new mechanical ventilation modes are proposed without any clinical evaluation. "Dual-controlled" modes, such as AutoFlow™, are supposed to improve patient- ventilator interfacing and could lead to fewer alarms. We performed a long-term clinical evaluation of the efficacy and safety of AutoFlow during assist-controlled ventilation, focusing on ventilator alarms.

METHODS

Forty-two adult patients, receiving mechanical ventilation for more than 2 days with a Dräger Evita 4 ventilator were randomized to conventional (n = 21) or AutoFlow (n = 21) assist-controlled ventilation. Sedation was given using a nurse-driven protocol. Ventilator-generated alarms were exhaustively recorded from the ventilator logbook with a computer. Daily blood gases and ventilation outcome were recorded.

RESULTS

A total of 403 days of mechanical ventilation were studied and 45,022 alarms were recorded over a period of 8074 hours. The course of respiratory rate, minute ventilation, Fio(2), positive end-expiratory pressure, Pao(2)/Fio(2), Paco(2), and pH and doses and duration of sedation did not differ between the 2 groups. Outcome (duration of mechanical ventilation, ventilator-associated pneumonia, course of Sequential Organ Failure Assessment score, or death) was not different between the 2 groups. The number of alarms per hour was lower with AutoFlow assist-controlled ventilation: 3.3 [1.5 to 17] versus 9.1 [5 to 19], P < 0.0001 (median [quartile range]). In multivariate analysis, a low alarm rate was associated with activation of AutoFlow and a higher midazolam dose.

CONCLUSIONS

This first long-term clinical evaluation of the AutoFlow mode demonstrated its safety with regard to gas exchange and patient outcome. AutoFlow also allowed a very marked reduction in the number of ventilator alarms.

Mechanical ventilation in Duchenne patients with chronic respiratory insufficiency: clinical implications of 20 years published experience

Chronic respiratory insufficiency is inevitable in the course of disease progression in patients with Duchenne muscular dystrophy (DMD). Without mechanical ventilation (MV), morbidity and mortality are highly likely towards the end of the second decade of life. The present review reports evidence and clinical implications regarding DMD patients treated with MV. There is no doubt that nocturnal hypercapnia precedes daytime hypercapnia. Historical comparisons have provided evidence that non-invasive intermittent positive pressure ventilation (NIPPV) at night is effective and improves quality of life and survival by 5-10 years. By contrast, the optimal criteria and timing for initiation of NIPPV are inconsistent. A recent randomized study however demonstrated the benefits of commencing NIPPV as soon as nocturnal hypoventilation is detected (Ward S, et al., Randomised controlled trial of non-invasive ventilation (NIV) for nocturnal hypoventilation in neuromuscular and chest wall disease patients with daytime normocapnia. Thorax 2005; 60: 1019-24). The respective role of the three hypotheses of the indirect action of nocturnal NIPPV on daytime blood gases may be complimentary; the main improvement may be due to improved ventilatory response to CO2. The ultimate time to offer full time ventilation with the most advantageous interface is lacking in evidence. Full time NIV is possible with a combination of a nasal mask during the night and a mouthpiece during the day, however tracheostomy may be provided when mechanical techniques of cough-assistance are useless to treat chronic cough insufficiency.

Effects of tidal volume on work of breathing during lung-protective ventilation in patients with acute lung injury and acute respiratory distress syndrome*

OBJECTIVE

To assess the effects of step-changes in tidal volume on work of breathing during lung-protective ventilation in patients with acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS).

DESIGN

Prospective, nonconsecutive patients with ALI/ARDS.

SETTING

Adult surgical, trauma, and medical intensive care units at a major inner-city, university-affiliated hospital.

PATIENTS

Ten patients with ALI/ARDS managed clinically with lung-protective ventilation.

INTERVENTIONS

Five patients were ventilated at a progressively smaller tidal volume in 1 mL/kg steps between 8 and 5 mL/kg; five other patients were ventilated at a progressively larger tidal volume from 5 to 8 mL/kg. The volume mode was used with a flow rate of 75 L/min. Minute ventilation was maintained constant at each tidal volume setting. Afterward, patients were placed on continuous positive airway pressure for 1-2 mins to measure their spontaneous tidal volume.

MEASUREMENTS AND MAIN RESULTS

Work of breathing and other variables were measured with a pulmonary mechanics monitor (Bicore CP-100). Work of breathing progressively increased (0.86 +/- 0.32, 1.05 +/- 0.40, 1.22 +/- 0.36, and 1.57 +/- 0.43 J/L) at a tidal volume of 8, 7, 6, and 5 mL/kg, respectively. In nine of ten patients there was a strong negative correlation between work of breathing and the ventilator-to-patient tidal volume difference (R = -.75 to -.998).

CONCLUSIONS

: The ventilator-delivered tidal volume exerts an independent influence on work of breathing during lung-protective ventilation in patients with ALI/ARDS. Patient work of breathing is inversely related to the difference between the ventilator-delivered tidal volume and patient-generated tidal volume during a brief trial of unassisted breathing.

The effect of back-up rate during non-invasive ventilation in young patients with cystic fibrosis

OBJECTIVE

The aim of the study was to evaluate the effect of the back-up rate on respiratory effort during non-invasive mechanical ventilation.

DESIGN

An in vitro study evaluated the inspiratory trigger in seven domiciliary ventilators. Then, a prospective, randomized, crossover trial compared the effect on respiratory effort of three different back-up rates during pressure support (PS) and assist-control/volume-targeted (AC/VT) ventilation.

SETTING

A research unit and a tertiary referral pediatric center.

PATIENTS

Ten patients with cystic fibrosis (CF).

INTERVENTIONS

During the in vivo study, the back-up rate was progressively increased to the maximum that patients could tolerate (Fmax) and respiratory effort, as judged by pressure/time product of the diaphragm (PTPdi/min), was compared between the two ventilatory modes.

RESULTS

Differences were observed between trigger pressure, trigger time delay, trigger pressure/time product and the slope between flow and pressure in the seven ventilators. PS and AC/VT ventilation were associated with a decrease in respiratory effort (PTPdi/min was 518+/-172, 271+/-119 and 291+/-138 cmH(2)O. s(-1). min(-1), for spontaneous breathing, PS and AC/VT ventilation, respectively, p=0.05). During the two modes, increasing the back-up rate to Fmax resulted in a greater reduction in PTPdi/min (p=0.001), which was more pronounced during AC/VT ventilation, due to the automatic adjustment of the inspiratory/expiratory time ratio.

CONCLUSIONS

Increasing the back-up rate during PS and AC/VT ventilation decreases respiratory effort in young patients with CF, but this effect was more marked with AC/VT ventilation.

Respiratory sensations in asthma: physiological and clinical implications

Dyspnea is a cardinal symptom of asthma and may arise from several pathophysiological mechanisms, including pulmonary hyperinflation, stimulation of vagal receptors, and, rarely, chemoreceptor stimulation. The language that patients use to describe their breathlessness may provide important clues about the physiology underlying symptoms in a particular patient. Several physiological derangements may contribute to dyspnea in a given individual. The variability in the severity of breathlessness for any given degree of airflow obstruction may relate to differences in the relative importance of these physiological changes and/or to a range of perceptual abilities in asthmatic patients. One hypothesis that is under current investigation is that defective perception of asthma symptoms may lead to undertreatment and the potential for greater morbidity and mortality from asthma.

Weaning from mechanical ventilation

For the ventilator-dependent patient, weaning should be accomplished by withdrawing support safely, efficaciously, and efficiently. Success depends largely on physiologic determinants of respiratory system function, avoidance of ventilator-associated complications, and attention to patient readiness. Recent clinical trials, predictors of weaning, current techniques of weaning, the concept of reloading the respiratory pump, and determinants of ventilator dependency are all discussed.

Comparison of oxygen cost of breathing with pressure-support ventilation and biphasic intermittent positive airway pressure ventilation

OBJECTIVE

To assess the oxygen cost of breathing with either pressure-support ventilation (PSV) or biphasic intermittent positive airway pressure ventilation (BIPAP).

DESIGN

Prospective, randomized, crossover study.

SETTING

Medical intensive care unit of a university hospital.

PATIENTS

Twenty clinically stable and spontaneously breathing patients after long-term mechanical ventilation.

INTERVENTIONS

Patients were randomized to start on either PSV or BIPAP, and measurements were performed after an adaptation period of 30 mins. Immediately after, the ventilatory mode was changed and after another 30-min adaptation period, the same measurements were performed.

MEASUREMENTS AND MAIN RESULTS

Indirect calorimetry was performed during each ventilatory mode for a period of 30 mins. Oxygen consumption, energy expenditure, CO2 production, and respiratory quotient did not differ significantly between the two ventilatory modes, regardless of the patients' randomization. There were no statistically significant differences with regard to respiratory rate, minute volume, and blood gas analysis. All patients tolerated both ventilatory modes without any signs of discomfort.

CONCLUSIONS

Pressure support ventilation and BIPAP are both used for weaning patients gradually from the ventilator. BIPAP may be advantageous in patients not breathing sufficiently with PSV, since no patient effort is necessary with use of this ventilatory mode.

Patient-ventilator flow dyssynchrony: flow-limited versus pressure-limited breaths

OBJECTIVES

Patient-ventilator flow dyssynchrony occurs when ventilator flow delivery is insufficient to meet patient demands. If sufficiently severe, flow dyssynchrony can produce significant imposed loads on ventilatory muscles. Flow dyssynchrony can be improved by increasing ventilator flow delivery. We hypothesized that the variable flow pressure-limited breath would be a better approach for matching patient flow demands than adjusting a set flow on a conventional volume-cycled breath.

DESIGN

Clinical interventional study.

SETTING

Medical intensive care unit.

PATIENTS

Sixteen stable, mechanically ventilated patients receiving volume-cycled assist-control ventilation.

INTERVENTIONS

Flow dyssynchrony was produced by reducing the set flow by 50%. Dyssynchrony was quantified by measuring the esophageal pressure time product during the assisted breath. Two strategies were then employed in an attempt to reduce the dyssynchrony. One strategy was to increase flow back to the initial set flow and then further increase flow by an additional 25% (VI strategy). The other strategy was to use a pressure-limited breath feature coupled to a volume assist breath (the P strategy). With the P strategy, the pressure limit was set at 75% and 100% of the static elastic recoil pressure at end-inspiration.

MEASUREMENTS AND MAIN RESULTS

Pressure time product, intrinsic positive end-expiratory pressure, and the ventilatory pattern were measured with each strategy and were analyzed by analysis of variance. Induced baseline flow dyssynchrony, as measured by the pressure time product, was > 5 cm H2O/sec in ten of 16 patients. This dyssynchrony was significantly reduced by both the VI strategy and the P strategy, although the P strategy appeared to be more effective in those patients with the greatest baseline dyssynchrony. Baseline inspiratory time was also shortened by both the VI strategy and the P strategy; the VI strategy shortened baseline inspiratory time more than the P strategy. Baseline tidal volume, frequency, and intrinsic positive end-expiratory pressure were only minimally affected by either strategy.

CONCLUSION

The pressure-limited, variable-flow approach to ventilator gas delivery appears to be more responsive to a vigorous patient effort than a fixed-flow approach.

Physiological effects and optimisation of nasal assist-control ventilation for patients with chronic obstructive pulmonary disease in respiratory failure

BACKGROUND

A study was undertaken to investigate the effects of non-invasive assist-control ventilation (ACV) by nasal mask on respiratory physiological parameters and comfort in acute on chronic respiratory failure (ACRF).

METHODS

Fifteen patients with chronic obstructive pulmonary disease (COPD) were prospectively and randomly assigned to two non-invasive ventilation (NIV) sequences in spontaneous breathing (SB) and ACV mode. ACV settings were always optimised and therefore subsequently adjusted according to patient's tolerance and air leaks.

RESULTS

ACV significantly decreased all the total inspiratory work of breathing (WOBinsp) parameters, pressure time product, and oesophageal pressure variation in comparison with SB mode. The ACV mode also resulted in a significant reduction in surface diaphragmatic electromyographic activity to 36% of the control values and significantly improved the breathing pattern. SB did not change the arterial blood gas tensions from baseline values whereas ACV significantly improved both the PaO2 from a mean (SD) of 8.45 (2.95) kPa to 13.31 (2.15) kPa, PaCO2 from 9.52 (1.61) kPa to 7.39 (1.39) kPa, and the pH from 7.32 (0.03) to 7.40 (0.07). The respiratory comfort was significantly lower with ACV than with SB.

CONCLUSIONS

This study shows that the clinical benefit of non-invasive ACV in the management of ACRF in patients with COPD results in a reduced inspiratory muscle activity providing an improvement in breathing pattern and gas exchange. Despite respiratory discomfort, the muscle rest provided appears sufficient when ACV settings are optimised.

Comparative physiologic effects of noninvasive assist-control and pressure support ventilation in acute hypercapnic respiratory failure

STUDY OBJECTIVE

To compare the effects of noninvasive assist-control ventilation (ACV) and pressure support ventilation (PSV) by nasal mask on respiratory physiologic parameters and comfort in acute hypercapnic respiratory failure (AHRF).

DESIGN

A prospective randomized study.

SETTING

A medical ICU.

PATIENTS AND INTERVENTIONS

Fifteen patients with COPD and AHRF were consecutively and randomly assigned to two noninvasive ventilation (NIV) sequences with ACV and PSV mode, spontaneous breathing (SB) via nasal mask being used as control. ACV and PSV settings were always subsequently adjusted according to patient's tolerance and air leaks. Fraction of inspired oxygen did not change between the sequences.

MEASUREMENTS AND RESULTS

ACV and PSV mode strongly decreased the inspiratory effort in comparison with SB. The total inspiratory work of breathing (WOBinsp) expressed as WOBinsp/tidal volume (VT) and WOBinsp/respiratory rate (RR), the pressure time product (PTP), and esophageal pressure variations (deltaPes) were the most discriminant parameters (p<0.001). ACV most reduced WOBinsp/VT (p<0.05), deltaPes (p<0.05), and PTP (0.01) compared with PSV mode. The surface diaphragmatic electromyogram activity was also decreased >32% as compared with control values (p<0.01), with no difference between the two modes. Simultaneously, NIV significantly improved breathing pattern (p<0.01) with no difference between ACV and PSV for VT, RR, minute ventilation, and total cycle duration. As compared to SB, respiratory acidosis was similarly improved by both modes. The respiratory comfort assessed by visual analog scale was less with ACV (57.23+/-30.12 mm) than with SB (75.15+/-18.25 mm) (p<0.05) and PSV mode (81.62+/-25.2 mm) (p<0.01) in our patients.

CONCLUSIONS

During NIV for AHRF using settings adapted to patient's clinical tolerance and mask air leaks, both ACV and PSV mode provide respiratory muscle rest and similarly improve breathing pattern and gas exchange. However, these physiologic effects are achieved with a lower inspiratory workload but at the expense of a higher respiratory discomfort with ACV than with PSV mode.

Pressure-support ventilation in children with severe asthma

OBJECTIVE

To review the efficacy of pressure-support ventilation in the management of children with status asthmaticus requiring mechanical ventilation.

DESIGN

A case series.

SETTING

A university hospital.

SUBJECTS

Children requiring mechanical ventilation due to respiratory failure despite medical therapy during an episode of acute asthma.

INTERVENTIONS

Mechanical ventilation with pressure-support ventilation.

MEASUREMENTS AND MAIN RESULTS

Respiratory parameters (ventilatory settings, minute ventilation, respiratory rate, airway pressures) and blood gases were determined before, on initiation, and for 6 hrs after pressure-support ventilation. Spontaneous ventilation with an initial respiratory rate of 45 breaths/min (range 31 to 46) and an inspiration/expiration ratio (I/E) of 1:1.2 (range 1:1.1 to 1:2) was readily established in each patient. Arterial pH normalized (7.41, range 7.39 to 7.43) within 6 hrs (4.25, range 2 to 6) of the time at which ventilation was begun and the Paco2 decreased (p < .02) to 44 torr (range 39 to 47) (5.9 kPa, range 5.2 to 6.3) during pressure support ventilation.

CONCLUSION

Pressure-support ventilation permitted patient-cycled spontaneous ventilation in children with asthma. The ability of patients to determine their own respiratory pattern and to maintain forced exhalation during pressure-support ventilation may have important advantages in children with severe asthma who require mechanical ventilation.

Inspiratory work of breathing in ventilated preterm infants

In ventilated newborns, part of the inspiratory work of breathing (WOB) may be due to the inspiratory efforts preceding inspiratory ventilator flow. This study was designed to quantify the contribution of these efforts to WOB. WOB was evaluated in six intubated preterm infants ventilated by the Dräger Babylog 8000. The ventilatory modes studied were intermittent mandatory ventilation (IMV), continuous positive airway pressure (CPAP), and assist-control ventilation at 10 (ACV10) and 15 (ACV15) cmH2O peak pressure. Mouth flow (V) and esophageal pressure (Pe) were recorded, and WOB was estimate from the area delineated by the esophageal pressure-volume curve, where volume is the time integral of V. Calculation of WOB started either at the onset of the infant's inspiratory flow (WOBi), or at the beginning of the infant's inspiratory muscle efforts, detected on Pe and confirmed on the V tracing (WOBm). WOBm was found to be significantly higher than WOBi under all ventilatory conditions studied. The difference in work of breathing (delta W) between WOBm and WOBi did not depend on the type of ventilatory mode. When delta W was related to WOBm, it amounted to about 30% of WOBm in IMV and CPAP, and 60% in ACV (P < 0.05, ACV15 vs. IMV). These results suggest that, in preterm infants connected to a ventilator, inspiratory efforts preceding flow inspiration might account for a large fraction of the inspiratory work of breathing.