Downward-facing dog is one of the best-known yoga poses. It is also one of the more gentle of inversions in yoga. However, Anyone with uncontrolled high blood pressure should avoid being in any pose in which the head is lower than the heart. Therefore, the traditional downward-facing dog is contraindicated for people with severe hypertension.
I encourage my students to get into their downward dog from a child's pose. Keeping their arms forward and hands planted on the ground, shoulder distance apart, and with their feet remaining where they are, I cue them to shift their body forward and lift the bottom up and back. The head, neck, and shoulders should relax as they pull the heels to the ground. At times, I see students whose feet are too far away from the hands, or too close. By starting in the child's pose, the hands and feet should be in proper alignment, hands shoulder distance apart and feet hip distance apart.
Downward-facing dog is a wonderful toner for the back muscles and helps to lengthen the hamstrings. It's also a great way to get the blood flowing to your brain.
According to one of the yogi masters, B.K.S. Iyengar, Adho Mukha Svanasana, as it's called in Sanskrit, has many benefits. In his book, "Light on Yoga, Iyengar recommended this pose for people with asthma as well as those with backaches, or arthritis of the dorsal region and shoulders.
As always, consult your yoga therapist, medical practitioner, or physical therapist to determine which poses are best suited (or contraindicated) for you.