The Low-Down on High Blood Pressure Part I - What do those numbers mean?

Posted on March 17, 2017 by Joe Jensen

Hypertension, or high blood pressure, is the most common reason for both doctor visits and the use of prescription medications. Nearly 1 out of 3 Americans reported to have this condition - and this number is only going to increase as our population ages. And yet, despite the frequent occurrence of high blood pressure and its severe consequences, experts estimate that only 50% of people with high blood pressure maintain proper control. How is that possible?

To answer that question, we must start with the heart.

Many people are familiar with a “normal” blood pressure of 120/80, but what do those numbers actually mean? When the heart squeezes, blood is propelled through arteries (carrying blood away from the heart) and veins (carrying blood back toward the heart). During that squeeze, also known as systole, pressure in the arteries rises. The pressure created during this phase is represented by the first number in your blood pressure. When the heart is relaxing, also known as diastole, it is filling with blood to prepare for another squeeze. The pressure during this phase, represents the second number in your blood pressure reading.

To measure those two numbers, we use the familiar and somewhat uncomfortable blood pressure cuff. The cuff is inflated to a pressure above what is commonly the systolic, or top number, of the measurement. A stethoscope, or a computerized version, is placed under or within the blood pressure cuff to listen for heart sounds. When the cuff pressure is higher than the systolic pressure, no sounds are heard because those arteries are collapsed and no blood gets through. Once the pressure equalizes, heart sounds are heard, and that pressure is recorded as the systolic pressure. The cuff continues to deflate until heart sounds are no longer heard because there is not enough pressure to transfer the sound. That pressure is the diastolic pressure when the heart is relaxed. At that point the pins and needles feeling of blood flow being restored to the arm is felt, but that’s it! Measurement complete. No harm done.

How high is too high?

To classify someone with normal blood pressure, high blood pressure, or something in between (known cleverly as prehypertension), three total physician visits with blood pressure readings must be made. The average of these three numbers is the final result:

Normal: Systolic <120 and Diastolic <80, <120/80

Prehypertension: Systolic 120-139 or Diastolic 80-89, 120/80-139/89

Hypertension: Systolic >140 or Diastolic >100

There are two difficulties with measuring blood pressure in this way. First, these measurements are only snapshots in time. It’s far more important to understand what the blood pressure reads throughout a typical day and night as more data leads to better understanding of blood pressure, and a doctor can then make better decisions to control the pressure. Second, a doctor’s office will never be known as a place of calm and relaxation. Simply being in a doctor’s office tends to raise anxiety and blood pressure. In fact somewhere between 20-25% of people have white coat hypertension which means their blood pressure is elevated in the clinic and normal elsewhere.

These two challenges led to a better way to measure blood pressure called Ambulatory Blood Pressure Monitoring (ABPM). ABPM works with a small blood pressure cuff that stays on a patient’s arm for 24 hours, taking blood pressure every 15-30 minutes. Most patients tolerate the devices quite well. While people with white coat hypertension don’t actually need to be treated thanks to ABPM, there is an opposite phenomenon called Masked Hypertension that ABPM can also recognize. These people have normal readings in the office but high measures elsewhere leading to “hidden” and under-treated high blood pressure. The ABPM provides the physician with the most accurate blood pressure measurement:

Normal: Systolic <130 and Diastolic <80, <130/80

Hypertension: Systolic >135 and Diastolic >85, >130/85

Understanding the normal vs abnormal of blood pressure is only the beginning. Part II will dive deeper into risk factors, the different types of hypertension, and why it matters. Part III will explore the treatment of high blood pressure specifically focusing on the most common medications people take, how these pills work, and any side effects these medications may have. Part IV will then attempt to answer the initial question posed above: Why are only 50% of people with hypertension achieving appropriate control? Stay tuned.