Depending on the cause, chest tightness can develop suddenly and disappear quickly, such as during hyperventilation or when breathing in very cold air. An intense feeling of tightness in the chest that occurs in a sudden, severe episode may be due to a heart attack or pulmonary embolism.
http://www.localhealth.com/article/chest-tightness

Breathing like this leads to a feeling of tightness in the chest. While not medically dangerous these symptoms can be very frightening and lead to an increase in anxiety which then creates a vicious circle.
http://www.hypnoanalysiskent.co.uk/what-can-hypnotherapy-tre...

If with intake of Mirtazapine, any of the following severe side effects are experienced, it is essential to seek medical attention immediately: mouth sores; red, swollen, blistered, or peeling skin; symptoms of infection (sore throat, fever, chills,); severe headache or dizziness; exaggerated feeling of well-being; sluggishness; aggressiveness; impulsiveness; irritability; panic attacks; suicidal thoughts or actions; worsening of depression; restlessness; and seizures. Most of all if there is an experience of difficulty of breathing; a feeling of tightness in the chest; ...
http://www.sideeffectsfacts.com/mirtazapine-side-effects

But others include unfocused worry or anxiety and a feeling of doom and utter despair, your heart pounding rapidly in your chest, sweating and clammy hands, headaches, severe shortness of breath and a feeling of tightness in the chest, and dizziness.
http://clearfromfear.blogspot.co.uk/