Pressure support compensation for inspiratory work due to endotracheal tubes and demand continuous positive airway pressure

Web Applications for Teaching the Respiratory System: Content Validation

The subject of respiratory mechanics has complex characteristics, functions, and interactions that can be difficult to understand in training and medical education contexts. As such, education strategies based on computational simulations comprise useful tools, but their application in the medical area requires stricter validation processes. This paper shows a statistical and a Delphi validation for two modules of a web application used for respiratory system learning: (I) “Anatomy and Physiology” and (II) “Work of Breathing Indexes”. For statistical validation, population and individual analyses were made using a database of healthy men to compare experimental and model-predicted data. For both modules, the predicted values followed the trend marked by the experimental data in the population analysis, while in the individual analysis, the predicted errors were 9.54% and 25.38% for maximal tidal volume and airflow, respectively, and 6.55%, 9.33%, and 11.77% for rapid shallow breathing index, work of breathing, and maximal inspiratory pressure, respectively. For the Delphi validation, an average higher than 4 was obtained after health professionals evaluated both modules from 1 to 5. In conclusion, both modules are good tools for respiratory system learning processes. The studied parameters behaved consistently with the expressions that describe ventilatory dynamics and were correlated with experimental data; furthermore, they had great acceptance by specialists.

Dynamic computed tomography evaluation of the nasopharynx in normal Beagle dogs

Pharyngeal collapsibility has been used as diagnostic criteria in dogs, whereas the normal range and quantitative method have not been studied. Dynamic and static computed tomography (CT) was performed in 23 normal Beagle dogs to quantify the nasopharyngeal collapsibility at different locations. Using dynamic CT, maximum and minimum nasopharyngeal cross-sectional areas (CSAs) were measured at the level of the cranial end of the soft palate, pterygoid hamulus, foramen lacerum, bony labyrinth, and caudal end of the soft palate. The ratio of all maximum and minimum CSA to nasopharyngeal CSA at the level of the caudal hard palate (rCSAmax and rCSAmin) and the nasopharyngeal collapsibility were calculated. The differences of rCSAmax, rCSAmin, and nasopharyngeal collapsibility were analyzed at various locations. The nasopharyngeal collapsibility at the level of foramen lacerum, bony labyrinth, and caudal end of soft palate were higher than the others. At the level of the caudal end of the soft palate, rCSAmin was lower than that of the foramen lacerum and bony labyrinth, whereas rCSAmax at foramen lacerum was higher than that of the caudal end of the soft palate. These results indicated that the nasopharynx at the level of foramen lacerum and caudal end of the soft palate were considered notable locations for evaluating collapsibility. Dynamic CT could show the nasopharyngeal dynamic profile and will be an adequate modality for evaluating nasopharynx. Our results will be helpful for further comparative studies in dogs with and without nasopharyngeal collapse.

Spontaneous Breathing and Imposed Work During Pediatric Mechanical Ventilation: A Bench Study

OBJECTIVES

To calculate imposed work of breathing during simulated spontaneous breathing at a given tidal volume across the range of normal length or shortened pediatric endotracheal tube sizes and endotracheal tubes with an intraluminal catheter in situ.

DESIGN

In vitro study.

SETTING

Research laboratory.

INTERVENTIONS

A bench model (normal compliance, no airway resistance) simulating sinusoid flow spontaneous breathing used to calculate imposed work of breathing for various endotracheal tube sizes (3.0-7.5 mm). Imposed work of breathing was calculated by integrating inspiratory tidal volume over the end-expiratory difference between the positive end-expiratory pressure and the tracheal pressure. Measurements were taken at different combinations of set spontaneous tidal volume (2.5, 5.0, 7.5, and 10 mL/kg), age-appropriate inspiratory times, length of endotracheal tube, and presence of intraluminal catheter.

MEASUREMENTS AND MAIN RESULTS

Overall median imposed work of breathing (Joules/L) was not significantly different between the four age groups: 0.047 Joules/L (interquartile range, 0.020-0.074 Joules/L) for newborns, 0.077 Joules/L (interquartile range, 0.032-0.127 Joules/L) for infants, 0.109 Joules/L (interquartile range, 0.0399-0.193 Joules/L) for small children, and 0.077 Joules/L (interquartile range, 0.032-0.132 Joules/L) for adolescents. Shortening the endotracheal tubes resulted in a significant difference in reduction in overall imposed work of breathing, but the absolute reduction was most notable in small children (0.030 Joules/L) and the least effect in neonates (0.016 Joules/L). Overall imposed work of breathing increased in each age group when an intraluminal catheter was in situ: 91.09% increase in imposed work of breathing in neonates to 0.168 Joules/L, 84.98% in infants to 0.142 Joules/L, 81.98% in small children to 0.219 Joules/L, and 55.45% in adolescents to 0.140 Joules/L.

CONCLUSIONS

Calculated imposed work of breathing were not different across the range of endotracheal tube sizes. The low imposed work of breathing values found in this study might be appreciated as clinically irrelevant. Our findings add to the change in reasoning that it is appropriate to perform spontaneous breathing trials without pressure support. Nonetheless, our findings on the measured imposed work of breathing values need to be confirmed in a clinical study.

Tracheal tube size in adults undergoing elective surgery - a narrative review

Tracheal tubes are routinely used in adults undergoing elective surgery. The size of the tracheal tube, defined by its internal diameter, is often generically selected according to sex, with 7-7.5 mm and 8-8.5 mm tubes recommended in women and men, respectively. Tracheal diameter in adults is highly variable, being narrowest at the subglottis, and is affected by height and sex. The outer diameter of routinely used tracheal tubes may exceed these dimensions, traumatise the airway and increase the risk of postoperative sore throat and hoarseness. These complications disproportionately affect women and may be mitigated by using smaller tracheal tubes (6-6.5 mm). Patient safety concerns about using small tracheal tubes are based on critical care populations undergoing prolonged periods of tracheal intubation and not patients undergoing elective surgery. The internal diameter of the tube corresponds to its clinical utility. Tracheal tubes as small as 6.0 mm will accommodate routinely used intubation aids, suction devices and slim-line fibreoptic bronchoscopes. Positive pressure ventilation may be performed without increasing the risk of ventilator-induced lung injury or air trapping, even when high minute volumes are required. There is also no demonstrable increased risk of aspiration or cuff pressure damage when using smaller tracheal tubes. Small tracheal tubes may not be safe in all patients, such as those with high secretion loads and airflow limitation. A balanced view of risks and benefits should be taken appropriate to the clinical context, to select the smallest tracheal tube that permits safe peri-operative management.

Not All COPD Patients Benefit from Prophylactic Noninvasive Ventilation After Scheduled Extubation: An Exploratory Study

Background

Prophylactic noninvasive ventilation (NIV) after scheduled extubation can benefit patients with chronic respiratory disorders, among which chronic obstructive pulmonary disease (COPD) is a significant example. However, it is not known whether all COPD patients benefit from prophylactic NIV.

Methods

We performed a post hoc analysis of prospectively collected data. COPD patients who successfully completed a spontaneous breathing trial were enrolled. In the prophylactic NIV group, NIV was applied immediately after extubation. In the usual care group, conventional oxygen therapy was used. Patients were followed up to 90 days post-extubation.

Results

Among patients with PaCO₂ > 45 mmHg, 128 and 40 received prophylactic NIV and usual care, respectively. Prophylactic NIV led to lower rates of re-intubation (4% vs 30% at 72 h and 11% vs 35% at 7 days, both p < 0.01) and hospital mortality (18% vs 40%, p < 0.01) than usual care. The proportion of 90-day mortality was also lower in the prophylactic NIV group (log rank test, p = 0.04). Among patients with PaCO₂ ≤ 45 mmHg, 32 and 21 received prophylactic NIV and usual care, respectively. In this cohort however, prophylactic NIV neither reduced re-intubation (6% vs 5% at 72 h, p > 0.99, and 9% vs 14% at 7 days, p = 0.67) nor hospital mortality (19% vs 24%, p = 0.74). The proportion of 90-day mortality did not differ between the two groups (log rank test, p = 0.79).

Conclusion

This exploratory study shows that prophylactic NIV benefits COPD patients with PaCO₂ > 45 mmHg, but it may not benefit those with PaCO₂ ≤ 45 mmHg. Further study with a larger sample size is required to confirm this.

Adherence to Evidence-Base Endotracheal Intubation Practice Patterns by Intensivists and Emergency Department Physicians

BACKGROUND

Endotracheal intubation outside the operating room (OR) is mainly performed by intensive care (IC) physicians and emergency department (ED) physicians. We hypothesized that difference in practice patterns exists between these two groups of physicians.

METHODS

A retrospective chart review was performed on all endotracheal intubations that were performed out of OR over a fi ve year period at our health care facility. Practice patterns of IC and ED physicians were compared regarding use of (a) video laryngoscopy, (b) paralytic agents, (c) waveform capnography, and (d) use of larger size of endotracheal tube (internal diameter ≥ 8 mm).

RESULTS

A total of 201 patients underwent out of OR intubations over a 5 year period. IC physicians used more often than ED physicians video laryngoscopy (67% vs. 49%; p = 0.008), waveform capnography (99% vs. 86%; p = 0.001) and larger size endotracheal tubes (95% vs. 60%; p < 0.001). Conversely, paralytic agents were used less frequently by IC than ED physicians (12% vs. 51%; p < 0.001). The success of fi rst intubation attempt was higher by IC than ED physicians (82% vs. 67%; p = 0.018).

CONCLUSIONS

IC physicians more often adhered to currently considered preferable practices for endotracheal intubation than ED physicians in this single center retrospective study. Although larger scale studies are needed to unveil the effects of different practice patterns on short and long term outcomes, the present study identifi es opportunity to bridge practice gaps that could lead to improved outcomes.

Noninvasive ventilation for avoidance of reintubation in patients with various cough strength

Background

Reintubation is associated with high mortality. Identification of methods to avoid reintubation is needed. The aim of this study was to assess whether prophylactic noninvasive ventilation (NIV) would benefit patients with various cough strengths.

Methods

We prospectively enrolled 356 patients who successfully passed a spontaneous breathing trial in a respiratory intensive care unit. Before extubation, cough peak flow was measured. After extubation, attending physicians determined whether the patients would receive prophylactic NIV or conventional oxygen treatment (control group). Patients were followed up to 90 days postextubation or death, whichever came first.

Results

The median value of cough peak flow was 70 L/minute. Among the patients with cough peak flow ≤70 L/minute, 108 received NIV and 72 received conventional oxygen treatment. In this cohort, NIV reduced reintubation (9 % vs. 35 % at postextubation 72 h, p < 0.01; and 24 % vs. 49 % at postextubation 7 days, p < 0.01) and postextubation 90-day mortality (43 % vs. 61 %, p = 0.02) compared with the control group. Further, use of NIV was an independent protective factor for reintubation (OR = 0.19, p < 0.01 at 72 h postextubation; and OR = 0.33, p < 0.01 at 7 days postextubation) and for death at 90 days postextubation (OR = 0.40, p = 0.02). Among patients with cough peak flow >70 L/minute, 71 received NIV and 105 received conventional oxygen treatment. In this cohort, NIV did not reduce reintubation (6 % vs. 6 % at 72 h postextubation, p > 0.99; and 9 % vs. 9 % at 7 days postextubation, p > 0.99) or postextubation 90-day mortality (21 % vs. 15 %, p = 0.32) compared with the control group. Further, use of NIV was not associated with reintubation or postextubation 90-day mortality.

Conclusion

In a planned extubated population, prophylactic NIV benefited patients with weak cough but possibly not in patients with strong cough.

Pediatric extubation readiness tests should not use pressure support

PURPOSE

Pressure support is often used for extubation readiness testing, to overcome perceived imposed work of breathing from endotracheal tubes. We sought to determine whether effort of breathing on continuous positive airway pressure (CPAP) of 5 cmH2O is higher than post-extubation effort, and if this is confounded by endotracheal tube size or post-extubation noninvasive respiratory support.

METHODS

Prospective trial in intubated children. Using esophageal manometry we compared effort of breathing with pressure rate product under four conditions: pressure support 10/5 cmH2O, CPAP 5 cmH2O (CPAP), and spontaneous breathing 5 and 60 min post-extubation. Subgroup analysis excluded post-extubation upper airway obstruction (UAO) and stratified by endotracheal tube size and post-extubation noninvasive respiratory support.

RESULTS

We included 409 children. Pressure rate product on pressure support [100 (IQR 60, 175)] was lower than CPAP [200 (120, 300)], which was lower than 5 min [300 (150, 500)] and 60 min [255 (175, 400)] post-extubation (all p < 0.01). Excluding 107 patients with post-extubation UAO (where pressure rate product after extubation is expected to be higher), pressure support still underestimated post-extubation effort by 126-147 %, and CPAP underestimated post-extubation effort by 17-25 %. For all endotracheal tube subgroups, ≤3.5 mmID (n = 152), 4-4.5 mmID (n = 102), and ≥5.0 mmID (n = 48), pressure rate product on pressure support was lower than CPAP and post-extubation (all p < 0.0001), while CPAP pressure rate product was not different from post-extubation (all p < 0.05). These findings were similar for patients extubated to noninvasive respiratory support, where pressure rate product on pressure support before extubation was significantly lower than pressure rate product post-extubation on noninvasive respiratory support (p < 0.0001, n = 81).

CONCLUSIONS

Regardless of endotracheal tube size, pressure support during extubation readiness tests significantly underestimates post-extubation effort of breathing.

The optimum timing to wean invasive ventilation for patients with AECOPD or COPD with pulmonary infection

COPD is characterized by a progressive decline in lung function and mental and physical comorbidities. It is a significant burden worldwide due to its growing prevalence, comorbidities, and mortality. Complication by bronchial-pulmonary infection causes 50%-90% of acute exacerbations of COPD (AECOPD), which may lead to the aggregation of COPD symptoms and the development of acute respiratory failure. Non-invasive or invasive ventilation (IV) is usually implemented to treat acute respiratory failure. However, ventilatory support (mainly IV) should be discarded as soon as possible to prevent the onset of time-dependent complications. To withdraw IV, an optimum timing has to be selected based on weaning assessment and spontaneous breathing trial or replacement of IV by non-IV at pulmonary infection control window. The former method is more suitable for patients with AECOPD without significant bronchial-pulmonary infection while the latter method is more suitable for patients with AECOPD with acute significant bronchial-pulmonary infection.

Setting the Ventilator in the NICU

Success in providing respiratory support to the neonate requires a clear understanding of the context in which it is being applied. Perhaps more than for any other age group, the array of different situations in which ventilation is applied to the newborn infant is extremely broad, with in each case different pathophysiological disturbances and often the need to use a specific approach to apply ventilation optimally. Table 42.1 provides a list of the more common situations in which conventional ventilation is used in the neonate and includes some considerations regarding ventilator settings for each situation. For each situation, a suggested mode of ventilation is indicated, along with target ranges for positive end-expiratory pressure (PEEP) and tidal volume (V_T). Further discussion of the physiological rationale and available evidence for ventilator settings is set out below.

Our paper 20 years later: how has withdrawal from mechanical ventilation changed?

Withdrawal from mechanical ventilation (or weaning) is one of the most common procedures in intensive care units. Almost 20 years ago, we published one of the seminal papers on weaning in which we showed that the best method for withdrawal from mechanical ventilation in difficult-to-wean patients was a once-daily spontaneous breathing trial with a T-piece. Progress has not stood still, and in the intervening years up to the present several other studies, by our group and others, have shaped weaning into an evidence-based technique. The results of these studies have been applied progressively to routine clinical practice. Currently, withdrawal from mechanical ventilation can be summarized as the evaluation of extubation readiness based on the patient's performance during a spontaneous breathing trial. This trial can be performed with a T-piece, which is the most common approach, or with continuous positive airway pressure or low levels of pressure support. Most patients can be disconnected after passing the first spontaneous breathing trial. In patients who fail the first attempt at withdrawal, the use of a once-daily spontaneous breathing trial or a gradual reduction in pressure support are the preferred weaning methods. However, new applications of standard techniques, such as noninvasive positive pressure ventilation, or new methods of mechanical ventilation, such as automatic tube compensation, automated closed-loop systems, and automated knowledge-based weaning systems, can play a role in the management of the patients with difficult or prolonged weaning.

The decision to extubate in the intensive care unit

The day of extubation is a critical time during an intensive care unit (ICU) stay. Extubation is usually decided after a weaning readiness test involving spontaneous breathing on a T-piece or low levels of ventilatory assist. Extubation failure occurs in 10 to 20% of patients and is associated with extremely poor outcomes, including high mortality rates of 25 to 50%. There is some evidence that extubation failure can directly worsen patient outcomes independently of underlying illness severity. Understanding the pathophysiology of weaning tests is essential given their central role in extubation decisions, yet few studies have investigated this point. Because extubation failure is relatively uncommon, randomized controlled trials on weaning are underpowered to address this issue. Moreover, most studies evaluated patients at low risk for extubation failure, whose reintubation rates were about 10 to 15%, whereas several studies identified high-risk patients with extubation failure rates exceeding 25 or 30%. Strategies for identifying patients at high risk for extubation failure are essential to improve the management of weaning and extubation. Two preventive measures may prove beneficial, although their exact role needs confirmation: one is noninvasive ventilation after extubation in high-risk or hypercapnic patients, and the other is steroid administration several hours before extubation. These measures might help to prevent postextubation respiratory distress in selected patient subgroups.

Comparison of the effects of two humidifier systems on endotracheal tube resistance

PURPOSE

To compare the effects of two humidifier systems on endotracheal tube (ETT) resistance during mechanical ventilation, either an active heated humidifier (HH) or a passive heat and moisture exchanger (HME) was selected using current clinical recommendations.

METHODS

This was a prospective clinical cohort study performed in an intensive care unit. Gas conditioning was performed using the HH in 22 patients and the HME in another 22. Patients were matched for endotracheal tube diameter, days of mechanical ventilation, simplified acute physiology score II (SAPS II), and fluid balance.

RESULTS

Used-ETT resistance was measured immediately after extubation. Unused-ETT resistance was calculated with an identical, clean ETT. No differences were found between the HH and HME groups in ETT diameter (7.9 ± 0.4 vs. 7.9 ± 0.3 mm; p = 0.98), days of mechanical ventilation (11.3 ± 7.7 vs. 9.5 ± 4.5; p = 0.34), SAPS II (41.0 ± 13.6 vs. 42.0 ± 11.7; p = 0.79), or fluid balance (-2,552 ± 6,268 vs. -2,579 ± 5,422 mL; p = 0.98). ETT resistance increased from intubation to extubation: from 6.8 ± 1.1 to 10.6 ± 4.3 cmH(2)O L(-1) s(-1) in the HH group, (p < 0.001) and from 6.8 ± 1.1 to 10.2 ± 3.8 cmH(2)O L(-1) s(-1) in the HME group (p < 0.001), which is a 53% average increase in resistive load.

CONCLUSIONS

We did not find differences between the two types of humidifiers in terms of airflow resistance during prolonged mechanical ventilation when the devices were selected on the basis of individual clinical needs. The increase in resistive load is physiologically relevant.

Impact of endotracheal tube size on preextubation respiratory variables

PURPOSE

Many parameters have been evaluated to predict successful extubation. These are all affected by extrapulmonary variables. The purpose of this study was to evaluate the effect of endotracheal tube (ETT) size on preextubation predictors of successful extubation.

MATERIALS AND METHODS

Twenty-two intubated and mechanically ventilated subjects were recruited when ready for extubation. Subjects were ventilated with T-piece, continuous positive airway pressure (CPAP) of 5 cm H(2)O, and pressure support ventilation (PSV) of 5 cm H(2)O in randomized order for 15 minutes each. Pulmonary mechanics-including respiratory frequency (f), tidal volume (V(T)), f/V(T) ratio, negative change in esophageal pressure, pressure time product (PTP), work of breathing, and the airway occlusion pressure 100 milliseconds after the onset of inspiratory flow-were measured using a microprocessor-based monitor at the end of each interval. After extubation, measurement of pulmonary mechanics was repeated at 15 and 60 minutes.

RESULTS

In patients with 7.0- or 7.5-mm ETT compared with patients with 8.0-mm ETT, (1) f was significantly higher during all ventilatory modes and 15 minutes after extubation; (2) V(T) during PSV and CPAP was significantly lower; (3) mean f/V(T) was significantly higher (122 ± 57 vs 69 ± 35, P = .026); and (4) PTP was significantly higher during CPAP, PSV, and 15 minutes after extubation. There was a nonsignificant trend toward increased negative change in esophageal pressure, work of breathing, and airway occlusion pressure 100 milliseconds after the onset of inspiratory flow in the smaller-ETT group.

CONCLUSIONS

The ETT size has a significant impact on f, V(T), f/V(T) ratio, and PTP.

Increases in endotracheal tube resistance are unpredictable relative to duration of intubation

BACKGROUND

Accumulated secretions after intubation can affect the resistance of an endotracheal tube (ETT). Our objective was to measure extubated patient tubes and size-matched controls to evaluate differences in resistance.

METHODS

New ETTs, with internal diameters of 7.0 through 8.5 mm, were tested as controls to establish the resistance of each size group as measured by pressure drop. Measurements were obtained using a mass flowmeter and pressure transducer. Pressure drop was measured at three flow rates. Seventy-one patient ETTs were evaluated after extubation by an identical method and compared with controls.

RESULTS

In each control group, pressure drop was tightly clustered with low variation and no overlap between sizes. A total of 73 to 79% of the patient ETTs had a pressure drop of > 3 SDs of size-matched controls at all flow rates. Pressure drop in 48 to 56% (across three flow rates) of extubated tubes was equivalent to the next smaller size of controls. At 60 and 90 L/min, 10% and 15% of patient tubes, respectively, had the pressure drop of a control tube three sizes smaller. The pressure drop was unpredictable relative to the duration of intubation.

CONCLUSIONS

Organized secretions can significantly increase resistance as measured by the pressure drop of ETTs. The degree of change was highly variable, occurs in all sizes, and was unrelated to the duration of intubation. The performance of an ETT may be comparable to new tubes one to four sizes smaller. This may impact the tolerance of ventilator weaning.

Respiratory management of inhalation injury

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

Ventilator Y-Piece Pressure Compared with Intratracheal Airway Pressure in Healthy Intubated Children

OBJECTIVE

Compare airway pressure measurements at the ventilator Y-piece of the breathing circuit (P( Y )) to intratracheal pressure measured at the distal end (P( T )) of the endotracheal tube (ETT) during mechanical ventilation and spontaneous breathing of intubated children.

METHODS

Thirty children (age range 29 days to 5 years) receiving general anesthesia were intubated with an ETT incorporating a lumen embedded in its sidewall that opened at the distal end to measure P( T ). Peak inflation pressure (PIP) was measured at P( Y ) and P( T ) during positive pressure ventilation. Just before extubation, all measurements were repeated and imposed resistive work of breathing (WOBi) was calculated at both sites while breathing spontaneously.

RESULTS

Average PIP was approximately 25% greater at P( Y ) (19.7 +/- 3.4 cm H(2)O) vs. P( T ) (15.0 +/- 2.9 cm H(2)O), p < 0.01. During spontaneous inhalation P( T ) was 59% lower ({bond}8.5 +/- 4.0 cm H(2)O) vs. P( Y ) ({bond}3.5 +/- 2.0 cm H(2)O), p < 0.01. WOBi measured at P( Y ) (0.10 +/- 0.02 Joule/L) was 86% less than WOBi measured at P( T ) (0.70 +/- 0.40 Joule/L), p < 0.01.

CONCLUSIONS

In healthy children P( Y ) significantly overestimates PIP in the trachea during positive pressure ventilation and underestimates the intratracheal airway pressure during spontaneous inhalation. During positive pressure ventilation P( T ) better assesses the pressure generated in the airways and lungs compared to P( Y ) because P( T ) also includes the difference in airway pressure across the ETT tube due to resistance. During spontaneous inhalation, P( T ) reflects the series resistance of the ETT and ventilator circuit, while P( Y ) reflects only the resistance of the ventilator circuit, accounting for the smaller decreases in pressure. Additionally, P( Y ) underestimates the total WOBi load on the respiratory muscles. Thus, P( T ) is a more accurate reflection of pulmonary airway pressures than P( Y ) and suggests that it should be incorporated into ventilator systems to more accurately trigger the ventilator and to reduce work of breathing.

Weaning from mechanical ventilation with pressure support in patients failing a T-tube trial of spontaneous breathing

OBJECTIVE

Evidence that PS may facilitate weaning from mechanical ventilation (MV), although not confirmed by randomized trials, prompted us to investigate whether patients could be weaned with PS after failing a T-tube trial.

DESIGN AND SETTING

This was a prospective, non-randomized study in two French intensive care units.

PATIENTS AND PARTICIPANTS

One hundred eighteen patients were enrolled and underwent a T-tube trial, after which 87 were extubated. Thirty-one underwent a further trial with PS, after which 21 were extubated.

INTERVENTIONS

All patients under MV >24 h meeting the criteria for a weaning test underwent a 30-min T-tube trial. If this was successful, they were immediately extubated. Otherwise, a 30-min trial with +7 cm H2O PS was initiated with an individualized pressurization slope and trigger adjustment. If all weaning criteria were met, the patients were extubated; otherwise, MV was reinstated.

MEASUREMENTS AND RESULTS

The extubation failure rate at 48 h did not differ significantly between the groups: 11/87 (13%) versus 4/21 (19%), P=0.39. The groups were comparable with regard to endotracheal tube diameter, MV duration, the use of non-invasive ventilation (NIV) after extubation, initial severity score, age and underlying pathology, except for COPD. A significantly higher percentage of patients with COPD was extubated after the trial with PS (8/21-38%) than after a single T-tube trial (11/87-13%) (P=0.003).

CONCLUSIONS

Of the patients, 21/118 (18%) could be extubated after a trial with PS, despite having failed a T-tube trial. The reintubation rate was not increased. This protocol may particularly benefit patients who are most difficult to wean, notably those with COPD.

Intratracheal pressure: a more accurate reflection of pulmonary airway pressure in pediatric patients with respiratory failure

OBJECTIVES

Peak inflation pressure (PIP) on many ventilators (P(vent)), measured distal to the exhalation limb or Y-piece of the breathing circuit, is assumed as the pressure applied to the airways and lungs. However, in vitro studies show P(vent) data are spurious. There are no studies evaluating the accuracy of P(vent) data for pediatric patients with acute respiratory failure. We hypothesized that intratracheal airway pressure (P(T)) is more accurate than P(vent) and that by using P(vent), abnormally increased imposed resistive work of breathing (WOBi) may go undetected.

DESIGN

Prospective and descriptive study.

SETTING

A pediatric intensive care unit at a university hospital.

PATIENTS

Twenty-one pediatric patients with respiratory failure requiring mechanical ventilation.

INTERVENTIONS

All patients were intubated with a commercially available endotracheal tube (ETT) with a pressure measuring the lumen opening at the distal end used for measuring P(T). Pressure/flow sensors positioned between the ETT and Y-piece measured tidal volume (V(T)) and flow rate. P(vent) data were recorded as displayed on the ventilator. WOBi was measured by integrating P(T) and V(T) data.

RESULTS

PIP at P(vent) and P(T) were 26 +/- 8 cm H(2)O and 19 +/- 7 cm H(2)O, respectively (p < .05). P(T) measurements averaged 27% less than P(vent). The relationship between P(vent)-P(T) (pressure drop across the breathing circuit and ETT) and flow rate during spontaneous inhalation was highly correlated (r = .80, p < .002), indicating the greater the flow rate, the greater the pressure drop and WOBi. WOBi, ranging from 0.04-1.5 J/L, was measured in 52% of the patients.

CONCLUSIONS

P(vent) significantly overestimates PIP. Moreover, P(vent) data does not allow for recognition of increased WOBi for many patients. Clinicians need to be aware of the limitations of P(vent) data and consider using ETTs that allow measurement of P(T), a more accurate reflection of pulmonary airway pressure.

Endotracheal tube intraluminal diameter narrowing after mechanical ventilation: use of acoustic reflectometry

OBJECTIVE

To quantify the incidence and degree of endotracheal tube intraluminal obstruction after mechanical ventilation and its relation to time of intubation.

DESIGN

Prospective observational study.

SETTING

A 14-bed medical-surgical intensive care unit at a university-affiliated teaching hospital.

PATIENTS

Ninety-four endotracheal tubes used in 80 patients requiring mechanical ventilation for more than 12 h.

INTERVENTIONS AND RESULTS

Acoustic reflectometry was performed in every endotracheal tube after patient extubation to measure its volume reduction. The intraluminal volumes of used endotracheal tubes in mechanically ventilated patients were significantly lower than those of unused tubes of the same size (5.52+/-0.92 ml(3) versus 6.54+/-0.79 ml(3), p<0.05). The mean difference in endotracheal tube segment volumes was 15.2% (range 0-66%). Volume reduction was above 10% in 60.8% of the tubes. In 22% of endotracheal tubes the remaining inner diameter was less than 7 mm. Reduction below this figure was less frequent (9.3%) in tubes 8 mm or more (p<0.05). The percentage of endotracheal tube volume reduction was not associated with the duration of intubation (r=-0.09, p= n.s.) Peak pressure measured before extubation did not predict obstruction (r=0.11, p= n.s.).

CONCLUSIONS

Inadvertent endotracheal tube obstruction was common in patients requiring mechanical ventilation and may be significant as early as at 24 h. Moderate obstruction in endotracheal tube lumens should be suspected in cases of difficulties in weaning, even in patients who were ventilated for less than 1 day.

Breathing pattern variability: a weaning predictor in postoperative patients recovering from systemic inflammatory response syndrome

OBJECTIVE

To investigate whether breathing pattern variability can serve as a potential weaning predictor for postoperative patients recovering from systemic inflammatory response syndrome (SIRS).

DESIGN AND SETTING

A prospective measurement of retrospectively analyzed breathing pattern variability in a surgical intensive care unit.

PATIENTS

Seventy-eight mechanically ventilated SIRS patients who had undergone abdominal surgery were included when they were ready for weaning. They were divided into success (n=57) and failure (n=21) groups based upon their weaning outcome.

MEASUREMENTS AND RESULTS

Before weaning, tidal volume, total breath duration, inspiratory time, expiratory time, and peak inspiratory flow were continuously monitored for 30 min, while patients received 5 cmH2O pressure support weaning trial. After the patients successfully completed the trial, they were extubated. Successful weaning was defined as patients free from the ventilator for over 48 h, whereas a weaning failure was considered as reinstitution of mechanical ventilation within 48 h of extubation. The coefficient of variation and two values of standard deviation (SD1 and SD2; indicators of the dispersion of data points in the plot) obtained from the Poincaré plot of five respiratory parameters in the failure group were significantly lower than those in the success group. The area under the receiver operating characteristic curve of these variability indices was within the range of 0.73-0.80, indicating the accuracy of prediction.

CONCLUSIONS

Small breathing pattern variability is associated with a high incidence of weaning failure in postoperative patients recovering from SIRS, and this variability may potentially serve as a weaning predictor.

Endotracheal tube intraluminal volume loss among mechanically ventilated patients*

OBJECTIVE

To measure endotracheal tube intraluminal volume loss among mechanically ventilated patients.

DESIGN

Prospective observational study.

SETTING

Medical intensive care unit (19 beds) of an urban university-affiliated teaching hospital.

PATIENTS

A total of 101 patients with acute respiratory failure requiring >24 hrs of mechanical ventilation.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Acoustic reflectometry was employed to measure the intraluminal volume of 13-cm endotracheal tube segments. The endotracheal tube segment volumes were statistically smaller among endotracheal tubes used in patients compared with unused endotracheal tubes (5.4 +/- 0.7 vs. 6.0 +/- 0.6 mL, p <.001). The average percentage difference in endotracheal tube segment volumes, between the unused endotracheal tubes and the endotracheal tubes used in patients, was 9.8% (range, 0-45.5%). The percentage difference in the endotracheal tube segment volumes increased significantly with increasing duration of tracheal intubation (r2 =.766, p <.001). The minimum diameter of the endotracheal tube segments was also statistically smaller among endotracheal tubes used in patients compared with the unused endotracheal tubes (7.5 +/- 0.4 vs. 6.7 +/- 1.2 mm, p <.001).

CONCLUSIONS

Endotracheal tube intraluminal volume loss is common among patients with acute respiratory failure requiring mechanical ventilation and increases with prolonged tracheal intubation.

Comparison between tube compensation and pressure support ventilation techniques on respiratory mechanics

In the intubated patient, the presence of an endotracheal tube increases the work of breathing during spontaneous breathing. The tube compensation technique was developed as a new ventilator mode that can compensate for that additional the work of breathing. We investigated the respiratory parameters during the pressure support ventilation 0, 5, 10 cmH2O and tube compensation 100% modes of the Puritan Bennett 840 ventilator in ten postoperative patients who had undergone radical surgery for oesophageal cancer. Measurements were performed just before extubation. The tidal volume, respiratory rate and other respiratory parameters were measured with a Ventrak respiratory monitor, and the duty ratio, mean inspiratory flow, and rapid shallow breathing index were calculated. In particular, we performed a comparison between pressure support ventilation 5 cmH2O and tube compensation 100%, because pressure support ventilation 5 cmH2O is the usual ventilating mode before the extubation in our intensive care unit. The tidal volume of pressure support ventilation 10 cmH2O was significantly larger and the respiratory rate was significantly lower than the other three modes. There was no significant difference in the minute volume, tidal volume, and respiratory rate between pressure support ventilation 5 cmH2O and tube compensation 100%. The duty ratio of pressure support ventilation 10 cmH2O was significantly smaller than the other three modes. There was no significant difference in the duty ratio and rapid shallow breathing index between pressure support ventilation 5 cmH2O and tube compensation 100%. It was concluded that the assist levels of pressure support ventilation 5 cmH2O and tube compensation 100% were almost equal for clinical purposes.

Extubation after breathing trials with automatic tube compensation, T-tube, or pressure support ventilation

BACKGROUND

Automatic tube compensation (ATC) is a new option to compensate for the pressure drop across the endotracheal or tracheostomy tube (ETT), especially during ventilator-assisted spontaneous breathing. While several benefits of this mode have so far been documented, ATC has not yet been used to predict whether the ETT could be safely removed at the end of weaning, from mechanical ventilation.

METHODS

We undertook a systematic trial using a randomized block design. During a 2-year period, all eligible patients of a medical intensive care unit were treated with ATC, conventional pressure support ventilation (PSV, 5 cmH2O), or T-tube for 2-h. Tolerance of the breathing trial served as a basis for the decision to remove the endotracheal tube. Extubation failure was considered if reintubation was necessary or if the patient required non-invasive ventilatory assistance (both within 48 h).

RESULTS AND CONCLUSIONS

After the inclusion of 90 patients (30 per group) we did not observe significant differences between the modes. Twelve patients failed the initial weaning trial. However, half of the patients who appeared to fail the spontaneous breathing trial on the T-tube, PSV, or both, were successfully extubated after a succeeding trial with ATC. Extubation was thus withheld from four and three of these patients while breathing with PSV or the T-tube, respectively, but to any patient breathing with ATC. It seems that ATC can be used as an alternative mode during the final phase of weaning from mechanical ventilation. Furthermore, this study may promote a larger multicenter trial on weaning with ATC compared with standard modes.

Automatic tube compensation in patients after cardiac surgery: effects on oxygen consumption and breathing pattern

OBJECTIVE

To evaluate patients without prior pulmonary disease after cardiac surgery and to determine whether resistive unloading by automatic tube compensation, pressure support ventilation, and continuous positive airway pressure has different effects on oxygen consumption, breathing pattern, gas exchange, and hemodynamics.

DESIGN

Prospective, randomized, controlled study.

SETTING

Tertiary care, postoperative intensive care unit.

PATIENTS

Twenty-one patients scheduled for open heart coronary artery bypass graft surgery.

INTERVENTIONS

Each patient was ventilated with all three modes in random order.

MEASUREMENTS AND MAIN RESULTS

Patients were ventilated in three modes, each applied for 30 mins according to computer-generated randomization: pressure support ventilation with 5 cm H2O, continuous positive airway pressure, and automatic tube compensation. Oxygen consumption was calculated by means of indirect calorimetry. The hypnotic state of the patients was monitored by Bispectral Index. For hemodynamic measurements, a fiberoptic pulmonary artery catheter was inserted. The main finding of our study was that oxygen consumption and breathing pattern (tidal volume and respiratory rate) did not differ significantly during automatic tube compensation and pressure support ventilation compared with continuous positive airway pressure (oxygen consumption, 170 +/- 29 vs. 170 +/- 26 vs. 174 +/- 29 mL.min.m, respectively; tidal volume, 466 +/- 132 vs. 484 +/- 125 vs. 470 +/- 119 mL, respectively; respiratory rate, 16 +/- 4 vs. 15 +/- 4 vs. 16 +/- 4 breaths/min, respectively). Automatic tube compensation and pressure support ventilation had no clinical effects on gas exchange and hemodynamic variables compared with continuous positive airway pressure. None of the variables differed significantly during the three ventilatory settings.

CONCLUSION

In postoperative tracheally intubated patients with normal ventilatory demand, automatic tube compensation and pressure support ventilation with 5 cm H2O lead to identical oxygen consumption, breathing patterns, gas exchange, and hemodynamics. We, therefore, suggest that this group of patients does not need any additional positive pressure support from the ventilator to overcome the additional work of breathing imposed by the endotracheal tube during the weaning phase from mechanical ventilation.

Tracheal pressure control provides automatic and variable inspiratory pressure assist to decrease the imposed resistive work of breathing

OBJECTIVE

To evaluate the operation of a continuous positive airway pressure system by using tracheal airway pressure (PT) as the control signal for system operation (i.e., tracheal pressure control).

DESIGN

Repeated measures.

SETTING

University research laboratory.

SUBJECTS

Twelve anesthetized, spontaneously breathing swine.

INTERVENTIONS

Subjects were intubated and connected to a tracheal pressure control system (5 cm H2O continuous positive airway pressure). Varying inspiratory flow demands and degrees of partial endotracheal tube occlusion (25%, 50%, and 75%) were studied. Tracheal pressure control was compared with a conventionally controlled system (pressure from breathing circuit Y-piece [PY] used as control signal) during endotracheal tube occlusion.

MEASUREMENTS AND RESULTS

Imposed resistive work of breathing (work to spontaneously inhale through endotracheal tube and ventilator circuit), work by ventilation system assisting inhalation, PT, PY, tidal volume, and inspiratory flow demands were measured. As inspiratory flow demands increased (range, 0.2-2.3 L/sec), pressure assist increased automatically (range, 5-40 cm H2O) as well as work of breathing by ventilation system assisting inhalation (range, 0.2-2.5 J/L). Imposed resistive work of breathing was nullified at the lower and was negligible at the higher flow demands. During endotracheal tube occlusion with a conventionally controlled system, PY was unchanged, whereas PT decreased (up to -15 cm H2O) and imposed resistive work of breathing increased (up to 1.05 J/L). With tracheal pressure control, PY increased automatically (range, 8-52 cm H2O), whereas PT varied slightly (range, 2 to -4.6 cm H2O). Imposed resistive work of breathing was negligible (range, 0-0.2 J/L). Breathing circuit pressure (PY), not pulmonary airway pressure (PT), increased significantly during tracheal pressure control.

CONCLUSIONS

Tracheal pressure control results in automatic and variable levels of pressure assist to decrease imposed resistive work of breathing under conditions of varying spontaneous inspiratory flow demands and endotracheal tube occlusion. Conventional systems are potentially flawed when PY is used as the control signal because they do not function in this manner and do not accurately assess pulmonary airway pressure.

Evaluation of tube compensation in the Bennett 840 ventilator--a new ventilatory mode to support spontaneous breathing

Respiratory care patients frequently require intubation with an endotracheal tube (ETT). Unfortunately, the ETT introduces a pressure drop (deltaPETT) that depends on the respiratory flow rate, thus increasing the work of breathing (WOB). Pressure support ventilation (PSV) cannot adequately compensate for this added WOB, because the degree of inspiratory assistance by PSV is fixed. Therefore, a technique called tube compensation (TC) has been developed to address deltaPETT. We examined the performance of TC and compared it with PSV of 5 cm H2O. The experimental system was constructed from a simulator, a test-lung, flow sensors, and a Bennett 840, and the respiratory parameters were studied. ETTs with IDs 6.5 and 8.0 mm were used. The quadratic approximation obtained for deltaPETT in the 6.5-mm ETT was 2.316 x flow + 7.910 x flow2, while that for the 8.0-mm ETT was 1.881 x flow + 3.353 x flow2. The maximum inspiratory flow (MIF) increased significantly with increasing TC, but tidal volume and inspiratory time did not show marked changes. The MIF for TC of 100% was larger than that for PSV of 5 cm H2O, when the 6.5-mm ID was used, but there was no significant difference between these modes when an ID of 8.0 mm was used. For both the 6.5 and 8.0-mm IDs, the PV loop corresponding to 100% TC was larger than that for PSV of 5 cm H2O. TC only compensated for the WOB caused by the ETT, whereas PSV compensated for the WOB caused by the ETT and the demand valve system. In clinical use, the differences between TC and PSV will demand attention.

Total versus tube-related additional work of breathing in ventilator-dependent patients

BACKGROUND

In tracheally intubated or tracheostomized spontaneously breathing patients, tube resistance can highly increase the patient's work of breathing. In this study we focused upon the relationship between total (WOBtot) and tube-related additional inspiratory work of breathing (WOBadd) and compared different ventilatory modalities for proper tube compensation.

METHODS

In ten tracheostomized spontaneously breathing patients we measured WOBtot and WOBadd in the continuous positive airway pressure (CPAP) mode, under inspiratory pressure support of 5, 10, and 15 cmH2O in the pressure support ventilation (PSV) mode, and under flow-adjusted pressure support in the automatic tube compensation (ATC) mode. WOBadd and WOBtot were calculated on the basis of measured tracheal pressure and esophageal pressure, respectively. Inspiratory peak tracheal pressure above PEEP was taken as an estimate of pressure support beyond mere tube compensation (i.e., overcompensation).

RESULTS

The percentage of the tube-related WOBadd on WOBtot in the CPAP mode was 52%. It decreased with increasing pressure support in the PSV mode from 32% (PSV 5 cmH2O) to 17% (PSV 15 cmH2O). WOBadd was only 15% of WOBtot in the ATC mode. In contrast to the other ventilatory modes, reduction of WOBadd in the ATC mode was achieved with the smallest amount of overcompensation, i.e. with minimal pressure assist beyond mere tube compensation.

CONCLUSION

In tracheally intubated or tracheostomized spontaneously breathing patients, adequate compensation of tube resistance (i.e. with minimal overcompensation and minimal undercompensation) is best done by the ATC mode.

Stress test and gastric-arterial PCO2 measurement improve prediction of successful extubation

OBJECTIVE

Correct early prediction of successful extubation decreases morbidity and mortality. The use of single time point measurements and surrogate markers of true extubation success limits weaning studies. Our aim was to determine whether a "stress test" improves prediction of extubation outcome and to determine the most predictive variables.

DESIGN

Observational study.

SETTING

Intensive care unit of a teaching hospital.

PATIENTS

A convenience sample of 68 patients judged to be ready for extubation.

INTERVENTIONS

We decreased pressure support from 5 to 0 cm H2O for 1 hr before extubation (stress test) while patients were on 5 cm H2O continuous positive airway pressure.

MEASUREMENTS AND MAIN RESULTS

We measured respiratory frequency, tidal volume, ratio of respiratory frequency to tidal volume (f/VT), airway occlusion pressure after onset of inspiration (P0.1), and gastric-arterial PCO2 (deltaPg-aCO2 both on 5 cm H2O and 0 cm H2O pressure support. Then all patients were extubated. Failure of extubation was defined as reintubation within 24 hrs. Seventeen patients (25%) failed extubation. With pressure support of 5 cm H2O and continuous positive airway pressure of 5 cm H2O, most predictors were not different between patients who failed and patients who were successfully extubated. After the stress test, deltaPg-aCO2 was 2 (-5; 15) mm Hg (median; quartiles) in successfully extubated patients vs. 28 (-9; 48) in failures (p = .0003), tidal volume was 473 (387; 558) vs. 400 (323; 435) mL (p = .02), and P0.1 was 2.8 (2; 4.1) vs. 4.1 (2.7; 5.3) mm Hg (p = .03), respectively. The stress test increased specificity of deltaPg-aCO2 from 0.45 to 0.94 and positive predictive value from 0.85 to 0.97. The specificity and positive predictive values for f/VT after the stress test were 0.23 and 0.78.

CONCLUSIONS

A simple stress test improves prediction of extubation outcome. deltaPg-aCO2 has superior specificity and positive predictive value compared with other variables. The use of true clinical outcome (i.e., extubation) instead of the use of surrogate markers (e.g., tachypnea) distinguishes these results from previous studies.

Effect of an additional 1-hour T-piece trial on weaning outcome at minimal pressure support

PURPOSE

The purpose of this article was to investigate the effect of an additional 1-hour T-piece trial at the level of minimum pressure support (PSmin) on weaning outcome.

MATERIALS AND METHODS

Open, randomized, prospective study of 44 patients who had received mechanical ventilation for more than 3 days.Thirty-six patients satisfied the protocol. There were 42 weaning trials. The patients were randomized into an intervention group (additional 1-hour period of T-piece trial) and a control group (extubation directly) at PSmin. Blood gas analysis and estimation of respiratory and hemodynamic variables were performed at the 15 cm H2O level of pressure support. Measurements were repeated at PSmin and during weaning process (in intervention group).

RESULTS

Mean PSmin level was 7.6 (+/- 1.9) cm H2O. There were no differences in total ventilation time (TVT), acute physiology and chronic health evaluation (APACHE) II score, nutritional indices, and respiratory mechanics on PSmin between the two groups. The weaning success rate and the reintubation rate were similar for the intervention group (55% and 18%, respectively) and control group (70% and 20%, respectively). Work of breathing, pressure time product, and tidal volume significantly worsened after a 1-hour T-piece trial when compared with those values measured at PSmin in the intervention group (P < .05). For the combined patient sample, TVT and tidal volume at PSmin were significantly different between the patients with weaning success (246 +/- 195 hours, 0.43 +/- 0.11 L) and those with weaning failure (407 +/- 248 hours, 0.35 +/- 0.10 L) (P < .05 in each).

CONCLUSION

There were no advantages in weaning outcome by the addition of a 1-hour T-piece trial compared with prompt extubation at PSmin.

Prediction of post-extubation work of breathing

OBJECTIVE

To evaluate which mode of preextubation ventilatory support most closely approximates the work of breathing performed by spontaneously breathing patients after extubation.

DESIGN

Prospective observational design.

SETTING

Medical, surgical, and coronary intensive care units in a university hospital.

PATIENTS

A total of 22 intubated subjects were recruited when weaned and ready for extubation.

INTERVENTIONS

Subjects were ventilated with continuous positive airway pressure at 5 cm H2O, spontaneous ventilation through an endotracheal tube (T piece), and pressure support ventilation at 5 cm H2O in randomized order for 15 mins each. At the end of each interval, we measured pulmonary mechanics including work of breathing reported as work per liter of ventilation, respiratory rate, tidal volume, negative change in esophageal pressure, pressure time product, and the airway occlusion pressure 100 msec after the onset of inspiratory flow, by using a microprocessor-based monitor. Subsequently, subjects were extubated, and measurements of pulmonary mechanics were repeated 15 and 60 mins after extubation.

MEASUREMENTS AND MAIN RESULTS

There were no statistical differences between work per liter of ventilation measured during continuous positive airway pressure, T piece, or pressure support ventilation (1.17+/-0.67 joule/L, 1.11+/-0.57 joule/L, and 0.97+/-0.57 joule/L, respectively). However, work per liter of ventilation during all three preextubation modes was significantly lower than work measured 15 and 60 mins after extubation (p < .05). Tidal volume during pressure support ventilation and continuous positive airway pressure (0.46+/-0.11 L and 0.44+/-0.11 L, respectively) were significantly greater than tidal volume during both T-piece breathing and spontaneous breathing 15 mins after extubation (p < .05). Negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product were significantly higher after extubation than during any of the three preextubation modes (p < .05).

CONCLUSIONS

Work per liter of ventilation, negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product all significantly increase postextubation. Tidal volume during continuous positive airway pressure or pressure support ventilation overestimates postextubation tidal volume.

Breathing pattern associated with respiratory comfort during automatic tube compensation and pressure support ventilation in normal subjects

BACKGROUND

Automatic tube compensation (ATC) is a new option to support spontaneously breathing tracheally intubated patients. We have previously demonstrated an increased respiratory comfort compared to pressure support ventilation (PSV) in volunteers. Here we characterized the breathing pattern during ATC associated with respiratory comfort in comparison to PSV. Furthermore, we studied whether ATC can be substituted by a simple modification of PSV.

METHODS

We exposed 10 volunteers breathing through a 7.5 mm endotracheal tube via mouthpiece to PSV with 1) immediate and 2) delayed pressure rise and to 3) ATC. Immediate changes of the respiratory pattern after mode shifts were analyzed in detail. Furthermore, the volunteers were instructed to indicate changes in comfort after transitions between these modes as increased, unchanged, or decreased.

RESULTS

Decreased comfort was associated with a substantial increase of tidal volume, minute ventilation, gas flow, and pressure. No differences in respiratory comfort were perceived between immediate and delayed pressure rise during PSV.

CONCLUSION

PSV resulted in excessive tidal volumes and airflow, which was perceived as discomfort. This cannot be avoided by a delayed pressure rise but can be by the more comfortable ATC. ATC seems to adapt better to the ventilatory demand than PSV.

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.

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.

Effects of pressure-support ventilation on recovery from acute diaphragmatic fatigue in rabbits

OBJECTIVES

To examine the effects of pressure-support ventilation on recovery from acute diaphragmatic fatigue.

DESIGN

Prospective laboratory trial.

SETTING

Experimental laboratory.

SUBJECTS

Twenty-one healthy, adult New Zealand white rabbits.

INTERVENTIONS

Diaphragmatic fatigue was induced with 50-Hz phrenic nerve stimulation for 30 mins. Recovery was compared between pressure-support ventilation 0 cm H2O (SB), 10 cm H2O (P10), and 20 cm H2O (P20) for 90 mins immediately after the end of the fatigue inducing procedure.

MEASUREMENTS AND MAIN RESULTS

After the fatigue-inducing procedure, pressure-support ventilation reduced transdiaphragmatic pressure and integrated diaphragmatic electromyogram both at P20 and P10, but not in SB. Recovery was assessed by airway occlusion pressure (Poccl) generated by high- (100 Hz) and low- (20 Hz) frequency phrenic nerve stimulation. Poccl at 100 Hz was lower in P10 and P20 than in SB (74.6 +/- 6.2 [SEM] %, 66.9 +/- 3.3%, and 94.8 +/- 3.6% of the baseline at 90 mins for P10, P20, and SB, respectively), while those at 20 Hz showed no differences between the three groups.

CONCLUSION

Recovery from acute diaphragmatic fatigue might be disturbed with pressure-support ventilation.

Contribution of the endotracheal tube and the upper airway to breathing workload

The influence of the endotracheal tube (ETT) during a T-piece trial remains controversial. Our aim was to compare the work of breathing of 14 successfully extubated patients at the end of a 2-h trial (T) and after extubation (E) of the trachea, and to assess, using the acoustic reflection method, the resistance of the endotracheal tube and of the supraglottic airway as well as their related work. We found that the work of breathing of the patients was identical between T and E (1.72 +/- 0.59 versus 1.63 +/- 0.45 J/L; p = 0.50 and 23.5 +/- 10.6 versus 22.6 +/- 9.7 J/min; p = 0.70). There was no significant difference between the beginning and the end of the T-piece trial (1.57 +/- 0.53 versus 1.72 +/- 0.59 J/ L, p = 0.10). The work caused by the ETT amounted to 11.0 +/- 3.9% of the total work of breathing. The supraglottic airway resistance was in the normal range and was significantly smaller than the endotracheal tube resistance (0.79 +/- 0.4 versus 1.43 +/- 0.31 cm H2O x s/L; p = 0.008, flow = 0.25 L/s). We conclude that a 2-h trial of spontaneous breathing through an endotracheal tube well mimics the work of breathing performed after extubation, in patients who pass a weaning trial and do not require reintubation.

Respiratory comfort of automatic tube compensation and inspiratory pressure support in conscious humans

OBJECTIVE

To compare the new mode of ventilatory support, which we call automatic tube compensation (ATC), with inspiratory pressure support (IPS) with respect to perception of respiratory comfort. ATC unloads the resistance of the endotracheal tube (ETT) in inspiration by increasing the airway pressure, and in expiration by decreasing the airway pressure according to the non-linear pressure-flow relationship of the ETT.

DESIGN

Prospective randomized single blind cross-over study.

SETTING

Laboratory of the Section of Experimental Anaesthesiology (Clinic of Anaesthesiology; University of Freiburg).

SUBJECTS

Ten healthy volunteers.

INTERVENTIONS

The subjects breathed spontaneously through an ETT of 7.5 mm i.d. Three different ventilatory modes, each with a PEEP of 5 cmH2O, were presented in random order using the Dräger Evita 2 ventilator with prototype software: (1) IPS (10 cmH2O, 1 s ramp), (2) inspiratory ATC (ATC-in), (3) inspiratory and expiratory ATC (ATC-in-ex).

MEASUREMENTS AND MAIN RESULTS

Immediately following a mode transition, the volunteers answered with a hand sign to show how they perceived the new mode compared with the preceding mode in terms of gain or loss in subjective respiratory comfort: "better", "unchanged" or "worse". Inspiration and expiration were investigated separately analyzing 60 mode transitions each. Flow rates were continuously measured. The transition from IPS to either type of ATC was perceived positively, i.e. as increased comfort, whereas the opposite transition from ATC to IPS was perceived negatively, i.e. as decreased comfort. The transition from ATC-in to ATC-in-ex was perceived positively whereas the opposite mode transition was perceived negatively in expiration only. Tidal volume was 1220 +/- 404 ml during IPS and 1017 +/- 362 ml during ATC. The inspiratory peak flow rate was 959 +/- 78 ml/s during IPS and 1048 +/- 197 ml/s during ATC.

CONCLUSIONS

ATC provides an increase in respiratory comfort compared with IPS. The predominant cause for respiratory discomfort in the IPS mode seems to be lung over-inflation.

NéoGanesh: a working system for the automated control of assisted ventilation in ICUs

Automating the control of therapy administered to a patient requires systems which integrate the knowledge of experienced physicians. This paper describes NéoGanesh, a knowledge-based system which controls, in closed-loop, the mechanical assistance provided to patients hospitalized in intensive care units. We report on how new advances in knowledge representation techniques have been used to model medical expertise. The clinical evaluation shows that such a system relieves the medical staff of routine tasks, improves patient care, and efficiently supports medical decisions regarding weaning. To be able to work in closed-loop and to be tested in real medical situations, NéoGanesh deals with a voluntarily limited problem. However, embedded in a powerful distributed environment, it is intended to support future extensions and refinements and to support reuse of knowledge bases.

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.