This can happen in response to a decrease in blood volume (through dehydration or bleeding), or an unexpected change in blood flow. The most common cause of the latter is orthostatic hypotension (also called postural hypotension). Fever, hyperventilation, diarrhea and severe infections can also cause tachycardia, primarily due to increase in metabolic demands.An increase in sympathetic nervous system stimulation causes the heart rate to increase, both by the direct action of sympathetic nerve fibers on the heart and by causing the endocrine system to release hormones such as epinephrine (adrenaline), which have a similar effect. Increased sympathetic stimulation is usually due to physical or psychological stress. This is the basis for the so-called fight-or-flight response, but such stimulation can also be induced by stimulants such as ephedrine, amphetamines or cocaine. Certain endocrine disorders such as pheochromocytoma can also cause epinephrine release and can result in tachycardia independent of nervous system stimulation. Hyperthyroidism can also cause tachycardia. The upper limit of normal rate for sinus tachycardia is thought to be 220 bpm minus age. Inappropriate sinus tachycardia
Inappropriate sinus tachycardia (IST) is a rare but benign type of cardiac arrhythmia that may be caused by a structural abnormality in the sinus node. It can occur in seemingly healthy individuals with no history of cardiovascular disease. Other causes may include Autonomic nervous system deficits, autoimmune response, or drug interactions. Although symptoms might be distressing, treatment is not generally needed. Ventricular
Ventricular tachycardia (VT or V-tach) is a potentially life-threatening cardiac arrhythmia that originates in the ventricles.
If there is doubt about the interpretation of the x-ray, stress x-rays, tomograms or CT scan may be used for confirmation. Cause
The reasons for non-union are
avascular necrosis (the blood supply was interrupted by the fracture)
the two ends are not apposed (that is, they are not next to each other)
infection (particularly osteomyelitis)
the fracture is not fixed (that is, the two ends are still mobile)
soft-tissue imposition (there is muscle or ligament covering the broken ends and preventing them from touching each other)

Risk factors
Related to the person:
Age: Common in old age
Nutritional status : poor
Habits : Nicotine and alcohol consumption
Metabolic disturbance : Hyperparathyroidism
Can be found in those with NF1
Genetic predisposition
Causes related to fracture:
Related to the fracture site
Soft tissue interposition
Bone loss at the fracture
Infection
Loss of blood supply
Damage of surrounding muscles
Related to treatment
Inadequate reduction
Insufficient immobilization
Improperly applied fixation devices. Types of Nonunion
There are typically three types of nonunion described. Hypertrophic nonunion
In a hypertrophic nonunion, the fracture site contains adequate blood supply but the fracture ends fail to heal together. X-rays show abundant callus formation. This type of nonunion is thought to occur when the body has adequate biology, such as stem cells and blood supply, but inadequate stability, meaning the bone ends are moving too much. Typically, the treatment consists of increasing stability of the fracture site with surgical implants. Atrophic nonunion
In an atrophic nonunion, x-rays show little to no callus formation. This is usually due to impaired bony healing, for example due to vascular causes (e.g. impaired blood supply to the bone fragments) or metabolic causes (e.g. diabetes or smoking).