Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal (such as with exercise) or abnormal (such as with electrical problems within the heart). Complications
Tachycardia can lead to fainting.When the rate of blood flow becomes too rapid, or fast  blood flow passes on damaged endothelium, it  increases the friction within vessels resulting in turbulence and other disturbances. According to the Virchows triad, this is one of the three conditions that can lead to thrombosis (i.e., blood clots within vessels). Causes
Some causes of tachycardia include:

Diagnosis
The upper threshold of a normal human resting heart rate is based on age. Cutoff values for tachycardia in different age groups are fairly well standardized; typical cutoffs are listed below:
1–2 days: Tachycardia >159 beats per minute (bpm)
3–6 days: Tachycardia >166 bpm
1–3 weeks: Tachycardia >182 bpm
1–2 months: Tachycardia >179 bpm
3–5 months: Tachycardia >186 bpm
6–11 months: Tachycardia >169 bpm
1–2 years: Tachycardia >151 bpm
3–4 years: Tachycardia >137 bpm
5–7 years: Tachycardia >133 bpm
8–11 years: Tachycardia >130 bpm
12–15 years: Tachycardia >119 bpm
>15 years – adult: Tachycardia >100 bpmHeart rate is considered in the context of the prevailing clinical picture. When the heart beats excessively or rapidly, the heart pumps less efficiently and provides less blood flow to the rest of the body, including the heart itself.
Doxapram hydrochloride (marketed as Dopram, Stimulex or Respiram) is a respiratory stimulant. Administered intravenously, doxapram stimulates an increase in tidal volume, and respiratory rate. Mechanism of action
Doxapram stimulates chemoreceptors in the carotid bodies of the carotid arteries, which in turn,  stimulates the respiratory centre in the brain stem. Appearance
Doxapram is a white to off-white, odorless, crystalline powder that is stable in light and air. It is soluble in water, sparingly soluble in alcohol and practically insoluble in ether. Injectable products have a pH from 3.5-5. Benzyl alcohol or chlorobutanol is added as a preservative agent in the commercially available injections. Uses
Doxapram is used in intensive care settings to stimulate the respiratory rate in patients with respiratory failure. It may be useful for treating respiratory depression in patients who have taken excessive doses of drugs such as buprenorphine or fentanyl analogues which may fail to respond adequately to treatment with naloxone.It is equally as effective as pethidine in suppressing shivering after surgery.Doxapram has been used as an anesthetic reversal agent when taking care of captive sharks but it must be used with caution since "animals can be extremely excitatory and dangerous under the influence of this drug". Side effects
Side effects include high blood pressure, panic attacks, rapid heart rate, tremor, sweating, and vomiting. Convulsions have been reported. Its use is relatively contraindicated in people with coronary heart disease, epilepsy, and high blood pressure.