Age‐related differences in the distortion of the sheep lung in response to localised pleural stress

Abstract

'As a means of relief from the leaping heart when passion is excited . . . they [the engendered sons of God] contrived and implanted the form of the lung -soft and bloodless . . . as a kind of padding around the heart . . .' (Plato, (Cournand, 1982) Although our understanding of the physiology of the lung has greatly changed since the time of Plato, the interaction between the heart and the lungs is still recognised to affect both respiratory and cardiac function. For example, cardiogenic gas mixing, though partially a result of instantaneous changes in the rate of O 2 consumption and CO 2 production , largely results from direct mechanical interactions between the heart and the lung. Lung gas volume decreases during diastole and increases during systole as the filling and emptying heart displaces the lungs Lloyd, 1989). Just as the heart influences lung gas volume, the presence of the lung surrounding the heart also significantly limits ventricular filling and thus limits cardiac output via the Frank-Starling mechanism. Brookhart & Boyd (1947) observed that the interaction between the heart and the lung results in intra-thoracic pressure within the cardiac fossa exceeding that in the lateral pleural space, and that an increase in lung volume is accompanied by an increase in extra-cardiac pressure. It was subsequently recognised that this increase in extra-cardiac pressure is the mechanism whereby the application of positive endexpiratory pressure and increasing lung volume act to constrain ventricular filling and limit cardiac output (Fewell et al. 1980a,b;Kingma et al. 1987).Not only does the constraint applied to the heart by the lungs change when airway pressure and lung volume are changed, it also changes with developmental age. In the fetus, the lungs are liquid filled and significantly constrain ventricular filling (Grant et al. 1992b;Grant & Walker, 1996). However, this constraint is largely eliminated immediately following the aeration of the lungs at birth (Grant et al. 1992a(Grant et al. , 1994. Within days of birth, the lungs once again significantly constrain the heart and produce up to 50% of the total constraint experienced by the neonatal heart (Grant et al. 1994); by contrast, in the adult only 25-30% of the total cardiac constraint arises from the lung (Kingma et al. 1987).Although developmental changes in the chest wall (Agostoni, 1959) and pericardium (Naimark, 1995;Naimark et al. 1998) 1. In order for diastolic filling to occur, the heart must displace the lung. Given the changes in lung structure and compliance that follow birth, we sought to determine whether the neonatal lung resists neighbouring structures encroaching into its space more than the adult lung and whether the lung surface making up the cardiac fossa resists distortion more than the lateral surface does.2. Pleural distortions, induced by applied pressures (P appl ) of 20-120 g cm _2 at airway pressures (P aw ) of 2.5-15 cmH 2 O, were recorded in isolated lungs of adult, neonatal (4-week-old) and newborn (1-week-old) sheep...

Right atrial pressure as a measure of ventricular constraint arising from positive end-expiratory pressure during mechanical ventilation of the neonatal lamb

“…Extracardiac constraint, arising from the combined action of the pericardium, the lungs, and the chest wall, limits ventricular filling (preload) and thus limits heart function throughout life (4, 10 -17). Although it is known that increasing lung volume (5) and Paw (3)(4)(5) amplifies extracardiac constraint in the adult, it is less predictable how Paw might alter extracardiac constraint during mechanical ventilation in the critically ill neonate because ventricular constraint depends on lung volume and lung compliance (18,19). The high Paw required to ventilate low compliant lungs may exaggerate ventricular constraint by increasing the rigidity of the lungs.…”