Etiology

• During fetal life the ductus arteriosus connects the pulmonary artery to the aorta allowing blood from the right ventricle to bypass the constricted pulmonary circulation and go directly to the placenta. At this stage blood flows from right to left.
• At birth the lungs inflate and there is a dramatic decrease in pulmonary resistance, at the same time as an increase in systemic resistance and elimination of placental flow. Blood now flows preferentially to the lungs and flow in the ductus is from left to right.
• Over the first few days of life the ductus closes in response to several factors including increased oxygen tension and changing prostaglandin levels; this occurs in the first 3-4 days.
• Until complete anatomic closure occurs the ductus can re-open if the calf develops Hypoxemia Hypoxemia and pulmonary hypertension.
• Patent ductus artieriosus (PDAs) therefore remain open either because of failure of anatomic closure or because other defects result in abnormal pressures, blood flow or oxygen tensions, such as neonatal high altitude disease, sepsis, Neonatal ARDS.

Pathophysiology

• The ductus arteriosus, a normal fetal vessel, fails to close in the first hours or days after birth    →   conduit between the descending aorta and the pulmonary trunk.

Typical

• Pulmonary vascular resistance is much lower than systemic vascular resistance so: 
  • Left to right shunt (aorta to pulmonary artery)    →     increased flow through pulmonary vasculature    →     increased return to left heart    →     volume overload of left heart    →     chronic left-sided CHF CHF.
• Dilation and eccentric hypertrophy of left atrium/ventricle can result in rare circumstances, mitral regurgitation (MR) due to stretching of the mitral annulus.
• MR as a consequence is much rarer than in other species because cattle have a solid supporting framework between the atria and ventricles that is initially made of cartilage and then ossifies. 

Rare

• Right-to-left shunt.
• Pulmonary hypertension    →   due to retention of fetal type pulmonary vasculature and/or massive pulmonary overcirculation with a left-right shunt resulting in reactive hypertension (a left-to-right shunt typical of PDA)    →   pulmonary overcirculation    →   pulmonary hypertension (as Eisenmenger's physiology)    →    right-left shunt.

Timecourse

• Depends on presence of other congenital abnormalities.

Epidemiology

• This is an extremely rare condition in cattle and as such there are insufficient publications to identify breed and gender predispositions.
  • One group of animals with an increased prevalence are cattle born at altitude with high altitude disease and increased pulmonary artery pressures.