The effect of pressure support ventilation on auto-PEEP in a patient with asthma

Acute iterative bronchospasm and "do not re-intubate" orders: sedation by an alpha-2 agonist combined with noninvasive ventilation

A male patient presented with bronchospasm and acute respiratory distress. The patient had presented 2 previous episodes of severe bronchospasm following abdominal surgery, leading twice to intubation, mechanical ventilation, and conventional sedation. As the patient positively rejected a third episode of intubation + mechanical ventilation, noninvasive ventilation (pressure support = 8 cm H₂O, positive end-expiratory pressure = 10 cm H₂O), inhaled therapy, and clonidine orally (≈ 4 μg/kg) were combined. Over 1 to 2 hours, the acute respiratory distress disappeared. Noninvasive ventilation was discontinued on the next morning (day 2). The patient was discharged from the critical care unit on day 3 on good condition but died at a later interval from iterative bronchospasm. Evidence-based documentation of the effects of alpha-2 agonists in the setting of acute bronchospasm in the emergency department or status asthmaticus in the critical care unit is awaited.

Relationship of flow and cross-sectional area to frictional stress in airway models of asthma

Frictional stress from air flowing through narrowed airways may damage the airway mucosa and thereby increase airway inflammation and airway obstruction. To investigate the levels of frictional stress that might occur in the airway, we measured the frictional stress in three physical airway models (model 1: normal state; models 2 and 3: narrowed states with cross-sectional area half and one-fourth of model 1, respectively) at tracheal expiratory flow rates of 1, 2, 3, 4, 5, 6, 7, and 8 L/sec. Frictional stress measured at stations in the trachea (A), two each in the left (B and C) and right (D and E) major bronchi, and one in the right secondary bronchus (F) indicated that at higher flow rates, high values of the frictional stress seen in model 3 (highest value being 139.2 N/m2 at 8 L/sec at station) could well damage the airway wall, especially during episodes of cough, and particularly when the mucosa is inflamed and friable as it is in asthmatic patients. Conversely, control of cough may have anti-inflammatory benefits in asthmatic patients.

Effects of positive end-expiratory pressure on transthoracic impedance--implications for defibrillation

The success of defibrillation is determined by trans-myocardial current. This current is inversely proportional to transthoracic impedance (TTI). We proposed that increasing lung volume using positive end-expiratory pressure (PEEP) would increase TTI. 12 healthy subjects aged 21-37 years (6 male) were recruited. TTI between self-adhesive defibrillation pads placed in the standard antero-apical position was measured at 30 kHz at end-expiration using Bodystat MultiScan 5000 equipment. PEEP at 2.5, 5.0, 10.0, 20.0 and 40.0 cm H2O was generated using a standard continuous positive airways pressure (CPAP) circuit. Data were analysed using linear regression and paired t-tests comparing baseline TTI with that at a given PEEP. Mean (+/- S.D.) baseline TTI was 65.7 +/- 6.9 omega. TTI increased linearly with increasing PEEP (r2 = 0.99; P < 0.001). Mean percentage increases in TTI were 0.6% at 2.5 cm H2O PEEP (P = 0.05), 1.5%, at 5.0 cm H2O PEEP (P < 0.001), 3.1% at 10.0 cm H2O PEEP (P < 0.001), 5.6% at 20.0 cm H2O PEEP (P < 0.001) and 10.4% at 40.0 cm H2O PEEP (P < 0.001). PEEP increases TTI, proportionately reducing transthoracic current during defibrillation. Clinically, high levels of PEEP (20-40 cm H2O) may occur during ventilation of patients at cardiac arrest and in acute asthmatics. To maximise peak defibrillation current, PEEP should either be minimised prior to defibrillation or consideration should be given to earlier use of high energy levels for defibrillation.

Noninvasive positive pressure ventilation in status asthmaticus

OBJECTIVE

In asthmatic patients with acute respiratory failure (ARF), placing an endotracheal tube is associated with a high rate of complications and results in increased airway resistance. In acute asthma, mask-continuous positive airway pressure (CPAP) decreases airway resistance and the work of breathing (WOB), but does not improve gas exchange. In COPD with ARF, adding intermittent positive pressure ventilation to mask-CPAP results in an additional improvement in WOB and is highly effective in correcting gas exchange abnormalities. In our medical ICU, noninvasive positive pressure ventilation (NPPV) is used as first-line interventional therapy in eligible patients with hypercapnic ARF. We report our experience with NPPV in 17 episodes of asthma and ARF over a 3-year period.

METHODS

A face mask was secured with head straps, avoiding a tight fit, and connected to a ventilator (PB-7200). Initial ventilatory settings included CPAP at 4 +/- 2 cm H2O to offset intrinsic positive end-expiratory pressure and pressure support ventilation (PSV) at 14 +/- 5 cm H2O aiming at a respiratory rate less than 25 breaths/min and an exhaled tidal volume of 7 mL/kg or more. PSV was then adjusted following arterial blood gas results.

RESULTS

Mean age was 35.4 +/- 11.3 years; 10 patients were female. The mean (+/- SE) for different physiologic values are reported at initiation, less than 2 h, 2 to 6 h, and 12 to 24 h into NPPV. pH was 7.25 +/- 0.01, 7.32 +/- 0.02 (p = 0.0012), 7.36 +/- 0.02 (p < 0.0001), and 7.38 +/- 0.02; PaCO2 was 65 +/- 2, 52 +/- 3(p = 0.002), 45 +/- 3(p < 0.0001), and 45 +/- 4; PaO2 fraction of inspired oxygen was 315 +/- 41, 403 +/- 47, 367 +/- 47, and 472 +/- 67 (p = 0.06); and respiratory rate was: 29.1 +/- 1, 22 +/- 1 (p < 0.0001), 20 +/- 1, and 17 +/- 1. NPPV was well tolerated, and only two patients required sedation. Initial delivered minute ventilation was 16 +/- 4 L/min. The mean (+/- SD) peak inspiratory pressure to ventilate in the NPPV-treated patients was 18 +/- 5 cm H2O and always less than 25 cm H2O. There was no complication or problem with expectorating of secretions. Oral intake (liquid diet) was preserved. Two patients required intubation (35 min and 89 h into NPPV) for worsening PaCO2. Duration of NPPV was 16 +/- 21 h. All patients survived. Length of hospital stay was 5 +/- 4 days.

CONCLUSIONS

In asthmatic patients with ARF, NPPV via a face mask appears highly effective in correcting gas exchange abnormalities using a low inspiratory pressure (< 25 cm H2O). A randomized study is in progress to assess fully the role of NPPV in status asthmaticus.

Pressure-support ventilation in children with severe asthma

OBJECTIVE

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

DESIGN

A case series.

SETTING

A university hospital.

SUBJECTS

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

INTERVENTIONS

Mechanical ventilation with pressure-support ventilation.

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSION

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

Noninvasive positive-pressure ventilation in patients with acute respiratory failure

This article provides a systematic review of the literature on the application of noninvasive ventilation in various forms of hypercapnic and hypoxemic respiratory failures. A description of the underlying pathophysiology is followed by a review of physiologic data explaining the mechanisms of action of noninvasive ventilation. A critical review of clinical studies is presented with specific suggestions. The methodology of correctly implementing and monitoring noninvasive ventilation in patients with acute respiratory failure, critical to success, is detailed.

An asthmatic weaned from a ventilator using a laryngeal mask

A case is described in which a patient was unable to be weaned from ventilatory support whilst the trachea remained intubated. She was successfully weaned using a laryngeal mask.