IntroductionPulmonary hypertension (PH) in modern genetic strains of chicken broilers is a world wide distributed entity, which has a recognized economical impact (Maxwell et al., 1997;Pavlidis et al., 2007).Also, the commercial chicken has been a model to study PH in man, because encountered pathological changes in that animal closely resemble the human condition.Under natural atmospheric conditions, two determining causes for PH are known: hypobaric hypoxia and low temperatures exposure (i.e.birds exposure to temperatures below 16º C (Pakdel et al., 2005;Pan et al., 2005).The incidence and clinical evolution of PH in various strains of commercial chickens maintained under a natural hypoxic tropical environment in the Bogotá plane at 2638 m above sea level (masl) have been evaluated over several years, using direct observation of animals, electrocardiography, morphometric studies, hematocrit and hemoglobin changes, histochemical, immunohistochemical and molecular procedures.Morphometric cardiac and pulmonary changes allowed in post-mortem studies to define mass cardiac index values (CI) to differentiate non-pulmonary hypertensive chickens (NPHC) from the pulmonary hypertensive ones (PHC) (Hernández, 1982(Hernández, , 1987;;Cárdenas et al., 1985;Areiza, 2010, Vásquez, 2010).CI is defined as the right ventricular muscle mass weight expressed as a percentage of the total ventricular muscle weight (Alexander and Jensen, 1959).It is now clear that PHC have a CI value of 25 and higher and, below 22, birds can be safely allocated in the NPHC group (Gómez et al., 2007;Areiza, 2010;Vásquez, 2010).The remodeling process of pulmonary arterioles has been studied by morphometric analysis of smooth muscle and adventitia layers in PHC (Useche et al., 1981 Sandino and Hernández, 2005).Electrocardiography was found to have a predictive value to define which animals would develop PH.Also, it determined that there is not a defined time course in the evolution of PH in chickens.The latter has been corroborated with daily field observations (Pulido, 1996).Several studies showed that low temperatures and pulmonary diseases have an enhancing effect in the occurrence of PH (Mejía, 1982;Hernández, 1984).Hypobaric hypoxia and low temperature are determinants of PH occurrence in the pulmonary vasculature of broilers.Sensitivity is species dependent and the domestic chicken appears to be www.intechopen.comAltitude m above sea level Hemoglobine g/100 mm Hematocrit % PVC Red blood cells millions/cc * Mean SD Mean SD Mean SD 225 8.54 1.26 27.30 2.88 4.72 0.97 2638 NPHC 10.82 1.36 33.91 3.15 4.40 0.71 2638 PHC 13.15 2.30 40.10 8.30 2.43 1.31 SD: standard deviation.NPHC and PHC: non-pulmonary and pulmonary hypertensive chickens * Red blood cells values should be multiplied by 10000 Cárdenas et al., 1985.