Variations of regional lung function in acute respiratory failure and during anaesthesia

Mechanical ventilation strategies in massive chest trauma

Patients in extremis because of trauma-related massive chest injury require expedient evaluation and prompt intervention. The initial pathophysiology relates to the significant intrapulmonary shunting caused by disruption of pulmonary capillaries and extravasation into the alveolar spaces. Disproportionate or unilateral lung involvement needs measures more technical than general supportive care. Independent lung ventilation (mostly with unilateral lung involvement) and other strategies like inhaled nitric oxide, prone positioning, partial liquid ventilation, and extracorporeal membrane oxygenation (ECMO) have had good results. Intensivists confronted with this clinical subset may consider using these strategies as alternative/adjunctive options for optimizing respiratory and hemodynamic status in the supportive management of trauma-related acute lung injury (ALI) and adult respiratory distress syndrome (ARDS).

Negative pressure ventilation via diaphragmatic pacing: a potential gateway for treating systemic dysfunctions

Programmed diaphragmatic pacing using implanted neuromodulators represents an emerging method for providing pulmonary support using negative pressure ventilation. The implantable, rechargeable, programmable and miniaturized nature of diaphragmatic pacers may obviate many of the management issues associated with noninvasive positive pressure ventilation devices. Closed loop systems may facilitate the implementation of diaphragmatic pacing for the treatment of many indications. They may allow for wider adoption of ventilatory support in central sleep apnea and improve quality of life in diseases of chronic hypoventilation, such as amyotrophic lateral sclerosis. In addition, it might alleviate subclinical hypoventilation--a condition that may affect a significant proportion of the aging population. Diaphragmatic pacing could also reduce sympathetic bias, which may contribute to a wide range of diseases associated with autonomic dysfunction.

Clinical review: Independent lung ventilation in critical care

Independent lung ventilation (ILV) can be classified into anatomical and physiological lung separation. It requires either endobronchial blockade or double-lumen endotracheal tube intubation. Endobronchial blockade or selective double-lumen tube ventilation may necessitate temporary one lung ventilation. Anatomical lung separation isolates a diseased lung from contaminating the non-diseased lung. Physiological lung separation ventilates each lung as an independent unit. There are some clear indications for ILV as a primary intervention and as a rescue ventilator strategy in both anatomical and physiological lung separation. Potential pitfalls are related to establishing and maintaining lung isolation. Nevertheless, ILV can be used in the intensive care setting safely with a good understanding of its limitations and potential complications.

Detection of local lung air content by electrical impedance tomography compared with electron beam CT

The aim of the study was to validate the ability of electrical impedance tomography (EIT) to detect local changes in air content, resulting from modified ventilator settings, by comparing EIT findings with electron beam computed tomography (EBCT) scans obtained under identical steady-state conditions. The experiments were carried out on six anesthetized supine pigs ventilated with five tidal volumes (VT) at three positive end-expiratory pressure (PEEP) levels. The lung air content changes were determined both by EIT (Goe-MF1 system) and EBCT (Imatron C-150XP scanner) in six regions of interest, located in the ventral, middle, and dorsal areas of each lung, with respect to the reference air content at the lowest VT and PEEP, as a change in either local electrical impedance or lung tissue density. An increase in local air content with VT and PEEP was identified by both methods at all regions studied. A good correlation between the changes in lung air content determined by EIT and EBCT was revealed. Mean correlation coefficients in the ventral, middle, and dorsal regions were 0.81, 0.87, and 0.93, respectively. The study confirms that EIT is a suitable, noninvasive method for detecting regional changes in air content and monitoring local effects of artificial ventilation.

Beneficial effects of the prone position on the incidence of barotrauma in oleic acid-induced lung injury under continuous positive pressure ventilation

BACKGROUND

Factors that contribute to ventilator-induced barotrauma are not well defined. This study investigates the effects of posture on ventilator-induced barotrauma, as well as arterial oxygenation.

METHODS

Twelve dogs with oleic acid lung injury, lying in the prone position (n = 6) and right lateral position (n = 6), were mechanically ventilated with continuous positive pressure ventilation for 24 hours. The incidence of pneumothorax and arterial oxygenation were investigated in each position.

RESULTS

All animals lying in the lateral position developed pneumothorax in the nondependent thorax, while only one animal in the prone position developed pneumothorax in the left thorax (P < 0.05). Postmortem examination revealed that in the lateral group, the nondependent lung consisted of large areas of emphysematous regions and neutrophil infiltration regions, while the dependent lung was composed of extensive atelectasis and neutrophil infiltration. Lung ruptures occurred in the nondependent lung at the regions of either emphysematous change or severe neutrophil infiltration. In the prone group, in contrast, both lungs were inflated fairly homogeneously with only small areas of atelectatic and emphysematous changes. In the dog in whom pneumothorax developed, lung ruptures were limited to the regions of severe neutrophil infiltration in the left lung. No animal in either group had a peak airway pressure more than 20 cmH2O. The peak airway pressures were 17.5 +/- 1.3 cmH2O in the lateral group and 16.6 +/- 2.1 cmH2O in the prone group (P > 0.05). Tidal volume was comparable between the lateral group (13 +/- 2 ml/kg) and the prone group (12 +/- 1 ml/kg) before pneumothorax occurred (P > 0.05). Arterial oxygenation was much better in the prone group than in the lateral group during the experiment (P < 0.05).

CONCLUSION

It was concluded that in lung injury dogs, the prone position has beneficial effects on the incidence of ventilator-induced barotrauma and arterial oxygenation when compared with the lateral position. Ventilator-induced barotrauma may occur at lower airway pressure.

Liquid ventilation improves pulmonary function, gas exchange, and lung injury in a model of respiratory failure

OBJECTIVE

The authors evaluated gas exchange, pulmonary function, and lung histology during perfluorocarbon liquid ventilation (LV) when compared with gas ventilation (GV) in the setting of severe respiratory failure.

BACKGROUND

The efficacy of LV in the setting of respiratory failure has been evaluated in premature animals with surfactant deficiency. However, very little work has been performed in evaluating the efficacy of LV in older animal models of the adult respiratory distress syndrome (ARDS).

METHODS

A stable model of lung injury was induced in 12 young sheep weighing 16.4 +/- 3.0 kg using right atrial injection of 0.07 mL/kg of oleic acid followed by saline pulmonary lavage and bijugular venovenous extracorporeal life support (ECLS). For the first 30 minutes on ECLS, all animals were ventilated with gas. Animals were then ventilated with either 15 mL/kg gas (GV, n = 6) or perflubron ([PFC], LV, n = 6) over the ensuing 2.5 hours. Subsequently, ECLS was discontinued in five of the GV animals and five of the LV animals, and GV or LV continued for 1 hour or until death.

MAIN FINDINGS

Physiologic shunt (Qps/Qt) was significantly reduced in the LV animals when compared with the GV animals (LV = 31 +/- 10%; GV = 93 +/- 4%; p < 0.001) after 3 hours of ECLS. At the same time point, pulmonary compliance (CT) was significantly increased in the LV group when compared with the GV group (LV = 1.04 +/- 0.19 mL/cm H2O/kg; GV = 0.41 +/- 0.02 mL/cm H2O/kg; p < 0.001). In addition, the ECLS flow rate required to maintain the PaO2 in the 50- to 80-mm Hg range was substantially and significantly lower in the LV group when compared with that of the GV group (LV = 14 +/- 5 mL/kg/min; GV = 87 +/- 15 mL/kg/min; p < 0.001). All of the GV animals died after discontinuation of ECLS, whereas all the LV animals demonstrated effective gas exchange without extracorporeal support for 1 hour (p < 0.01). Lung biopsy light microscopy demonstrated a marked reduction in alveolar hemorrhage, lung fluid accumulation, and inflammatory infiltration in the LV group when compared with the GV animals.

CONCLUSION

In a model of severe respiratory failure, LV improves pulmonary gas exchange and compliance with an associated reduction in alveolar hemorrhage, edema, and inflammatory infiltrate.

Disconnection of the ventilatory system does not prevent pleural lesions during sternotomy

In previous studies pleurotomy has seldom been reported as a complication of sternotomy and, therefore, the incidence is unknown. Factors increasing or decreasing the risk of pleurotomy also have not been studied properly. In a prospective, randomized trial, performed during 14 consecutive months from 1988 until 1989, the incidence of pleurotomy and its possible risk factors were studied in 712 patients undergoing median sternotomy for cardiac and mediastinal procedures. The overall incidence of pleurotomy was 14.7%. Chronic obstructive pulmonary disease, the use of positive end-expiratory pressure, and continuation or discontinuation of the ventilatory system did not affect the incidence. A surgeon-related risk factor could be significantly identified (P < 0.001). In conclusion, disconnection of the ventilatory system during sternotomy has been shown to have no influence on the pleurotomy rate and its continued use is no longer valid.

Selective positive end-expiratory pressure and cardiac function in dogs

Effects of general (G) versus selective (S) right (R) and left (L) positive end-expiratory pressure (PEEP) were compared during differential lung ventilation in 11 anaesthetized dogs in the supine position. GPEEP 20 cmH2O decreased cardiac output (1 min-1) from 2.9 +/- 0.2 (mean +/- SE) to 1.7 +/- 0.5 (p less than 0.05), RPEEP from 2.8 +/- 0.2 to 2.2 +/- 0.2 (p less than 0.05) while LPEEP caused no significant change in cardiac output. GPEEP increased pleural pressure more than SPEEP. Pleural pressure was asymmetric during SPEEP. Both SPEEP and GPEEP increased pericardial pressure uniformly, but the increase was less marked with SPEEP. During GPEEP 20 cmH2O transmural left ventricular end-diastolic pressure (LVEDP) decreased markedly. SPEEP caused less marked reductions in transmural LVEDP. Qualitatively similar, but less marked changes were observed with PEEP 10 cmH2O. In conclusion, cardiac output decreased less with selective PEEP than with general PEEP. This was explained by less increase in pleural and pericardial pressure, and accordingly less decrease in LV transmural filling pressure.

Differential ventilation with low-flow CPAP and CPPV in the treatment of unilateral chest trauma

A case of severe unilateral chest trauma with bronchopleural fistula is presented. Ventilatory therapy consisted of asynchronous independent lung ventilation (AILV). The injured lung was ventilated with intermittent positive pressure ventilation (IPPV) [tidal volume (TV) = 200 ml, f = 25/min, I:E = 0.5, minute volume (MV) = 5.0 l/min, FiO2 = 0.4], and the unaffected lung was ventilated with continuous positive pressure ventilation (CPPV) (TV = 600 ml, f = 12/min, I:E = 0.5, MV = 7.2 l/min, PEEP = 0.5 kPa, FiO2 = 0.4). Adequate gas exchange was obtained (PaO2 = 14.5 +/- 2.3 kPa, PaCO2 = 5.5 +/- 0.7 kPa), but high air leakage volumes persisted. Thus, differential low-flow CPAP (V = 5.0-7.5 l/min, PEEP = 0.5 kPa, FiO2 = 0.4) of the injured lung and CPPV (TV = 600 ml, f = 12/min, MV = 7.2 l/min, I:E = 0.5, PEEP = 0.5 kPa, FiO2 = 0.4) of the unaffected lung was applied for 36 hours. Further deterioration of pulmonary function was prevented, and the bronchopleural fistula closed after several hours. After another period of AILV the patient was treated with conventional mechanical ventilation, and finally weaned with high-flow CPAP.

[Artificial respiration technics]

In general there are two distinguishable methods of artificial ventilation: assisted spontaneous ventilation and controlled ventilation. Spontaneous ventilation can be supported by CPAP or PEEP, in order to improve oxygenation, and by IMV to improve CO2 elimination. Furthermore, high frequency low pressure ventilation may be used versus low frequency high pressure ventilation. Conventional IPPV may be supported by continuous endexspiratory pressure. In special cases IRV may be applied. High frequency low pressure ventilation methods may be used intra- and postoperatively as well as post-traumatically.