Intrinsic Dynamic Positive End-Expiratory Pressure in Stable Patients with Chronic Obstructive Pulmonary Disease.

BACKGROUND: Assessment of intrinsic dynamic positive end-expiratory pressure (PEEPi,dyn) may be clinically important in stable patients with chronic obstructive pulmonary disease (COPD), but epidemiological data are scant. OBJECTIVES: The aim of our study was (i) to assess the PEEPi,dyn in a large population of stable patients with COPD and (ii) to evaluate the correlations with some noninvasive measurements routinely assessed. METHOD: Retrospective analysis of lung mechanics, dynamic volumes, arterial blood gases, dyspnoea by means of the Medical Research Council (MRC) scale, the COPD Assessment Test score, and maximal inspiratory/expiratory pressures in 87 hypercapnic and 62 normocapnic patients. RESULTS: The mean PEEPi,dyn was significantly higher in hypercapnic than normocapnic patients (2.8 ± 2.2 vs. 1.9 ± 1.6 cm H2O, respectively, p = 0.0094). PEEPi,dyn did not differ according to Global Initiative for Chronic Obstructive Lung Disease stage, MRC score, or use or not of long-term oxygen therapy. There were significant although weak correlations between PEEPi,dyn and airway obstruction, hyperinflation, respiratory muscle function, arterial CO2 tension, and number of exacerbations/year. The transdiaphragmatic pressure was the strongest variable associated to PEEPi,dyn (R = 0.5713, p = 0.001). CONCLUSION: In stable patients with COPD, PEEPi,dyn is higher in hypercapnic patients and weakly correlated to noninvasive measures of lung and respiratory muscle function.

3D airway model to assess airway dead space.

High flow therapy works partly by washout of airway dead space, the volume of which has not been quantified in newborns. This observational study aimed to quantify airway dead space in infants and to compare efficacy of washout between high flow devices in three-dimensional (3D) printed airway models of infants weighing 2.5-3.8 kg. Nasopharyngeal airway dead space volume was 1.5-2.0 mL/kg in newborns. A single cannula device produced lower carbon dioxide (CO2) levels than a dual cannula device (33.7, 31.2, 23.1, 15.9, 10.9 and 6.3 mm Hg vs 36.8, 35.5, 32.1, 26.8, 23.1 and 18.8 mm Hg at flow rates of 1, 2, 3, 4, 6 and 8 L/min, respectively; p<0.0001 at all flow rates). Airway pressure was 1 mm Hg at all flow rates with the single cannula but increased at higher flow rates with the dual cannula.Relative nasopharyngeal airway dead space volume is increased in newborns. In 3D-printed airway models, a single cannula high flow device produces improved CO2 washout with lower airway pressure.

Expiratory Flow Limitation During Mechanical Ventilation.

Expiratory flow limitation (EFL) is present when the flow cannot rise despite an increase in the expiratory driving pressure. The mechanisms of EFL are debated but are believed to be related to the collapsibility of small airways. In patients who are mechanically ventilated, EFL can exist during tidal ventilation, representing an extreme situation in which lung volume cannot decrease, regardless of the expiratory driving forces. It is a key factor for the generation of auto- or intrinsic positive end-expiratory pressure (PEEP) and requires specific management such as positioning and adjustment of external PEEP. EFL can be responsible for causing dyspnea and patient-ventilator dyssynchrony, and it is influenced by the fluid status of the patient. EFL frequently affects patients with COPD, obesity, and heart failure, as well as patients with ARDS, especially at low PEEP. EFL is, however, most often unrecognized in the clinical setting despite being associated with complications of mechanical ventilation and poor outcomes such as postoperative pulmonary complications, extubation failure, and possibly airway injury in ARDS. Therefore, prompt recognition might help the management of patients being mechanically ventilated who have EFL and could potentially influence outcome. EFL can be suspected by using different means, and this review summarizes the methods to specifically detect EFL during mechanical ventilation.

Optimization of Mechanical Ventilation in a 31-Year-Old Morbidly Obese Man With Refractory Hypoxemia.

Morbidly obese, critically ill patients are prone to develop hypoxemic respiratory failure and ventilator dependency. The best method for recruiting the lungs of these patients and keeping alveoli open without causing injury remains unclear. We present the case of a 31-year-old patient with severe refractory hypoxemia reversed by lung recruitment maneuvers and subsequent application of positive end-expiratory pressure (PEEP) at a level determined by a decremental PEEP trial. The patient was extubated at a high PEEP level of 22 cm H2O followed by noninvasive ventilatory support after extubation. This case suggests that a recruitment maneuver followed by PEEP titration is necessary in obese patients for optimizing mechanical ventilation. Extubation to noninvasive ventilatory support with the identified optimal PEEP may decrease an inappropriate increased work of breathing and the risk of reintubation.

Compensating Artificial Airway Resistance via Active Expiration Assistance.

BACKGROUND: Artificial airway resistance as provided by small-lumen tracheal tubes or catheters increases the risk of intrinsic PEEP (PEEPi). We hypothesized that by active expiration assistance, larger minute volumes could be generated without causing PEEPi compared with conventional mechanical ventilation when using small-lumen tracheal tubes or a cricothyrotomy catheter. METHODS: We investigated the active expiration assistance in a physical model of the respiratory system and estimated its hypothetical performance in terms of maximal flow generated with endotracheal tubes ranging from 3.0 to 8.0 mm inner diameter (ID); with microlaryngeal tubes of 4.0, 5.0, and 6.0 mm ID; and with a cricothyrotomy catheter. Furthermore, we determined the minute volumes that could be achieved without generating PEEPi by ventilating a physical lung model using conventional mechanical ventilation or using active expiration assistance. RESULTS: The inspiratory and expiratory flow during active expiration assistance increased with increasing supply flow and decreased with decreasing ID of the connected endotracheal tubes (both P < .001). With small-lumen tracheal tubes, the active expiration assistance generated similar or higher minute volumes than conventional ventilation. Conventional mechanical ventilation with PEEPi <1 cm H2O was not achievable via a microlaryngeal tube of 4.0 mm ID and smaller lumen tubes. CONCLUSIONS: For mechanical ventilation via small-lumen tubes or thin catheters, active compensation of airway resistance might be a necessary means to generate adequate minute ventilation without causing PEEPi. Active expiration assistance can generate reasonable respiratory minute volumes via small-lumen tubes or thin catheters.

Assessment of Factors Related to Auto-PEEP.

BACKGROUND: Previous physiological studies have identified factors that are involved in auto-PEEP generation. In our study, we examined how much auto-PEEP is generated from factors that are involved in its development. METHODS: One hundred eighty-six subjects undergoing controlled mechanical ventilation with persistent expiratory flow at the beginning of each inspiration were enrolled in the study. Volume-controlled continuous mandatory ventilation with PEEP of 0 cm H2O was applied while maintaining the ventilator setting as chosen by the attending physician. End-expiratory and end-inspiratory airway occlusion maneuvers were performed to calculate respiratory mechanics, and tidal flow limitation was assessed by a maneuver of manual compression of the abdomen. RESULTS: The variable with the strongest effect on auto-PEEP was flow limitation, which was associated with an increase of 2.4 cm H2O in auto-PEEP values. Moreover, auto-PEEP values were directly related to resistance of the respiratory system and body mass index and inversely related to expiratory time/time constant. Variables that were associated with the breathing pattern (tidal volume, frequency minute ventilation, and expiratory time) did not show any relationship with auto-PEEP values. The risk of auto-PEEP ≥5 cm H2O was increased by flow limitation (adjusted odds ratio 17; 95% CI: 6-56.2), expiratory time/time constant ratio <1.85 (12.6; 4.7-39.6), respiratory system resistance >15 cm H2O/L s (3; 1.3-6.9), age >65 y (2.8; 1.2-6.5), and body mass index >26 kg/m(2) (2.6; 1.1-6.1). CONCLUSIONS: Flow limitation, expiratory time/time constant, resistance of the respiratory system, and obesity are the most important variables that affect auto-PEEP values. Frequency expiratory time, tidal volume, and minute ventilation were not independently associated with auto-PEEP. Therapeutic strategies aimed at reducing auto-PEEP and its adverse effects should be primarily oriented to the variables that mainly affect auto-PEEP values.

Role of nasal positive end expiratory pressure valve as an alternative treatment for obstructive sleep apnoea in Chinese patients.

BACKGROUND AND OBJECTIVE: As compliance of continuous positive airway pressure (CPAP) for treatment of obstructive sleep apnoea (OSA) is often suboptimal, a less cumbersome treatment is desirable. We explored the clinical usefulness of nasal positive end expiratory pressure (nPEEP) valves. METHODS: Symptomatic OSA patients (apnoea hypopnea index (AHI) >5/h by polysomnography (PSG) or >10/h by type III devices), who declined CPAP, were recruited. A nPEEP valve was attached to each nostril before bed. After successful acclimatization for 1 week, treatment was continued for 4 weeks. The nPEEP valves provided expiratory resistance to build up PEEP. PSG was performed at week 4. RESULTS: Among 196 subjects, 46 (23%) failed acclimatization and 14 (7%) withdrew. Among the 120 patients with a valid PSG, 72 (60%) and 75 (63%) had >50% reduction in mean (standard deviation) overall AHI 26 (16)/h to 18 (18)/h and mean supine AHI 31 (19)/h to 11(16)/h, respectively, P < 0.001. Compared with responders, patients with <50% reduction in AHI had a higher mean overall AHI (30/h vs 23/h, P = 0.03), higher mean supine AHI (35/h vs 26/h, P = 0.04), more severe mean oxygen desaturation nadir (76.7% vs 82.7%, P < 0.01) and longer mean period of desaturation <90% SaO2 (7.7 vs 2.4, P = 0.02). Breathing discomfort and dry mouth were the most common side effects. Compared with a dental device, there was a larger mean reduction in supine AHI using nPEEP (29 (14)/h vs 16 (17)/h). CONCLUSION: nPEEP valves were useful in selected patients with mild or positional-related OSA.

Intrapulmonary percussive ventilation superimposed on conventional mechanical ventilation: comparison of volume controlled and pressure controlled modes.

BACKGROUND: Previous bench studies suggest that dynamic hyperinflation may occur if intrapulmonary percussive ventilation (IPV) is superimposed on mechanical ventilation in volume controlled continuous mandatory ventilation (VC-CMV) mode. We tested the hypothesis that pressure controlled continuous mandatory ventilation (PC-CMV) can protect against this risk. METHODS: An ICU ventilator was connected to an IPV device cone adapter that was attached to a lung model (compliance 30 mL/cm H2O, resistance 20 cm H2O/L/s). We measured inspired tidal volume (VTI) and lung pressure (Plung). Measurements were first taken with IPV off and the ICU ventilator set to VC-CMV or PC-CMV mode with a targeted VTI of 500 mL. For each mode, an inspiratory time (TI) of 0.8 or 1.5 s and PEEP 7 or 15 cm H2O were selected. The experiments were repeated with the IPV set to either 20 or 30 psi. The dependent variables were differences in VTI (ΔVTI) and Plung with IPV off or on. The effect of VC-CMV or PC-CMV mode was tested with the ICU ventilators for TI, PEEP, and IPV working pressure using repeated measures of analysis of variance. RESULTS: At TI 0.8 s and 20 psi, ΔVTI was significantly higher in VC-CMV than in PC-CMV. PEEP had no effect on ΔVTI. At TI 1.5 s and 20 psi and at both TI values at each psi, mode and PEEP had a significant effect on ΔVTI. With the ICU ventilators at TI 1.5 s, PEEP 7 cm H2O, and 30 psi, ΔVTI (mean ± SD) ranged from -27 ± 25 to -176 ± 6 mL in PC-CMV and from 258 ± 369 to 369 ± 16 mL in VC-CMV. The corresponding ranges were -15 ± 17 to -62 ± 68 mL in PC-CMV and 26 ± 21 to 102 ± 95 mL in VC-CMV at TI 0.8 s, PEEP 7 cm H2O, and 20 psi. Similar findings pertained to Plung. CONCLUSIONS: When IPV is added to mechanical ventilation, the risk of hyperinflation is greater with VC-CMV than with PC-CMV. We recommend using PC-CMV to deliver IPV and adjusting the trigger variable to avoid autotriggering.

State of the evidence: mechanical ventilation with PEEP in patients with cardiogenic shock.

The need to provide invasive mechanical ventilatory support to patients with myocardial infarction and acute left heart failure is common. Despite the large number of patients requiring mechanical ventilation in this setting, there are remarkably few data addressing the ideal mode of respiratory support in such patients. Although there is near universal acceptance regarding the use of non-invasive positive pressure ventilation in patients with acute pulmonary oedema, there is more concern with invasive positive pressure ventilation owing to its more significant haemodynamic impact. Positive end-expiratory pressure (PEEP) is almost universally applied in mechanically ventilated patients due to benefits in gas exchange, recruitment of alveolar units, counterbalance of hydrostatic forces leading to pulmonary oedema and maintenance of airway patency. The limited available clinical data suggest that a moderate level of PEEP is safe to use in severe left ventricular (LV) dysfunction and cardiogenic shock, and may provide haemodynamic benefits as well in LV failure which exhibits afterload-sensitive physiology.

Auto-PEEP in respiratory failure.

Intrinsic positive end-expiratory pressure (auto-PEEP) is a common occurrence in patients with acute respiratory failure requiring mechanical ventilation. Auto-PEEP can cause severe respiratory and hemodynamic compromise. The presence of auto-PEEP should be suspected when airflow at end-exhalation is not zero. In patients receiving controlled mechanical ventilation, auto-PEEP can be estimated measuring the rise in airway pressure during an end-expiratory occlusion maneuver. In patients who trigger the ventilator or who are not connected to a ventilator, auto-PEEP can be estimated by simultaneous recordings of airflow and airway and esophageal pressure, respectively. The best technique to accurately measure auto-PEEP in patients who actively recruit their expiratory muscle remains controversial. Strategies that may reduce auto-PEEP include reduction of minute ventilation, use of small tidal volumes and prolongation of the time available for exhalation. In patients in whom auto-PEEP is caused by expiratory flow limitation, the application of low-levels of external PEEP can reduce dyspnea, reduce work of breathing, improve patient-ventilator interaction and cardiac function, all without worsening hyperinflation. Neurally adjusted ventilatory assist, a novel strategy of ventilatory assist, may improve patient-ventilator interaction in patients with auto-PEEP.

The pathophysiology of emphysema: considerations for critical care nursing practice.

Emphysema is caused by exposure to cigarette smoking as well as alpha(1)-antitrypsin deficiency. It has been estimated to cost the National Health Service (NHS) in excess of 800 million pounds per year in related health care costs. The challenges for Critical Care nurses are those associated with dynamic hyperinflation, Auto-PEEP, malnutrition and the weaning from invasive and non-invasive mechanical ventilation. In this paper we consider the impact of the pathophysiology of emphysema, its effects on other body systems as well as the impact acute exacerbations have when patients are admitted to the Intensive Care Unit.

Ventilating patients with acute severe asthma: what do we really know?

The goal of mechanical ventilation for patients with acute severe asthma is to ensure adequate oxygenation, ventilation, and gas exchange while simultaneously preventing hyperinflation, auto-positive end expiratory pressure, and subsequent barotrauma. Though existing evidence on the topic is relatively scarce, the application of current knowledge may guide our practice and prevent iatrogenic complications. To that end, this article describes selected ventilatory management strategies for the patient with acute severe asthma, such as the limitation of tidal volume size and respiratory rate, selection of specific inspiratory and expiratory ratios, the use of positive end expiratory pressure, and the application of helium-oxygen mixtures.

Auto-positive end-expiratory pressure: mechanisms and treatment.

Auto-positive end-expiratory pressure (auto-PEEP) is a common problem in patients receiving full or partial ventilatory support, as well as in those ready to be weaned from the ventilator. Physicians should be alert for it and take measures to reduce it, as it can have serious consequences.

Management of acute severe asthma.

Asthma is a chronic inflammatory disorder that results in recurrent episodes of reversible airflow obstruction. Lung hyperinflation results from obstruction or dynamic airway collapse during exhalation. Obstruction and dynamic hyperinflation both play a deleterious role in asthma. Patients who present with asthma have increased inspiratory work of breathing due to lung hyperinflation and auto-positive end-expiratory pressure (auto-PEEP). The goal of acute care treatment is to reverse bronchoconstriction and inflammation, thus reducing dynamic hyperinflation, so that breathing is restored to baseline, unlabored, quiet breathing.

Working with respiratory waveforms: how to use bedside graphics.

Respiratory waveform graphics packages are available on many ventilators. Despite the prevalence of the waveforms, accurate interpretation and clinical application are not widespread. In fact, many clinicians find the waveforms confusing and choose to ignore them. This article provides a straightforward description of how to interpret the waveforms and suggests ways that the information might be used to improve clinical outcomes.

Helium/oxygen mixture reduces the work of breathing at the end of the weaning process in patients with severe chronic obstructive pulmonary disease.

OBJECTIVE: To test the hypothesis that helium/oxygen mixture can reduce the work of breathing at the end of the weaning process in patients with chronic obstructive pulmonary disease. DESIGN: Prospective, randomized, crossover study. SETTING: Two medical intensive care units at two university tertiary care centers. PATIENTS: Thirteen patients with chronic obstructive pulmonary disease evaluated just before and after extubation. INTERVENTIONS: Helium/oxygen and air/oxygen mixtures were administered sequentially, for 20 mins each, in a randomized order, just before extubation. It was possible to repeat the study after extubation in five patients. MEASUREMENTS AND MAIN RESULTS: Before extubation, the helium/oxygen mixture induced no significant variation in the breathing pattern. By contrast, it reduced the work of breathing from 1.442 +/- 0.718 J/L (mean +/- sd) to 1.133 +/- 0.500 J/L (p <.05). This reduction was explained mainly by a reduction in the resistive component of the work of breathing from 0.662 +/- 0.376 to 0.459 +/- 0.256 J/L (p <.01). We also observed a slight reduction in the intrinsic positive end-expiratory pressure from 2.9 +/- 2.1 cm H(2)O to 2.1 +/- 1.8 cm H(2)O (p <.05). Similar results were also observed after extubation in five patients in whom the repetition of the study was possible. CONCLUSIONS: In spontaneously breathing intubated patients with chronic obstructive pulmonary disease recovering from an acute exacerbation, helium/oxygen mixture reduces the work of breathing as well as intrinsic positive end-expiratory pressure without modifying the breathing pattern.