The work of breathing: potential for clinical application and the results of studies performed on 100 normal males

Assessing breathing effort in mechanical ventilation: physiology and clinical implications

Recent studies have shown both beneficial and detrimental effects of patient breathing effort in mechanical ventilation. Quantification of breathing effort may allow the clinician to titrate ventilator support to physiological levels of respiratory muscle activity. In this review we will describe the physiological background and methodological issues of the most frequently used methods to quantify breathing effort, including esophageal pressure measurement, the work of breathing, the pressure-time-product, electromyography and ultrasound. We will also discuss the level of breathing effort that may be considered optimal during mechanical ventilation at different stages of critical illness.

The application of esophageal pressure measurement in patients with respiratory failure

This report summarizes current physiological and technical knowledge on esophageal pressure (Pes) measurements in patients receiving mechanical ventilation. The respiratory changes in Pes are representative of changes in pleural pressure. The difference between airway pressure (Paw) and Pes is a valid estimate of transpulmonary pressure. Pes helps determine what fraction of Paw is applied to overcome lung and chest wall elastance. Pes is usually measured via a catheter with an air-filled thin-walled latex balloon inserted nasally or orally. To validate Pes measurement, a dynamic occlusion test measures the ratio of change in Pes to change in Paw during inspiratory efforts against a closed airway. A ratio close to unity indicates that the system provides a valid measurement. Provided transpulmonary pressure is the lung-distending pressure, and that chest wall elastance may vary among individuals, a physiologically based ventilator strategy should take the transpulmonary pressure into account. For monitoring purposes, clinicians rely mostly on Paw and flow waveforms. However, these measurements may mask profound patient-ventilator asynchrony and do not allow respiratory muscle effort assessment. Pes also permits the measurement of transmural vascular pressures during both passive and active breathing. Pes measurements have enhanced our understanding of the pathophysiology of acute lung injury, patient-ventilator interaction, and weaning failure. The use of Pes for positive end-expiratory pressure titration may help improve oxygenation and compliance. Pes measurements make it feasible to individualize the level of muscle effort during mechanical ventilation and weaning. The time is now right to apply the knowledge obtained with Pes to improve the management of critically ill and ventilator-dependent patients.

Measurement validity of an electronic inspiratory loading device during a loaded breathing task in patients with COPD

We studied the validity of a recently introduced, handheld, electronic loading device in providing automatically processed information on external inspiratory work, power and breathing pattern during loaded breathing tasks in patients with COPD. Thirty-five patients with moderate to severe COPD performed an endurance breathing task against a fixed resistive inspiratory load that corresponded to 55 ± 13% of their maximal inspiratory pressure. Flow and pressure signals during this task were sampled and processed at 500 Hz by the handheld loading device and at 100 Hz with an external, laboratory system that provided the "gold standard" reference data. Intra Class Correlations between methods were 0.97 for average mean inspiratory power, 0.98 for average mean pressure, 0.98 for average duty cycle, and 0.99 for total work (all p < 0.0001). We conclude that the handheld device provides automatically processed and valid estimates of physical units of energy during loaded breathing tasks. This enables health care providers to quantify the load on inspiratory muscles during these tests in daily clinical practice.

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.

Work of breathing measurement in the critically ill patient

Weaning patients from mechanical ventilation in the intensive care unit can be difficult. In patients requiring prolonged ventilatory support it has been demonstrated that conventional weaning criteria are frequently incorrect. In this group measurement of respiratory work may be of benefit. Until recently, estimation of the work of breathing in patients receiving mechanical ventilation was logistically difficult. The availability of a computerized bedside monitoring device potentially allows easier estimation of the work of breathing at the bedside. The results of preliminary studies utilizing such monitoring are provocative: they highlight the phenomenon of nosocomial respiratory failure and challenge our clinical ability to determine patient workloads and timing of extubation. The potential benefits of work of breathing measurement, in particular the avoidance of respiratory muscle fatigue, earlier extubation, reduced duration of mechanical ventilation, reduction in ICU and hospital length of stay, and most importantly, a reduction in patient morbidity are yet to be demonstrated and concerns still exist about the monitor's accuracy.

Monitoring patient mechanics during mechanical ventilation

This article provides a review of respiratory mechanics that can be monitored in ventilator-dependent patients during passive and spontaneous breathing. Special focus is placed on resistance, compliance, and work of breathing. A description of methods and techniques, and a summary of clinical observations and applications in critically-ill patients are also included.

Use of the rapid/shallow breathing index as an indicator of patient work of breathing during pressure support ventilation

BACKGROUND

Measuring patient work of breathing (WOBpt) has been suggested to provide safe, aggressive weaning from mechanical ventilation. We compared WOBpt and pressure-time-product (PTP) to routine weaning parameters [breath rate (f), tidal volume (VT), frequency/tidal volume ratio (f/VT)] at different levels of pressure support ventilation (PSV).

METHODS

Fifteen patients in the surgical intensive care unit requiring prolonged weaning (more than 3 days) were entered in the study. A balloon-tipped esophageal catheter was placed and position confirmed by inspection of pressure and flow waveforms. Each patient was randomly assigned to breathe with 5, 10, 15, and 20 cm H2O of PSV. After 30 minutes, 40 breaths were recorded and analyzed. Measurement of WOBpt PTP, f, VT, and f/VT were made using the Bicore CP-100 monitor. Mean values for each parameter were calculated. PTP and WOBpt were plotted against f/VT to determine correlation coefficient.

RESULTS

PTP, WOBpt and f/VT decreased in a stepwise fashion as PSV was increased. The f/VT correlated most closely with WOBpt (r = 0.983) and PTP (r = 0.972). Monitoring f alone also correlated with WOBpt (r = 0.894) and PTP (r = 0.881). All patients were weaned from the ventilator (mean duration, 22 +/- 5.9 days). Nine patients required tracheostomy before final liberation from the ventilator (mean duration, 22 +/- 5.9 days). Nine patients required tracheostomy before final liberation from the ventilator.

CONCLUSIONS

Direct measurement of WOBpt is invasive, expensive, and' may be confusing to clinicians. Monitoring f/VT may be useful when changing PSV during weaning.

Changes in ventilatory mechanics and diaphragmatic function after lung volume reduction surgery in patients with COPD

BACKGROUND

Lung volume reduction (LVR) has recently been used to treat severe emphysema. About 25% of the volume of each lung is removed with this method. Little is known about the mechanism of functional improvement so a study was undertaken to investigate the changes in ventilatory mechanics and diaphragmatic function in eight patients after LVR.

METHODS

Measurements of work of breathing (WOB), intrinsic positive end expiratory pressure (PEEPi), dynamic compliance (Cdyn), and arterial carbon dioxide tension (PaCO2) were performed on the day before surgery and daily for seven days after surgery, as well as one, three, and six months after surgery. All measurements were performed on spontaneously breathing patients, simultaneously assessing oesophageal pressure via an oesophageal balloon catheter and air flow via a tightly adjusted mask. Diaphragmatic function was evaluated by measuring oesophageal and transdiaphragmatic pressure (Pdi) preoperatively and at one, three, and six months postoperatively.

RESULTS

Mean forced expiratory volume in one second (FEV1) was 23 (3.6)% predicted, and all patients were oxygen dependent before the-operation. One day after LVR the mean decrease in WOB was 0.93 (95% confidence interval (CI) 0.46 to 1.40) joule/l, the mean decrease in PEEPi was 0.61 (95% CI 0.35 to 0.87) kPa, and the mean increase in Cdyn was 182.5 (95% CI 80.0 to 284.2) ml/kPa. Similar changes were found seven days and six months after surgery. PaCO2 was higher on the day after the operation but was significantly reduced six months later. Pdi was increased three and six months after surgery.

CONCLUSIONS

Ventilatory mechanics improved immediately after LVR, probably by decompression of lung tissue and relief of thoracic distension. An improvement in diaphragmatic function three and six months postoperatively also contributes to improved respiratory function after LVR.

The influence of lung volume reduction surgery on ventilatory mechanics in patients suffering from severe chronic obstructive pulmonary disease

Recently, lung volume reduction [LVR] removal of about 20% of lung volume), has been performed to treat severe emphysema. Little is known, however, about the mechanism and time course of functional improvement, and the reasons that such patients can be tracheally extubated very early. Therefore, we studied changes in ventilatory mechanics in 12 patients after LVR. Measurements of work of breathing (WOB), intrinsic positive end-expiratory pressure (PEEPi), dynamic compliance (Cdyn), and mean airway resistance (Rawm) were performed the day before surgery, early postoperatively, and 1 and 3 mo after surgery. All measurements were performed on tracheally extubated patients, simultaneously assessing esophageal pressure via esophageal balloon catheter and air flow via tightly adjusted mask. Standard spirometry was assessed pre-operatively and 1 and 3 mo postoperatively. The patients presented with forced expiratory volume in 1 s (FEV1), of 670 +/- 50 mL and pathological values of WOB and PEEPi. All patients were successfully tracheally extubated within 5 h postoperatively. Immediately thereafter, a marked and sustained decrease in WOB, PEEPi, and Rawm was noted, as well as an increase in Cdyn. Ventilatory mechanics improved immediately after LVR, probably due to decompression of lung tissue, thereby enabling successful tracheal extubation.

Pulmonary flow-resistive work during hydrostatic loading

This paper focuses upon flow-resistive pulmonary work during upright immersion, and during changes in the air delivery pressure. Nine male non-smokers (aged 26.2 +/- 3.5 years), with normal lung function history, performed spontaneous respiration while seated in air (control) and during total immersion. During the immersed state subjects were supplied with air at four hydrostatic pressures: mouth pressure (PM; simulating a mouth-held demand regulator), lung centroid pressure (PLC; + 1.33 kPa relative to the sternal notch), and 0.98 kPa (10 cmH2O) above and below the lung centroid pressure. Inspiratory, expiratory and total flow-resistive pulmonary work were computed from the integration of transpulmonary pressure (difference between oesophageal and mouth pressure) with respect to lung volume change. When breathing air delivered at mouth pressure, immersion significantly elevated all total flow-resistive pulmonary work components (P less than 0.05). Each increment in breathing pressure resulted in a progressive reduction in expiratory and total flow-resistive pulmonary work, so that when air was provided at lung centroid pressure and lung centroid pressure +0.98 kPa both components were similar to control values (P greater than 0.05). Inspiratory was always less than expiratory pulmonary work. During immersion inspiratory pulmonary work was significantly reduced when air supply pressure was increased above mouth pressure (P less than 0.05). Subsequent pressure increments failed to produce further changes in inspiratory pulmonary work. The difference in response between the inspiratory and expiratory components of total flow resistive pulmonary work was attributed primarily to the volume-dependence of the expiratory component.

Mechanics of breathing during prolonged artificial ventilation

In three patients with lesions of peripheral regions of the central nervous system and paralyses of the respiratory muscles (malignant myasthenia, amyotrophic lateral sclerosis and residual manifestations after poliomyelitis), artificial ventilation has been performed continuously for periods from 7 years 4 months to 18 years 9 months using some degree of hyperventilation (PCO2 19.5--26.8 mmHg). The mechanics of breathing were studied by pneumotachography with synchronous recording of transpulmonary and oesophageal pressures using cuffed tubes. The findings were a decrease of total compliance of the respiratory apparatus (Ctot 23--28 ml x cm x H2O-1), a severe decrease in compliance of the lung tissue (CL 24--30 ml x cm x H2O-1), a sharp increase of bronchial resistance during inspiration (RresLI 15.0--23 cm x H2O l-1 s-1), and during expiration (RresLE 16.5--30.7 cm x H2O l-1 s-1), an elevation of compliance of the chest (Cw 358--555 ml x cm x H2O-1), a reduction of non-elastic resistance of the chest (Rres w 0.2--0.6 cm x H2O l-1 s-1). These parameters indicated progressive changes of the structures of the lung tissue and of the tracheobronchial tree which were most pronounced in the case of amyotrophic lateral sclerosis.

Work of nasal breathing: measurement of each nostril independently using a split mask

The work of nasal breathing was determined in human subjects as a measure of impedance to respiratory airflow. The nasal cavities were examined separately and simultaneously with a split mask; flow and pressure signals were fed to a microprocessor for on-line computation and printout of respired volumes and work of nasal breathing. An alternating resistive nasal cycle of 3--4 hours' duration was demonstrated in the majority of normal, resting subjects. Reciprocity of the resistive changes in each nasal cavity maintained a constant total nasal respiratory work load of about 0.2 Joules/litre. Moderate changes in breathing rate and tidal volume had little influence on work. Inspiratory work was 1.6 times that of expiration. Increases in resistance of the dependent nostril were seen when the lateral decubitus position was adopted. Increase in cephalic venous pressure and pathological nasal obstruction increased the work of nasal breathing.

A comparison of central venous pressure and pleural pressure in supine dogs

Respiration induced changes in central venous pressure were analyzed and compared with intrapleural pressure changes in dogs. During normal breathing intrapleural pressure changes were transmitted to the vena cava with distortion consisting of attenuation, addition of cardiac and mean pressure components, and a slight temporal delay. Attenuation and temporal delay increased in a regular manner as mean central venous pressure increased. Cardiac components could be removed by electronic filters. The presence of these distortions suggest the need for caution in interpreting intrapleural pressure changes from central venous pressure.