Hypotension can be defined as any BP that is below the normal expected for an individual in a given environment. There is no single numerical cut-off universally accepted as representing hypotension. For example, while <90mmHg may be considered hypotensive for someone with 'normal BP' of 120mmHg, many healthy young adults will have a resting BP at or potentially below this level and will not be considered hypotensive. As a result, it is difficult to estimate the prevalence. Background history and accompanying clinical findings should be considered, bearing in mind that many common illnesses present atypically in older adults, without characteristic symptoms such as pain.  Hypotension can be the first sign of a serious acute illness (such as MI, sepsis, or GI haemorrhage).  Once acute causes have been excluded, other chronic or recurring causes should then be investigated.
It is essential to ensure that an appropriate cuff size is being used for BP measurements.   Using a cuff that is too small (undercuffing) may cause overestimation of BP and may further mask a period of hypotension; while use of a cuff that is too large (overcuffing) can lead to falsely low BP readings with subsequent unwarranted investigation. 
If the patient is not hypotensive at the time of assessment, lying and standing BP recording should be carried out in order to determine if orthostatic hypotension is present. A systolic BP drop of 20 mmHg or a diastolic BP drop of 10 mmHg occurring within 3 minutes of orthostasis is generally considered to be of significance. 
Hypotension is usually due to:
Decreased effective circulating volume (hypovolaemia)
Impaired cardiac output due to heart pump dysfunction (cardiogenic)
Impaired cardiac output due to obstruction to cardiac filling (obstructive)
Impaired peripheral vasoconstriction/distributive change.
In many cases, more than one mechanism is present (e.g., a patient with chronic heart failure who presents with a GI bleed may have evidence of both hypovolaemia and reduced cardiac output).
Initial management considerations
Liaison with nursing staff with regard to maintenance of patient safety while investigations are underway is prudent. In particular, patients who are hypotensive in the setting of sepsis may have an accompanying delirium increasing their risk of falls and injury.   It is often difficult to exclude or confirm the presence of shock without close monitoring of vital signs, including BP, heart rate, and urinary output. Therefore, consideration should be given to the most appropriate setting required to monitor, nurse, and medically manage the hypotensive patient.
- Ruptured abdominal aortic aneurysm
- Ectopic pregnancy
- Retroperitoneal bleed
- Carotid sinus syndrome (cardio-inhibitory subtype)
- Severe hypothyroidism
- Tension pneumothorax
- Cardiac tamponade
- Situational syncope
- Carotid sinus syndrome (vasodepressor subtype)
- Diabetic autonomic neuropathy
- Multi-system atrophy
- Primary autonomic failure
- Adrenal supression (iatrogenic)
- Addison's disease
- Thiamine deficiency
- Vitamin B12 deficiency