Nonconventional methods of ventilation

Physiologic effects of alveolar recruitment and inspiratory pauses during moderately-high-frequency ventilation delivered by a conventional ventilator in a severe lung injury model

Background and aims

To investigate whether performing alveolar recruitment or adding inspiratory pauses could promote physiologic benefits (VT) during moderately-high-frequency positive pressure ventilation (MHFPPV) delivered by a conventional ventilator in a porcine model of severe acute respiratory distress syndrome (ARDS).

Methods

Prospective experimental laboratory study with eight pigs. Induction of acute lung injury with sequential pulmonary lavages and injurious ventilation was initially performed. Then, animals were ventilated on a conventional mechanical ventilator with a respiratory rate (RR) = 60 breaths/minute and PEEP titrated according to ARDS Network table. The first two steps consisted of a randomized order of inspiratory pauses of 10 and 30% of inspiratory time. In final step, we removed the inspiratory pause and titrated PEEP, after lung recruitment, with the aid of electrical impedance tomography. At each step, PaCO₂ was allowed to stabilize between 57–63 mmHg for 30 minutes.

Results

The step with RR of 60 after lung recruitment had the highest PEEP when compared with all other steps (17 [16,19] vs 14 [10, 17]cmH₂O), but had lower driving pressures (13 [13,11] vs 16 [14, 17]cmH₂O), higher P/F ratios (212 [191,243] vs 141 [105, 184] mmHg), lower shunt (23 [20, 23] vs 32 [27, 49]%), lower dead space ventilation (10 [0, 15] vs 30 [20, 37]%), and a more homogeneous alveolar ventilation distribution. There were no detrimental effects in terms of lung mechanics, hemodynamics, or gas exchange. Neither the addition of inspiratory pauses or the alveolar recruitment maneuver followed by decremental PEEP titration resulted in further reductions in VT.

Conclusions

During MHFPPV set with RR of 60 bpm delivered by a conventional ventilator in severe ARDS swine model, neither the inspiratory pauses or PEEP titration after recruitment maneuver allowed reduction of VT significantly, however the last strategy decreased driving pressures and improved both shunt and dead space.

Is high-frequency oscillatory ventilation more effective and safer than conventional protective ventilation in adult acute respiratory distress syndrome patients? A meta-analysis of randomized controlled trials

INTRODUCTION

Comprehensively evaluating the efficacy and safety of high-frequency oscillatory ventilation (HFOV) is important to allow clinicians who are using or considering this intervention to make appropriate decisions.

METHODS

To find randomized controlled trials (RCTs) comparing HFOV with conventional mechanical ventilation (CMV) as an initial treatment for adult ARDS patients, we searched electronic databases (including PubMed, MedLine, Springer Link, Elsevier Science Direct, ISI web of knowledge, and EMBASE) with the following terms: "acute respiratory distress syndrome", "acute lung injury", and "high frequency oscillation ventilation". Additional sources included reference lists from the identified primary studies and relevant meta-analyses. Two investigators independently screened articles and extracted data. Meta-analysis was conducted using random-effects models.

RESULTS

We included 6 RCTs with a total of 1,608 patients in this meta-analysis. Compared with CMV, HFOV did not significantly reduce the mortality at 30 or 28 days. The pooled relative risk (RR) was 1.051 (95% confidence interval (CI) 0.813 to 1.358). ICU mortality was also not significantly reduced in HFOV group, with a pooled RR of 1.218 (95% CI 0.925 to 1.604). The pooled effect sizes of HFOV for oxygenation failure, ventilation failure and duration of mechanical ventilation were 0.557 (95% CI 0.351 to 0.884), 0.892 (95% CI 0.435 to 1.829) and 0.079 (95% CI -0.045 to 0.203), respectively. The risk of barotrauma and hypotension were similar between the CMV group and HFOV group, with a RR of 1.205 (95% CI 0.834 to 1.742) and a RR of 1.326 (95% CI 0.271 to 6.476), respectively.

CONCLUSIONS

Although HFOV seems not to increase the risk of barotrauma or hypotension, and reduces the risk of oxygenation failure, it does not improve survival in adult acute respiratory distress syndrome patients.

The role of high-frequency oscillatory ventilation in the treatment of acute respiratory failure in adults

PURPOSE OF REVIEW

High-frequency oscillatory ventilation (HFOV) is increasingly used in adults with the acute respiratory distress syndrome (ARDS), who remain hypoxemic during conventional mechanical ventilation. In this review, we will summarize the trials evaluating HFOV in adults with ARDS and discuss issues relevant to the clinician regarding the use of HFOV.

RECENT FINDINGS

Several observational and randomized trials support the safety of HFOV and improvements in oxygenation in adult patients with severe ARDS, who remain hypoxemic during conventional mechanical ventilation.

SUMMARY

HFOV theoretically meets the goals of lung-protective ventilation. On the basis of the current evidence, HFOV is associated with improvements in oxygenation in severe, adult ARDS. However, whether HFOV influences mortality, length of ICU stay, ventilator-free days, quality-of-life factors and is cost-effective remains to be determined. Large, prospective, randomized controlled trials such as the ongoing OSCAR and OSCILLATE trials will help further define the role of HFOV in adult ARDS.

High-frequency oscillatory ventilation for adult patients with ARDS

High-frequency oscillatory ventilation (HFOV) is characterized by the rapid delivery of small tidal volumes (Vts) of gas and the application of high mean airway pressures (mPaws). These characteristics make HFOV conceptually attractive as an ideal lung-protective ventilatory mode for the management of ARDS, as the high mPaws prevent cyclical derecruitment of the lung and the small Vts limit alveolar overdistension. In this review, we will summarize the literature describing the use of HFOV in adult patients with ARDS. In addition, we will discuss recent experimental studies of HFOV that have advanced our understanding of its mechanical properties. We identified 2 randomized controlled trials (RCTs) and 12 case series evaluating HFOV in adults with ARDS. In these studies, HFOV appears to be safe and consistently improves oxygenation when used as a rescue mode of ventilation in patients with severe ARDS. The two RCTs comparing HFOV to conventional ventilation revealed encouraging results but failed to show a mortality benefit of HFOV over conventional ventilation. Further research is needed to identify optimal patient selection, technique, the actual Vt delivered, and the role of combining HFOV with other interventions, such as recruitment maneuvers, prone positioning, and nitric oxide.

Damage limitation in burn surgery

Burn injury differs from other types of trauma in the apparent lack of urgency for treatment. We argue that in order to limit physiological damage and the development of multi-organ failure, management of the burn wound must be immediate and aggressive. Supportive fluid treatment should be judicious in order to prevent excessive oedema causing wound extension. Some potential strategies utilising oral fluid resuscitation are discussed, and potential pharmacological interventions. When associated with other trauma, major burn injury has a detrimental effect on morbidity and mortality, and surgical management of both aspects of a patient's injuries are altered.

[Preoxygenation and upper airway patency control]

During preoperative assessment, risk factors of upper airway obstruction should be evaluated: respiratory insufficiency, low O(2) reserve, preoxygenation failure or difficult face mask ventilation. In healthy subjects, spontaneous breathing O(2) for 3 min is the reference method. Apnoea duration is longer after preoxygenation than after denitrogenation, even if FEO(2) and SpO(2) do not change during the two last minutes of preoxygenation. The apnea time is longer after 3 min spontaneous breathing than after four deep breaths for 1 min in most of the literature. Maximal breathing during 2 min can produce values comparable to those obtained with tidal volume breathing for 3 min. FEO(2) monitoring is helpful in the assessment of preoxygenation quality: In case of oxygenation impairment during anaesthesia induction, algorithm use is helpful. Because desperate emergencies will occur in association with anaesthesia, every location should have the immediate availability of Fastrach and trans tracheal ventilation. Every anaesthesiologist should be familiar with and well practised in a variety of airway management techniques. Teaching programs are organised in order to develop anaesthesiologist sensitisation and skill.

Non-conventional respiratory support modalities applicable in the older child. High frequency ventilation and liquid ventilation

HFV, LV, and several other novel therapies offer promise to adults and children that the mortality associated with respiratory failure may be affected. Although there are several forms of HFV, HFOV is presently gaining favor in the treatment of severe respiratory failure and has generally supplanted HFJV in pediatric critical care. HFOV has the advantage of having an active expiratory phase, which helps to minimize air trapping and better modulate mean lung volume. Ventilators with sufficient power to perform HFOV in adults are currently under investigation, although there is a growing experience in using current ventilators in larger patients. To date, however, demonstration of lowered mortality with HFOV is lacking although intermediate outcome indicators are improved. PLV also offers promise in the treatment of ARF through its drastic ability to improve oxygenation, ventilation, and compliance in many lung injury models. Human trials are presently underway, but the optimal delivery of this novel therapy still necessitates extensive investigation. TLV is likely even more removed from general clinical application given the necessity of developing a new generation of ventilators for the delivery of liquid tidal volumes. How these and other modalities may piece together to improve the condition of our patients who have respiratory failure remains to be seen, but certainly, present and future investigation will be intriguing for years to come.

Experimental study of double-lumen, two-stage endotracheal tube during conventional mechanical ventilation in rabbits

OBJECTIVE

To evaluate the effects of a double-lumen, two-stage endotracheal tube on gas exchanges (ventilatory efficiency) during conventional mechanical ventilation, using a ventilator in rabbits.

DESIGN

Prospective, randomized, crossover laboratory animal trial.

SETTING

Research laboratory in the Beijing Children's Hospital.

SUBJECTS

Five adult New Zealand rabbits, weighing 3.2 to 3.7 kg.

INTERVENTIONS

A new type of endotracheal tube-the double-lumen two-stage endotracheal tube-was designed and tested for ventilation efficiency in rabbits with normal and injured lungs. The new tube (size 3.0 mm) was made out of two Portex endotracheal tubes (size 3.0 mm) by adhering two vertical cross-sections at the distal end of the two tubes. The new tube and a conventional endotracheal tube of the same size (inner diameter 3.0 mm) were randomly used in pressure control ventilation. Each trial was maintained for 30 mins.

MEASUREMENTS AND MAIN RESULTS

Effects from the two endotracheal tubes on pulmonary mechanics, hemodynamics, and gas exchange were observed. We measured peak inspiratory pressure, positive end-expiratory pressure (PEEP), intrinsic PEEP, mean airway pressure, and arterial blood and mixed expired gas variables (Pao2, Paco2, pH, and mixed expired gas Pco2). The new endotracheal tube acutely increased CO2 elimination in all animals with normal and injured lungs. Paco2 decreased from 46 +/- 4 to 36 +/- 5 torr (6.1 +/- 0.5 to 4.8 +/- 0.7 kPa; p < .01) in normal lungs and from 48 +/- 5 to 36 +/- 5 (6.4 +/- 0.7 to 4.8 +/- 0.7 kPa; p < .01). Meanwhile, tidal volume fraction decreased from 0.48 +/- 0.07 to 0.35 +/- 0.05 (p < .01) and from 0.56 +/- 0.07 to 0.40 +/- 0.07 (p < .01) in normal and injured lungs, respectively. Intrinsic PEEP of the new tube slightly increased, but there were no significant differences in comparison with the conventional tube.

CONCLUSION

Compared with the conventional endotracheal tube, the new double-lumen, two-stage endotracheal tube reduced Paco2 by decreasing anatomical deadspace in rabbits with normal and injured lungs under pressure control ventilation, thus enhancing ventilatory efficiency and reducing ventilator-induced injury.

High-frequency pressure-control ventilation with high positive end-expiratory pressure in children with acute respiratory distress syndrome

OBJECTIVE

Animal models suggest that high-frequency ventilation with low tidal volumes and high positive end-expiratory pressure (PEEP) minimize secondary injury to the lung. We hypothesized that using a high-frequency pressure-control mode of ventilation with high PEEP in children with severe acute respiratory distress syndrome (ARDS) would be associated with improved survival.

DESIGN

The study was a retrospective and prospective clinical study at a 24-bed tertiary care pediatric critical care unit. Fifty-three patients with severe ARDS were studied during a 37-month period, 30 prospectively and 23 retrospectively. Severe ARDS was defined as (1) rapid onset of severe bilateral infiltrates of noncardiac origin, (2) partial pressure of oxygen (arterial)/fraction of inspired oxygen less than 200 on PEEP of 6 cm H2O or more for 24 hours or longer, and (3) Murray disease severity score greater than 2.5. All patients meeting these criteria underwent ventilation in the pressure-control mode; the protocol for ventilation had the following general guidelines: (1) fraction of inspired oxygen limited to 0.5, (2) mean airway pressure titrated with PEEP to maintain arterial partial pressure of oxygen of 55 mm Hg or greater (7.3 kPa), (3) peak inspiratory pressure minimized to allow hypercapnia (arterial partial pressure of carbon dioxide, 45 to 60 mm Hg (6.0 to 8.0 kPa), and (4) ventilator rates of 40 to 120/min. Percutaneous thoracostomy and mediastinal tubes were placed for treatment of air leak.

RESULTS

The survival rate was 89% (47/53) in children with severe ARDS. Nonsurvivors had significantly higher peak inspiratory pressures (75 vs 40 cm H2O, p = 0.0006), PEEP (23 vs 17 cm H2O, p = 0.0004), mean airway pressure (40 vs 28 cm H2O, p = 0.04), alveolar-arterial oxygen gradient (579 vs 540 mm Hg, p = 0.03), and oxygenation index (43 vs 19, p = 0.0008) than survivors. Air leak was present in 51% of patients; there was no difference in the incidence of air leak between survivors and nonsurvivors (p = 0.42).

CONCLUSIONS

The high-frequency positive-pressure mode of ventilation was safe and was associated with an improved survival rate (89%) for children with severe ARDS. Limitation of both inspired oxygen and tidal volume, along with aggressive treatment of air leak, may have contributed to the improved survival rate.

Intratracheal pulmonary ventilation provides effective ventilation in a near-drowning model

Overdistension of the lungs from high inspiratory pressure is increasingly recognized as a major contributor to lung injury and worsening respiratory failure in the child who requires prolonged mechanical ventilation. Many modes of ventilation (such as high-frequency ventilation) have been introduced in an attempt to decrease this lung injury. Recently, a new mode of tracheal ventilation, intratracheal pulmonary ventilation (ITPV), has been described. By using a catheter positioned at the carina with continuous gas flow, it is possible to achieve effective ventilation at very low pressures. The purpose of this study was to evaluate the usefulness of ITPV in a near-drowning model. Ten domestic Yorkshire swine underwent arterial, venous, and pulmonary arterial catheter as well as tracheotomy placement. All animals received 13 mL/kg of fresh water intratracheally to induce a pulmonary injury. Six pigs were ventilated for 4 hours using ITPV; the other four pigs received conventional mechanical ventilation (CMV). Circulatory and ventilatory pressures, hemodynamic variables, arterial blood gases, and end-tidal CO2 were measured before lung injury and every 30 minutes thereafter. Both proximal and distal peak and mean airway pressures were measured. The animals were ventilated as needed to maintain the arterial blood gases in the normal range. The authors found the expected changes in pulmonary compliance, oxygen requirement, and airway pressure after inducement of lung injury. The six animals treated with ITPV had significantly lower airway pressures than those of controls. Peak inspiratory pressures with ITPV were 8.2 +/- 1.9 cm H2O versus 17.8 +/- 3.7 with CMV (P < .001). Distal mean airway pressures using ITPV were 2.3 +/- 0.1 cm H2O versus 9.0 +/- 3.2 with CMV (P < .01). With respect to hemodynamic variables, there were no differences between experimental and control animals. In conclusion, ITPV can afford effective ventilation in a near-drowning model of lung injury at airway pressures significantly lower than those required with CMV. ITPV could be a very valuable addition to the currently available methods of mechanical ventilation.

Acute respiratory distress syndrome: diagnosis and management

This review will attempt to put together the voluminous studies and concepts that have been published during the past 25 years following the description of the acute respiratory distress syndrome (ARDS) regarding diagnosis and management. The initial discussion will focus on how to clinically diagnose ARDS based recommendations. This also gives the current definition of acute lung injury and when to call it ARDS. The radiographic and hemodynamic characteristics are discussed including oxygenation parameters. The management outlines the conventional as well as new therapies intended to improve survival of this devastating disease.

Efficacy of tracheal insufflation of oxygen during oleic acid-induced pulmonary edema

STUDY OBJECTIVES

To determine whether tracheal insufflation of oxygen (TRIO) might be useful in field resuscitation of casualties with lung dysfunction.

DESIGN

Physiological measurements of cardiac and respiratory function were compared before and after oleic acid lung injury.

SETTING AND PARTICIPANTS

Beagles were studied in a laboratory.

INTERVENTIONS

Oleic acid (0.06 mL/kg) was injected over four minutes into the central venous port of a pulmonary artery catheter. Measurements were made during 30 minutes of TRIO before and after acute lung injury.

MEASUREMENTS

Hemodynamic and respiratory measurements, including intravascular pressures, heart rate, cardiac output, blood gases, respiratory system compliance, and O2 consumption were recorded during conventional mechanical ventilation and TRIO.

RESULTS

Before acute lung injury, PaO2 (mean +/- SD) increased (P less than .05) from 96 +/- 7.4 (13 +/- 1.0 kPa) during conventional mechanical ventilation to 360 +/- 123 mm Hg (48 +/- 16.4 kPa) after TRIO. PaCO2 (mean +/- SD) increased (P less than .05) from 39.5 +/- 1.1 (5.3 +/- 0.1 kPa) to 102 +/- 27.4 mm Hg (13.6 +/- 3.6 kPa). Arterial and mixed venous pH values decreased in proportion to PCO2. After acute lung injury, compliance decreased. PAO2 decreased (P less than .05) to 58 +/- 8.4 mm Hg (7.7 +/- 1.1 kPa) during conventional mechanical ventilation and increased (P less than .05) to 84 +/- 19.6 mm Hg (11.2 +/- 2.6 kPa) after 30 minutes of TRIO.

CONCLUSION

Despite poor gas exchange after acute lung injury, TRIO maintained adequate oxygenation and may be useful for emergency ventilation even when pulmonary edema complicates resuscitation.