Hypertension: severe, uncontrolled: patients required higher blood pressures than normal to avoid brain hypoxia.

Clinical bottom line (level 4)

  1. Patients with severe uncontrolled hypertension suffered brain hypoxia if their mean arterial blood pressure fell below 70 mmHg (compared with 40 mmHg in controls).
  2. In patients with severe uncontrolled hypertension, the brain could typically autoregulate cerebral blood supply between 120 and 200 mmHg.
Strandgaard et al: British Medical Journal 1973; 1: 507-510
Expires August 2003

The study

Case-control study with objective outcomes, not adjusted for confounding factors, not validated in an independent set of patients.

Setting:

13 patients (aged mean 58y, 85% male)

Excluded if
  • overt heart failure


    • Cases: 10 patients (100% male, mean age 55): severe hypertension (160/ 110 to 250/ 145 mmHg) and grade II-IV hypertensive retinopathy
    Controls: 3 patients (33% male, mean age 70): normotensive, but other disease (dementia, chronic vertigo, chronic alcohol misuse)


    Outcomes studied:
    • Cerebral blood flow measured by arteriovenous oxygen difference method using the Fick principle. Transducers were placed into the brachial artery and jugular bulb to measure blood pressure. Oxygen concentration was measured by spectrophotometry of haemoglobin. PaCO2 was measured with a Severinghaus electrode. The blood pressure was varied with an intravenous infusion of angiotensin amide and trimethaphan. Neither of these two drugs have been shown to have a direct effect on cerebral blood flow.

    The evidence

    • range of mean blood pressure at which brain autoregulates oxygen supply:
      • hypertensive: 120-200 mmHg
      • control: 70-140 mmHg
    • mean blood pressure required to cause symptoms of hypoxia:
      • hypertensive: 68 mmHg
      • control: 40 mmHg
    • Brain hypoxia determined by symptoms - nausea, sleepiness, dizziness, discomfort with hyperventilation and a decrease in pCO2.

    Comments

    1. Small study with ill-defined inclusion/exclusion criteria, and a poorly matched control group.
    2. Hypertension mainly due to essential hypertension: two patients studied one week after hypertensive encephalopathy.
    3. This study has important implications for reducing the blood pressure in hypertensive crises - dropping the blood pressure too greatly (mean arterial bp > 120 mmHg) can lead to brain hypoxia. Care should therefore be taken to lower blood pressure gradually.

    Citation

    1. Strandgaard S, et al: Autoregulation of brain circulation in severe arterial hypertension. British Medical Journal 1973; 1: 507-510
    Contributor: Nick Shenker and Chris Ball, August 1999
    Reviewer:

    Clinical Question.
    Patient severe, uncontrolled hypertension
    Intervention or Exposure mean arterial blood pressure
    Outcome hypoxia occurs