Oxygen cost of breathing and diaphragmatic pressure-time index. Measurement in patients with COPD during weaning with pressure support ventilation

Kinetics of oxygen uptake during unassisted breathing trials in prolonged mechanical ventilation: a prospective pilot study

Few studies have investigated the measurement of oxygen uptake (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}}$$\end{document}V˙O₂) in tracheostomized patients undergoing unassisted breathing trials (UBTs) for liberation from mechanical ventilation (MV). Using an open-circuit, breath-to-breath method, we continuously measured \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}}$$\end{document}V˙O₂ and relevant parameters during 120-min UBTs via a T-tube in 49 tracheostomized patients with prolonged MV, and calculated mean values in the first and last 5-min periods. Forty-one (84%) patients successfully completed the UBTs. The median \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}}$$\end{document}V˙O₂ increased significantly (from 235.8 to 298.2 ml/min; P = 0.025) in the failure group, but there was no significant change in the success group (from 223.1 to 221.6 ml/min; P = 0.505). In multivariate logistic regression analysis, an increase in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}}$$\end{document}V˙O₂ > 17% from the beginning period (odds ratio [OR] 0.084; 95% confidence interval [CI] 0.012–0.600; P = 0.014) and a peak inspiratory pressure greater than − 30 cmH₂O (OR 11.083; 95% CI 1.117–109.944; P = 0.04) were significantly associated with the success of 120-min UBT. A refined prediction model combining heart rate, energy expenditure, end-tidal CO₂ and oxygen equivalent showed a modest increase in the area under the receiver operating characteristic curve of 0.788 (P = 0.578) and lower Akaike information criterion score of 41.83 compared to the traditional prediction model including heart rate and respiratory rate for achieving 48 h of unassisted breathing. Our findings show the potential of monitoring \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}}$$\end{document}V˙O₂ in the final phase of weaning in tracheostomized patients with prolonged MV.

Titration and characteristics of pressure-support ventilation use in Argentina: an online cross-sectional survey study

Objective

To identify common practices related to the use and titration of pressure-support ventilation (PC-CSV - pressure control-continuous spontaneous ventilation) in patients under mechanical ventilation and to analyze diagnostic criteria for over-assistance and under-assistance. The secondary objective was to compare the responses provided by physician, physiotherapists and nurses related to diagnostic criteria for over-assistance and under-assistance.

Methods

An online survey was conducted using the Survey Monkey tool. Physicians, nurses and physiotherapists from Argentina with access to PC-CSV in their usual clinical practice were included.

Results

A total of 509 surveys were collected from October to December 2018. Of these, 74.1% were completed by physiotherapists. A total of 77.6% reported using PC-CSV to initiate the partial ventilatory support phase, and 43.8% of respondents select inspiratory pressure support level based on tidal volume. The main objective for selecting positive end-expiratory pressure (PEEP) level was to decrease the work of breathing. High tidal volume was the primary variable for detecting over-assistance, while the use of accessory respiratory muscles was the most commonly chosen for under-assistance. Discrepancies were observed between physicians and physiotherapists in relation to the diagnostic criteria for over-assistance.

Conclusion

The most commonly used mode to initiate the partial ventilatory support phase was PC-CSV. The most frequently selected variable to guide the titration of inspiratory pressure support level was tidal volume, and the main objective of PEEP was to decrease the work of breathing. Over-assistance was detected primarily by high tidal volume, while under-assistance by accessory respiratory muscles activation. Discrepancies were observed among professions in relation to the diagnostic criteria for over-assistance, but not for under-assistance.

Assessment of Work of Breathing in Patients with Acute Exacerbations of Chronic Obstructive Pulmonary Disease

The assessment of the work of breathing (WOB) of patients with acute exacerbations of chronic obstructive pulmonary disease (COPD) is difficult, particularly when the patient first presents with acute hypercapnia and respiratory acidosis. Acute exacerbations of COPD patients are in significant respiratory distress and noninvasive measurements of WOB are easier for the patient to tolerate. Given the interest in using alternative therapies to noninvasive ventilation, such as high flow nasal oxygen therapy or extracorporeal carbon dioxide removal, understanding the physiological changes are key and this includes assessment of WOB. This narrative review considers the role of three different methods of assessing WOB in patients with acute exacerbations of COPD. Esophageal pressure is a very well validated measure of WOB, however the ability of patients with acute exacerbations of COPD to tolerate esophageal tubes is poor. Noninvasive alternative measurements include parasternal electromyography (EMG) and electrical impedance tomography (EIT). EMG is easily applied and is a well validated measure of neural drive but is more likely to be degraded by the electrical environment in intensive care or high dependency. EIT is less well validated as a tool for WOB in COPD but extremely well tolerated by patients. Each of the different methods assess WOB in a different way and have different advantages and disadvantages. For research into therapies treating acute exacerbations of COPD, combinations of EIT, EMG and esophageal pressure are likely to be better than only one of these.

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.

Controversies in Weaning from Mechanical Ventilation

Invasive mechanical ventilation can successfully support the patient with acute respiratory failure, but it is associated with considerable risks. Numerous complications of invasive mechanical ventilation have been identified, and these may contribute to increased mortality. Therefore after clinical improvement has occurred, considerable emphasis is placed on expeditiously freeing the patient from the ventilator. This process of getting a patient off mechanical ventilation has been variably termed weaning, liberation, or discontinuation (terms which may be used interchangeably), and can be further divided into “readiness testing” and “progressive withdrawal.” Over the last decade, new developments in our understanding of the process of weaning have provided investigators with the tools to address a number of key questions: How should readiness for weaning (and trials of spontaneous breathing) be determined? What is the role of weaning parameters in deciding when to initiate the weaning process? What is the best mode for conducting a spontaneous breathing trial and how should the patient be monitored? What are the mechanisms for weaning (and spontaneous breathing trial) failure? What is the best technique to facilitate progressive withdrawal? What other factors can facilitate liberation from mechanical ventilation? What are the risks of extubation failure and how can extubation outcome best be predicted? What is the role for protocols in facilitating weaning from mechanical ventilation?.

Nonventilatory interventions in the acute respiratory distress syndrome

Acute respiratory distress syndrome was first described in 1967. Acute respiratory distress syndrome and acute lung injury are diseases the busy intensivist treats almost daily. The etiologies of acute respiratory distress syndrome are many. A significant distinction is based on whether the insult to the lung was direct, such as in pneumonia, or indirect, such as trauma or sepsis. Strategies for managing patients with acute respiratory distress syndrome/acute lung injury can be subdivided into 2 large groups, those based in manipulation of mechanical ventilation and those based in nonventilatory modalities. This review focuses on the nonventlilatory strategies and includes fluid restriction, exogenous surfactant, inhaled nitric oxide, manipulation of production, or administration of eicosanoids, neuromuscular blocking agents, prone position ventilation, glucocorticoids, extracorporeal membrane oxygenation, and administration of beta-agonists. Most of these therapies either have not been studied in large trials or have failed to show a benefit in terms of long-term patient mortality. Many of these therapies have shown promise in terms of improved oxygenation and may therefore be beneficial as rescue therapy for severely hypoxic patients. Recommendations regarding the use of each of these strategies are made, and an algorithm for implementing these strategies is suggested.

Effects of pressure support ventilation and continuous positive airway pressure on diaphragm performance

Many patients who are on mechanical ventilation are on ventilator modes called pressure support ventilation (PSV) and continuous positive airway pressure (CPAP) particularly when they are being weaned. As the diaphragm is responsible for approximately 75% of breathing, it is important to promote diaphragm shortening to optimize weaning from mechanical ventilation. The purpose of our 1998 quasi-experimental study was to explore the effects of PSV and CPVP on diaphragm shortening. An animal model was utilized using four Sprague-Dawley rats from the same litter purchased from Sasco (Kansas City, USA). Also measured in this study were intrathoracic pressure (DeltaITP), positive inspiratory pressure, respiratory rate, tidal volume, end-tidal carbon dioxide, central venous pressure (CVP) and mean arterial pressure (MAP). Pressure support was increased in increments of 5 cm H2O at CPAP levels of 0, 2 and 4 cm H2O. A direct assessment of diaphragm shortening was achieved through the adherence of a miniaturized ultrasonic sensor to the inferior surface of the middle costal surface of the right hemidiaphragm of four Sprague-Dawley rats. Limitations of this study included a small sample size, anaesthetized rats and abdominal dissection for insertion of the ultrasonic sensor. As PSV was increased, there was a decrease in MAP, CVP, respiratory rate and end-tidal CO2. When increasing levels of CPAP were added to PSV, a decrease in diaphragm shortening was observed. These results support that higher levels CPAP may hinder diaphragmatic function thus prolong mechanical ventilation. The purpose of this pilot study was to explore the effects of PSV and CPAP on diaphragm shortening. Also measured were DeltaITP, positive inspiratory pressure, respiratory rate, tidal volume, end-tidal carbon dioxide, CVP and MAP. Pressure support was increased in increments of 5 cm H2O at CPAP levels of 0, 2 and 4 cm H2O. A direct assessment of diaphragm shortening was achieved through the adherence of a miniaturized ultrasonic sensor to the inferior surface of the middle costal surface of the right hemidiaphragm of four Sprague-Dawley rats. Limitations of this study included a small sample size, anaesthetized rats and abdominal dissection for insertion of the ultrasonic sensor. As PSV was increased, there was a decrease in MAP, CVP, respiratory rate and end-tidal CO2. When increasing levels of CPAP were added to PSV, a decrease in diaphragm shortening was observed.

Capnometric recirculation gas tonometry and weaning from mechanical ventilation

The aim of this study was to describe changes in regional intramucosal PCO(2) (Pr(CO(2)) measured with capnometric recirculation gas tonometry [CRGT]) in patients with acute respiratory failure, who proceed from mechanical ventilation to weaning. In addition, we compared the predictive power for the weaning outcome of CRGT measurements obtained during mechanical ventilation to the frequency/ tidal volume (f/VT) ratio. A total of 24 patients (31 weaning trials) were included in the study, but four of the 24 patients (17%) were excluded because of extubation failure. Of the remaining 27 weaning trials in 20 patients, 12 (44%) were unsuccessful. Changes observed in patients with weaning failure (increase in Pr(CO(2)) from 60.4 +/- 15.0 mm Hg in mechanical ventilation to 67.4 +/- 21.0 mm Hg, in weaning) were significantly different (p = 0.046) from those observed in patients with weaning success (fall in Pr(CO(2)) from 61.5 +/- 15.0 mm Hg in mechanical ventilation to 56.3 +/- 16.7 mm Hg in weaning). However, absolute values of Pr(CO(2)) were not significantly different between patients with weaning success and failure, neither during mechanical ventilation (success, 61.5 +/- 15.0 versus failure, 60.4 +/- 15.0 mm Hg, p = 0.848) nor during weaning (success, 56.3 +/- 16.7 versus failure, 67.4 +/- 21.0 mm Hg, p = 0.135). The best single predictor for weaning outcome was the f/VT ratio measured early during weaning (area under the curve: 0.844 +/- 0.081; adjusted odds ratio for threshold value </= 105: 42.0, 95% CI 3.8 to 469.1, p = 0.002). CRGT could confirm a significant increase in Pr(CO(2)) during weaning in patients who finally failed the weaning trial. However, differences between patients with weaning success and failure were small and CRGT did not replace or improve the predictive power of the f/VT ratio for weaning outcome.

Beneficial effects of helium:oxygen versus air:oxygen noninvasive pressure support in patients with decompensated chronic obstructive pulmonary disease

OBJECTIVE

To test the hypothesis that, in decompensated chronic obstructive pulmonary disease (COPD), noninvasive pressure support ventilation using 70:30 helium:oxygen instead of 70:30 air:oxygen could reduce dyspnea and improve ventilatory variables, gas exchange, and hemodynamic tolerance.

DESIGN

Prospective, randomized, crossover study.

SETTING

Medical intensive care unit, university tertiary care center.

PATIENTS

Nineteen patients with severe COPD (forced 1-sec expiratory volume of 0.83+/-0.3 l) hospitalized in the intensive care unit for noninvasive pressure support ventilation after initial stabilization with noninvasive pressure support for no more than 24 hrs after intensive care unit admission.

INTERVENTIONS

Noninvasive pressure support ventilation was administered in the following randomized crossover design: a) 45 min with air:oxygen or helium:oxygen; b) no ventilation for 45 min; and c) 45 min with air:oxygen or helium:oxygen.

MEASUREMENTS AND MAIN RESULTS

Air:oxygen and helium:oxygen decreased respiratory rate and increased tidal volume and minute ventilation. Helium:oxygen decreased inspiratory time. Both gases increased total respiratory cycle time and decreased the inspiratory/total time ratio, the reduction in the latter being significantly greater with helium:oxygen. Peak inspiratory flow rate increased more with helium:oxygen. PaO2 increased with both gases, whereas PaCO2 decreased more with helium:oxygen (values shown are mean+/-SD) (52+/-6 torr [6.9+/-0.8 kPa] vs. 55+/-8 torr [7.3+/-1.1 kPa] and 48+/-6 torr [6.4+/-0.8 kPa] vs. 54+/-7 torr [7.2+/-0.9 kPa] for air:oxygen and helium:oxygen, respectively; p<.05). When hypercapnia was severe (PaCO2 >56 torr [7.5 kPa]), PaCO2 decreased by > or =7.5 torr (1 kPa) in six of seven patients with helium:oxygen and in four of seven patients with air:oxygen (p<.01). Dyspnea score (Borg scale) decreased more with helium:oxygen than with air:oxygen (3.7+/-1.6 vs. 4.5+/-1.4 and 2.8+/-1.6 vs. 4.6+/-1.5 for air:oxygen and helium:oxygen, respectively; p<.05). Mean arterial blood pressure decreased with air:oxygen (76+/-12 vs. 82+/-14 mm Hg; p<.05) but remained unchanged with helium:oxygen.

CONCLUSION

In decompensated COPD patients, noninvasive pressure support ventilation with helium:oxygen reduced dyspnea and PaCO2 more than air:oxygen, modified respiratory cycle times, and did not modify systemic blood pressure. These effects could prove beneficial in COPD patients with severe acute respiratory failure and might reduce the need for endotracheal intubation.

Optimal pressure support level for beginning weaning in patients with COPD: measurement of diaphragmatic activity with step-by-step decreasing pressure support level

PURPOSE

The study objective was to determine an "optimal" individual pressure support (PS) level for beginning weaning with PS ventilation in patients with chronic obstructive pulmonary disease (COPD).

MATERIALS AND METHODS

Eleven COPD patients intubated and ventilated for acute respiratory failure and judged ready for weaning were studied. The technique consisted of lowering the PS level from a point that was characteristic for each patient and measurable under controlled mechanical ventilation, after setting the ventilator as recommended for COPD patients judged ready for weaning, that is, peak inflation pressure (PIP). This determination was based mainly on exploring the diaphragm with an electromyographic technique by defining the optimal PS level as the lowest PS level associated with no EMG evidence of diaphragmatic stress. Diaphragmatic electromyographic activity (diEMG) was recorded by a bipolar esophageal electrode (Disa-Denmark), and the high-frequency electrical component/low-frequency ratio (H/L) was calculated. The reference H/L was determined during a few spontaneous ventilatory cycles. Muscle stress was defined as a greater than 20% reduction in H/L compared with the reference value.

RESULTS

Optimal PS levels ranged from 4 to 24 cm H2O with a mean of 14+/-6 cm H2O. Two patients with optimal PS level at 4 cm H2O did not require weaning and were quickly extubated. For the nine other patients, optimal PS levels were found to be 70% of PIP; in none was it necessary during weaning to use PS levels higher than individual optimal PS levels.

CONCLUSIONS

Optimal PS level established with diEMG monitoring seems to be a useful index for beginning weaning in the PS ventilation mode in COPD patients. The hypothesis of beginning weaning with a PS level equal to 70% of PIP needs to be tested.

Influence of mechanical ventilation on blood lactate in patients with acute respiratory failure

OBJECTIVES

To determine whether mechanical ventilation (MV) may affect blood lactate concentration in patients with acute respiratory failure.

DESIGN

Prospective observational study with follow-up to hospital discharge.

SETTING

A 17-bed medical and coronary intensive care unit in a 650-bed general hospital.

PATIENTS

55 adult patients mechanically ventilated for acute respiratory failure between May 1996 and April 1997 were recruited.

MEASUREMENTS AND RESULTS

Arterial blood samples for determination of plasma lactate and blood gas analysis were taken just before tracheal intubation on spontaneous breathing, and 20 and 60 min after the initiation of controlled MV. Cuff systemic arterial pressure was measured before tracheal intubation and every 10 min during the first h of MV. Hyperlactatemia (arterial blood lactate > or = 2 mmol/l) was present in 21 of the 55 patients studied. After 20 min of MV, there was a decrease in blood lactate from 4.74 +/- 1.78 to 3.07 +/- 1.69 mmol/l (p < 0.01); 40 min later there was a further decrease to 2.63 +/- 1.35 mmol/l (p < 0.05). The decrease in blood lactate was also observed in those patients who after starting MV developed systemic arterial hypotension (p < 0.01). In patients with a normal lactate concentration at the entry to the study, lactate remained the same after 60 min on MV (NS).

CONCLUSIONS

Controlled MV decreases substantially the severity of hyperlactatemia in patients with acute respiratory failure, and any adverse circulatory effects of MV do not alter this beneficial outcome.

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.

Increased initial flow rate reduces inspiratory work of breathing during pressure support ventilation in patients with exacerbation of chronic obstructive pulmonary disease

OBJECTIVE

To investigate whether the level of initial flow rate alters the work of breathing in chronic obstructive pulmonary disease (COPD) patients ventilated in pressure support ventilation (PSV).

DESIGN

Prospective study.

SETTINGS

Medical ICU in University hospital.

PATIENTS

Eleven intubated COPD patients.

METHODS

We modulated the initial flow rate in order to achieve seven different sequences. In each sequence, the plateau pressure was reached within a predetermined time: 0.1, 0.25, 0.50, 0.75, 1, 1.25 or 1.50 s. The more rapidly the pressure plateau was achieved, the higher was the initial flow rate. In each patient, the pressure support level was an invariable parameter. The order of the seven sequences for each patient was determined randomly.

MEASUREMENTS AND RESULTS

Ten minutes after application of each initial flow rate, we measured the following parameters: inspiratory work of breathing, electromyogram (EMG) of the diaphragm (EMGdi), breathing pattern, and intrinsic positive end-expiratory pressure (PEEPi). Comparison between the means for each sequence and each variable measured was performed by two-way analysis of variance with internal comparisons between sequences by Duncan's test. The reduction of the initial flow rate induced a progressive increase in the values of the work of breathing, EMGdi, and mouth occlusion pressure (P 0.1). In contrast, the reduction of the initial flow rate did not induce any significant change in tidal volume, respiratory frequency or PEEPi.

CONCLUSIONS

As the objective of PSV is to reduce the work of breathing, it seems logical to use the highest initial flow rate to induce the lowest possible work of breathing in COPD ventilated patients.

The effects of neuromuscular paralysis on systemic and splanchnic oxygen utilization in mechanically ventilated patients

OBJECTIVE

To evaluate the effect of neuromuscular paralysis on systemic and splanchnic oxygen utilization in patients in respiratory failure during controlled mechanical ventilation.

SETTING

A university-affiliated teaching hospital.

INTERVENTION

Mechanically ventilated patients, who were undergoing hemodynamics monitoring and who had a gastric intramucosal pH (pHi) of less than 7.35, were studied. Prior to paralysis, the patients were sedated with lorazepam and morphine to standard end points, and the cardiac output and oxygenation were optimized. The patients were then paralyzed with doxacurium and the ventilator rate adjusted to keep the PaCO2 at baseline value. The hemodynamic and oxygenation profile and pHi were determined prior to paralysis and repeated 2 to 2.5 h later.

RESULTS

Eight patients were studied; their mean age was 63 +/- 8 years and acute physiology and chronic health evaluation II score was 22 +/- 4. The mean fraction of inspired oxygen, positive end-expiratory pressure, and venous admixture ratio prior to the study was 0.7 +/- 0.14, 11.8 +/- 2.4 cm H2O, and 26 +/- 9%, respectively. Prior to paralysis, the mean set assist controlled ventilation rate was 15 +/- 2 breaths/min and the patient rate was 23 +/- 5 breaths/min. With neuromuscular paralysis, the cardiac index fell from 4.6 +/- 2.2 to 4.3 +/- 2.4 L/min/m2 (p=0.1), the oxygen delivery fell from 537 +/- 129 to 471 +/- 95 mL/min/m2 (p=0.03), and the oxygen consumption and extraction ratio fell from 200 +/ 77 to 149 +/- 35 mL/min/m2 (p=0.03) and 36 +/- 5 to 31 +/- 10, respectively (p=0.2). The pHi increased from 7.21 +/- 0.16 to 7.29 +/- 0.1 (p=0.02).

CONCLUSION

In critically ill patients in respiratory failure, neuromuscular paralysis decreased whole body oxygen consumption and increased pHi. Presumably, by eliminating the work of breathing, there is a redistribution of blood flow from the respiratory muscles to the splanchnic and other nonvital vascular beds.

Oxygen cost of breathing in newborn infants with long-term ventilatory support

Oxygen consumption (VO2) was measured during controlled and spontaneous ventilation with continuous positive airway pressure in newborn infants in whom chronic lung disease later developed. The oxygen cost of breathing (the difference in VO2 between spontaneous and controlled ventilation) was significantly higher in infants with chronic lung disease than in control infants (20.1% +/- 7.5% and 4.8% +/- 4.9% of VO2 during spontaneous ventilation (p < 0.05), respectively).

Comparison of the effects of pressure support ventilation delivered by three different ventilators during weaning from mechanical ventilation

OBJECTIVE

To compare the effects of pressure support ventilation (PSV) delivered at the same level by three different ventilators on patients' work of breathing (WOB), breathing pattern and gas exchange.

DESIGN

Prospective, self-controlled clinical study.

SETTING

Intensive care unit of a tertiary university hospital.

PATIENTS

Nine intubated adult patients during weaning from mechanical ventilation.

INTERVENTIONS

Patients were randomly connected to one of three ventilators: the Siemens Servo 900 C (SC), the Ohmeda CPU 1 (CPU), and the Engström Erica (EE) during both zero cmH2O PSV and 15 cmH2O PSV.

MEASUREMENTS AND RESULTS

During zero PSV, there was no significant difference in terms of WOB, VT, VE, or auto-PEEP among the three ventilators, although there was a trend towards higher levels of WOB with EE. During 15 cmH2O PSV, WOB was significantly less with SC than with EE or CPU (0.47 +/- 0.48 J/l for SC, 1.0 +/- 0.48 for EE and 0.78 +/- 0.51 for CPU1, p = 0.003). WOB was 64% less than at zero PSV with SC but only 38% less with EE. This was associated with a different pressurization shape, as assessed by the interior surface of Paw-VT loops (1.23 +/- 0.09 J/l for SC, 0.9 +/- 0.02 for EE, and 0.79 +/- 0.18 for CPU; p < 0.001). At 15 cmH2O PSV, auto-PEEP was significantly lower with SC than with EE (1.7 +/- 2.1 cmH2O for SC, 4.7 +/- 3.6 for EE, and 2.8 +/- 0.3 for CPU; p = 0.04). External expiratory resistances, in cmH2O/l/s, were significantly higher with EE than with CPU or SC (12.9 +/- 3.2 EE, 7.5 +/- 2.4 CPU, 5.9 +/- 0.5 SC; p < 0.001).

CONCLUSION

During PSV, the different working principles of different mechanical ventilators profoundly affect patient's WOB. Among the various factors, velocity of pressurization of PSV may play a role in its efficacy in unloading the respiratory muscles.

P0.1 is a useful parameter in setting the level of pressure support ventilation

OBJECTIVE

The purpose of this study was to investigate whether changes in breathing pattern, neuromuscular drive (P0.1), and the work involved in breathing might help to set the individual appropriate level of pressure support ventilation (PSV) in patients with acute respiratory failure (ARF) requiring ventilatory assistance.

DESIGN

A prospective, interventional study.

SETTING

An 8-bed multidisciplinary intensive care unit (ICU).

PATIENTS

Ten patients with ARF due to adult respiratory distress syndrome (ARDS), sepsis or airway infection were included in the study. Chronic obstructive pulmonary disease (COPD) patients with acute exacerbation were excluded. None of these patients was in the weaning process.

INTERVENTIONS

We found a level of pressure support able to generate a condition of near-relaxation in each patient, as evidenced by work of breathing (WOB) values close to 0 J/1. This level was called PS 100 and baseline physiological measurements, namely, breathing pattern, P0.1 and WOB were obtained. Pressure support was then reduced to 85%, 70%, and 50% of the initial value and the same set of measurements was obtained.

MEASUREMENTS AND RESULTS

Flow (V) was measured by a flow sensor (Varflex) positioned between the Y-piece of the breathing circuit and the endotracheal tube. Tidal volume was obtained by numerical integration of the flow signal. Airway pressure (P(aw)) was sampled through a catheter attached to the flow sensor. Esophageal pressure (Pes) was measured with a nasogastric tube incorporating an esophageal balloon. The esophageal balloon and flow and pressure sensors were connected to a portable monitor (CP 100 Bicore) that provided real-time display of flow, volume, P(aw) and Pes tracings and loops of Pes/V, P(aw)/V and V/V relationships. The breathing pattern was analyzed from the flow signal. Patient work of breathing (WOB) was calculated by integration of the area of the Pes/V loop. Respiratory drive (P0.1) was measured at the esophageal pressure change during the first 100 ms of a breath, by the quasi-occlusion technique. When pressure support was reduced, we found that the respiration rate significantly increased from PS 100 to PS 85, but varied negligibly with lower pressure support levels. Tidal volume behaved in a similar way, decreasing significantly from PS 100 to PS 85, but hardly changing at PS 70 and PS 50. In contrast, WOB and P0.1 increased progressively with decreasing pressure support levels. The changes in WOB were significant at each stage in the trial, whereas P0.1 increased significantly from PS 100 at other stages. Linear regression analysis revealed a highly positive, significant correlation between WOB and P0.1 at decreasing PSV levels (r = 0.87), whereas the correlation between WOB and ventilatory frequency was less significant (r = 0.53). No other correlation was found.

CONCLUSIONS

During pressure support ventilation, P0.1 may be a more sensitive parameter than the assessment of breathing pattern in setting the optimal level of pressure support in individual patients. Although P0.1 was measured with an esophageal balloon in the present study, non-invasive techniques can also be used.

Pressure support ventilation attenuates the cardiopulmonary response to an acute increase in oxygen demand

Critically ill patients undergo interventions, such as chest physical therapy, that acutely increase metabolic rate. Previous observations revealed that chest physical therapy is accompanied by increases of 40 to 50% in oxygen consumption (Vo2) and 40% in minute ventilation contributes to the rise in Vo2 and its associated hemodynamic responses. This was done by increasing mandatory ventilatory support during the chest physical therapy session: In phase 1 the mandatory ventilation rate was increased by 35% and in phase 2 pressure support ventilation 15 cm h2O was added. In phase 1 (n = 12), the increase in mandatory rate did not attenuate the chest physical therapy induced rises in heart rate, arterial blood pressure and Vo2. The increase in minute ventilation when the mandatory rate was increased prevented a rise in PaCO2. In phase 2 (n = 15), no change in the increase in Vo2 with chest physical therapy was observed with the addition of pressure support. Yet the rises in heart rate and systemic and pulmonary artery pressures were attenuated, as was the increase in PaCO2. Respiratory rate did not increase as much with pressure support. There appears to be a role for pressure support ventilation in attenuating the pulmonary and hemodynamic responses to interventions that increase oxygen demand.

A comparison of the effects of assist-control, SIMV, and SIMV with pressure support on ventilation, oxygen consumption, and ventilatory equivalent

OBJECTIVE

To quantify the ventilatory efficiency of different modes of mechanical ventilation used to achieve full ventilatory support in normal subjects. Modes compared were assist-control, synchronized intermittent mandatory ventilation (SIMV), and SIMV with 10 cm H2O (0.98 kPA) of pressure support.

DESIGN

Prospective, randomized blocks repeated measures design. Subjects served as their own controls.

SETTING

A university affiliated pulmonary laboratory.

SUBJECTS

Ten healthy volunteers, aged 31-54 years.

OUTCOME MEASURES

Minute volume, respiratory rate, average tidal volume, oxygen consumption, and ventilatory equivalent.

INTERVENTION

Baseline spontaneous ventilation data collection was followed by mechanical ventilation by mouthpiece in each of three modes in a random sequence. All modes used a machine set rate of 12 breaths per minute, VT of 10 cc/kg of ideal body weight, inspiratory time of 1 second, square wave flow pattern and a sensitivity of -1 cm H2O (-0.09806 kPa) to achieve full ventilatory support. Data were collected continuously for 5 minutes and the mean values were reported. Ventilatory equivalent for oxygen is a measure of the efficiency of the ventilatory pump at various work loads and was calculated by dividing VE (BTPS) by the VO2 (STPD).

RESULTS

There were significant differences by mode of mechanical ventilation in average tidal volume (p = 0.02), minute volume (p = 0.02), oxygen consumption (p = 0.04), and ventilatory equivalent (p = 0.01) using ANOVA. There was no significant difference (p = 0.66) by mode of ventilation in respiratory rate. Pairwise follow-up comparisons for these variables found that SIMV with pressure support produced a significantly greater average tidal volume, minute volume, oxygen consumption, and ventilatory equivalent than SIMV alone. SIMV with pressure support also produced a significantly greater minute volume and ventilatory equivalent than assist-control. There were no significant differences between assist-control and SIMV. All three modes produced a lower ventilatory equivalent and higher oxygen consumption than spontaneous breathing.

CONCLUSIONS

SIMV with pressure support significantly increased minute volume and ventilatory equivalent when compared with assist-control or SIMV alone, and thus was the most efficient mode of full ventilatory support for our subjects. We found no difference in ventilatory efficiency between assist-control and SIMV. All three mechanical modes were less efficient for our subjects than spontaneous breathing. The inspiratory time of 1 second used in this study, although common in clinical practice, may be inadequate for some patients.

Synchronized intermittent mandatory ventilation with and without pressure support ventilation in weaning patients with COPD from mechanical ventilation

This prospective study compared two weaning modalities in COPD patients requiring mechanical ventilation (MV) for acute respiratory failure. Nineteen patients with COPD were studied when their precipitating illness was controlled. Although they satisfied the conventional bedside weaning criteria, they could not tolerate any reduction in the respirator rate below 10 cycles/min. At this time, patients were randomized into two groups receiving either synchronized intermittent mandatory ventilation (SIMV) with pressure support ventilation (PSV) (group 1) or SIMV alone (group 2). The volumetric support of ventilation (SIMV rate) was progressively decreased in both groups according to the patient's tolerance with a concurrent decrease in the barometric support of ventilation (PSV levels from 15 cm H2O to 6 cm H2O). At each step of SIMV rate, we found no difference between group 1 and group 2 in arterial blood gases, blood pressure, heart rate, airway occlusion pressure, maximal inspiratory pressure, and oxygen cost of breathing (OCB). At each step, however, group 1 patients showed significantly higher spontaneous tidal volume and lower spontaneous breathing frequency than did group 2 patients. We found a slight but not significant tendency to a shorter weaning period with than without PSV, but no difference in the weaning success. We concluded that (1) conventional weaning criteria might be inaccurate in COPD patients, (2) SIMV appeared very useful in weaning COPD patients from MV, (3) PSV marginally reduced the weaning period when added to SIMV, and (4) the OCB was not significantly improved with PSV.

Spontaneous breathing and total body oxygen consumption in children recovering from open-heart surgery

The effects of withdrawal of ventilatory support on cardiopulmonary function, oxygen consumption and carbon dioxide production were assessed in 25 infants and children within seven days (2.9 +/- 2.5 days; mean +/- SD) of an open heart operation, during weaning from ventilatory support. The average age of the patients was 3.4 +/- 3.5 years and weight 12.4 +/- 8.3 kg. Heart rate, blood pressure, arterial and central venous blood gas values, and oxyhemoglobin saturations were measured during controlled mechanical ventilation and during spontaneous breathing with continuous positive airway pressure. Simultaneously, VO2 and VCO2 were measured using indirect calorimetry. Withdrawal of ventilatory support effected an expected, significant decrease in arterial pH (7.42 +/- 0.10 to 7.37 +/- 0.06; p less than 0.001) and an increase in PaCO2 (34 +/- 6 to 40 +/- 5 mm Hg; p less than 0.0001), while arterial blood oxyhemoglobin saturation, heart rate, and blood pressure remained unchanged. A significant increase in central venous oxyhemoglobin saturation (67.9 +/- 11.9 to 74.8 +/- 8.3 percent; p less than 0.001) indicated improvement in systemic blood flow during spontaneous breathing. Average VO2 and VCO2 did not change significantly. A decrease in VO2 by more than 5 percent was seen in seven patients, an increase by more than 5 percent in nine, and a change within +/- 5 percent in nine patients. The change in VO2 was inversely related to the difference between measured and expected VO2 during mechanical ventilation (r = -0.73) and to body temperature (r = -0.69). The results indicate that factors other than the oxygen uptake by the respiratory muscles may have significant effects on total body VO2 in infants and children after open-heart surgery. Therefore, monitoring of VO2 during withdrawal of ventilatory support may not be an accurate indicator of respiratory work and oxygen cost of breathing in these patients.

Plasma catecholamines and oxygen consumption during weaning from mechanical ventilation

Previous studies on oxygen consumption (VO2) during weaning from mechanical ventilation assumed that an increase in VO2 (delta VO2) reflected oxygen consumption by respiratory muscles (VO2RESP), and proposed delta VO2 as a weaning predictor. We measured VO2 CO2 production (VCO2) and plasma catecholamines in 20 short-term ventilated patients during weaning by SIMV and CPAP. delta VO2 as a percentage of VO2 during spontaneous ventilation (delta VO2%) ranged from 4.8% to 41.5%. VCO2 also increased and correlated with VO2. Plasma adrenaline and noradrenaline increased significantly to levels known to produce considerable increases in metabolic rate. Mean arterial pressure and heart rate concomitantly increased, but spontaneous minute ventilation decreased. Thus, since the increased plasma catecholamines are calorigenic, the assumption that delta VO2 represents VO2RESP is incorrect. Although mean delta VO2% of successfully weaned patients was significantly less than that of failure-to-wean patients, the wide scatter of individual values in the latter group excludes delta VO2% as an accurate weaning predictor.