Functional lung unit in the pig
Introduction
Pulmonary vascular emboli may reduce or eliminate gas exchange in the affected lung region by obstructing blood flow to that region. However, this requires that the embolus be larger than a certain minimum size. Thus, small emboli lodging peripherally within the respiratory zone may not increase physiological dead space or cause a disturbance in ventilation-perfusion (a/) relationships because alveolar gas mixing and rich capillary interconnections will allow continued perfusion and gas exchange of the tissue distal to the site of obstruction. Using the multiple inert gas elimination technique, it has been shown that 150 μm (or larger) diameter beads injected into the pulmonary arterial tree in dogs produced regions with high a/ ratios, whereas beads of 100 μm or smaller diameter caused no increase in dead space or any development of high a/ regions (Young et al., 1980). A similar interpretation (cessation of gas exchange) was proposed by (Swinburne et al., 1982) on the observation of erroneously small lung water determinations, using inhaled C15O2, after injection of 175 μm spheres but not after 50 μm spheres. These findings suggest that the functional gas exchanging lung unit in the dog was supplied by a pulmonary arteriole of between 100 and 150 μm in diameter.
Another species that is frequently used for circulatory and respiratory studies is the pig. However, there are differences in lung morphology that may affect pulmonary gas exchange. One difference is that the muscle layer of the pulmonary artery is thicker in the pig than in the dog, and that the hypoxic pulmonary vasoconstrictor response is stronger (Tucker et al., 1975, Rendas et al., 1978). Another is that the pig lung has well-developed interlobular fibrous septa, not seen in the dog. This allows the dog to have collateral ventilation (Van Allen et al., 1931), mainly by communications between respiratory bronchioles (Martin, 1966), whereas the pig is devoid of such ventilation (Woolcock and Macklem, 1971, Sylvester et al., 1975). These collaterals enable gas mixing between units, which have no blood supply, e.g. after micro-embobilisation, and adjacent normal units. Thus, to impair gas exchange, a vessel must be obstructed that supplies all alveoli that can exchange gas via the collaterals (the ‘functional unit’). If there is no collateral ventilation, occlusion of a smaller vessel should suffice to produce a measurable gas exchange impairment. Pigs complete their pulmonary alveolarisation and growth of the pulmonary circulation in the first 8–12 weeks after birth (Hislop and Reid, 1977). Also, 2–3 month old pigs weigh ±20 kg and are similar in size to the dogs in previous studies of the functional lung unit (Young et al., 1980, Swinburne et al., 1982).
Accordingly, the purpose of the present study was to compare the size of the vessel supplying the functional lung unit in the pig with that in the dog of the same size, and to relate the findings to vascular dimensions and the presence or absence of collateral ventilation.
Section snippets
Preparation of animals
Twenty-two pigs (age 2–3 months, average weight of 20.5 kg) were studied in the supine position. All animals were initially anaesthetised with an i.m. injection of 1.5 ml Innovar Vet, 2.5 ml ketamine and 4 ml atropine. They were then brought to the laboratory where they received 300 mg pentobarbital i.v. and were intubated and connected to a Harvard ventilator. All were ventilated with air throughout the study at a respiratory frequency of 12 breaths min−1 and a tidal volume of ≈20 ml kg−1,
Baseline measurements
Cardiac output, pulmonary artery mean pressure and pulmonary vascular resistance were comparable to previously published data in normal pigs (Fredén et al., 1995) (Fig. 1). In all pigs, baseline ventilation-perfusion distributions measured before bead injection contained virtually all of the perfusion and most of the ventilation in a single narrow mode centred on a a/ ratio of 1. The dispersion of the perfusion and ventilation distributions against a/ ratios (log SDQ, log SDV) was 0.54
Discussion
This study suggests that the functional lung unit in the 20 kg pig is supplied by a vessel that is smaller by a factor of two than in the dog of similar size. Thus, the dispersion of the ventilation distribution, which is particularly sensitive to reduction in regional lung blood flow causing areas of greater than average a/ ratio, increased with increasing size of beads injected into the pulmonary vasculature at and above bead sizes of 92 μm. Linear interpolation of data (Fig. 4) suggests
Acknowledgements
This study was supported by National Heart, Lung and Blood Institute grant HL-17731, the Swedish Medical Research Council (5315) and the Swedish Heart–Lung Fund.
References (20)
Volumen inefficax Bemerkungen zur Frage des schädlichen Raumes
Ups. Läkarefören. Förhand.
(1938)- et al.
Limits of a/ distributions from analysis of experimental inert gas elimination
J. Appl. Physiol.
(1977) - et al.
Nitric oxide modulation of pulmonary blood flow distribution in lobar hypoxia
Anesthesiology
(1995) - et al.
Formation of the pulmonary vasculature
- et al.
Hypoxemia in acute pulmonary embolism
Chest
(1985) - et al.
Collateral ventilation may protect against high-altitude pulmonary hypertension
J. Appl. Physiol.
(1981) - et al.
Role of collateral ventilation in ventilation-perfusion balance
J. Appl. Physiol.
(1984) Pulmonary microembolism
Physiol. Rev.
(1983)- et al.
Determinants of hypoxemia during the acute phase of pulmonary embolism in humans
Am. Rev. Respir. Dis.
(1985) Respiratory bronchioles as the pathway for collateral ventilation
J. Appl. Physiol.
(1966)
Cited by (0)
- 1
Deceased.