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Physiopathology

Short-Term Effects of NMBAs

The majority of the studies that have reported the effects of NMBA-induced paralysis were conducted in very short periods of time (<24 h) during anaesthesia (i.e. in “normal lungs”) with muscle paralysis and mechanical ventilation without PEEP [19-21]. Tokics and colleagues demonstrated that patients displayed atelectasis. Shunt was placed in gravity-dependent lung regions that corresponded to the atelectatic areas. There were considerable V'/Q' ventilation-to-perfusion mismatches that resulted in ventilation primarily of the ventral lung regions and perfusion of the dorsal lung regions [20]. Hedenstierna and colleagues demonstrated that due to the reduction of the cross-sectional chest area and the cephalic ascension of the diaphragm, the thoracic volume was reduced. This reduction was accompanied by a decrease in the functional residual capacity (FRC) that was attributed (at least in part) to the loss of muscular tone [21, 22] and was thought to be a major cause of postoperative hypoxaemia. However, few studies have reported the effects of anaesthesia and NMBA-induced paralysis on lung mechanics. In healthy subjects, Brismar and colleagues found no further reduction in FRC or modification of the elastic properties of the lung or the chest wall following the addition of paralysis to anaesthesia [23]. In the intensive care setting, Conti and colleagues conducted a study of 13 patients affected by diseases involving both lungs and the chest wall who required mechanical ventilation for acute respiratory failure and were heavily sedated (Ramsay scores of 5) and failed to demonstrate any further modification of either the chest wall or lung elastance following paralysis. CT scans and analysis of the pressure- volume curves revealed that derecruitment was reduced after the application of a

Potential physiopathological effects that explain the benefits of NMBAs in the most severe forms of ARDS. V/Q ventilator/perfusion ratio, FRC functional residual capacity

Fig. 12.2 Potential physiopathological effects that explain the benefits of NMBAs in the most severe forms of ARDS. V/Q ventilator/perfusion ratio, FRC functional residual capacity

PEEP of 10 cmH2O [21, 24]. In summary, no data support any additional deleterious role of NMBAs in the onset of atelectasis in sedated ARDS patients.

 
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