Blood pressure measures the force exerted by circulating blood against the walls of the arteries. This measurement is expressed as two numbers in units of millimeters of mercury (mmHg). The top number, or systolic pressure, represents the pressure when the heart contracts and pushes blood out. The bottom number, or diastolic pressure, measures the pressure when the heart rests between beats. Medical standards for healthy blood pressure have changed significantly over time, driven by scientific research revealing hidden health risks.
The Mid-Century Acceptable Range
The acceptable blood pressure range in 1950 was markedly higher than today’s standards. Many medical practitioners considered a reading of up to 150/90 mmHg to be within a tolerable limit for a healthy adult. For older individuals, an unscientific guideline suggested the permissible systolic pressure was “100 plus the patient’s age.” This meant a 60-year-old could have a systolic reading of 160 mmHg without immediate concern from their physician.
Readings categorized as Stage 2 hypertension today were often labeled “benign essential hypertension” in the mid-century. For example, a blood pressure of 160/100 mmHg was frequently viewed as a common and relatively harmless condition. Medical textbooks from the era suggested that even pressures up to 200/100 mmHg did not necessitate the use of hypotensive drugs.
The Medical Understanding of Risk in 1950
The rationale behind these high acceptable thresholds was rooted in a medical philosophy that viewed elevated blood pressure as a necessary compensation mechanism. Physicians believed that in older patients with hardened arteries, a higher pressure was required to perfuse vital organs with oxygen and nutrients. Attempting to lower this seemingly adaptive high pressure, according to this view, could lead to a stroke or kidney failure.
This belief system created a reluctance to intervene unless the patient presented with severe, life-threatening symptoms, known as malignant hypertension. Furthermore, drug treatments available in the 1950s were largely ineffective or carried significant, poorly tolerated side effects. Early medications like ganglion blockers often caused severe side effects such as difficulty standing, constipation, and difficulty urinating. The risk-benefit calculation for treating asymptomatic, “mild” high blood pressure therefore leaned heavily against treatment.
Key Research That Lowered Blood Pressure Thresholds
The shift away from mid-century standards began with large-scale population studies providing data on long-term risk. The Framingham Heart Study, which began in 1948, was among the first to challenge the notion of “benign” hypertension. Early findings, published in the late 1950s, demonstrated a progressive link between moderately elevated blood pressure and an increased risk of cardiovascular disease. The Framingham data showed that the risk of heart disease and stroke increased steadily across the entire range of blood pressure readings.
A second major turning point came with the Veterans Administration Cooperative Study on Antihypertensive Agents in the 1960s. This was the first well-designed, placebo-controlled clinical trial to test the effect of drug treatment on moderately high blood pressure. The trial demonstrated that treating patients with diastolic pressures between 105 and 114 mmHg reduced the rate of major complications, such as stroke, heart failure, and kidney failure, by over 50 percent. This compelling evidence directly contradicted the medical wisdom that lowering blood pressure was dangerous.
The introduction of effective and better-tolerated treatments also facilitated the change in guidelines. The discovery of thiazide diuretics, such as chlorothiazide, in the late 1950s offered a convenient and less toxic way to reduce blood pressure. These landmark studies, combined with safer medications, proved that treating what was once considered “mild” high blood pressure provided substantial, life-saving benefits, paving the way for lower treatment thresholds.
The Modern Definition of Normal Blood Pressure
The current medical guidelines define normal blood pressure as less than 120 mmHg systolic and less than 80 mmHg diastolic. This standard is significantly lower than the acceptable limits of the 1950s. The contemporary classification system recognizes that cardiovascular risk begins to rise at levels previously ignored.
A reading of 120-129 mmHg systolic and less than 80 mmHg diastolic is now classified as “Elevated.” Hypertension is defined in two stages, beginning at a systolic pressure of 130-139 mmHg or a diastolic pressure of 80-89 mmHg (Stage 1). A blood pressure considered harmless in the mid-century, such as 160/100 mmHg, is now categorized as severe Stage 2 hypertension, requiring immediate therapeutic intervention.