Why is second blood pressure reading lower than the first

Oct. 1, 1999 (Atlanta) -- 120/95. 190/80. 180/110. Many patients have wondered, when they get their blood pressure taken, which is the most important number: the first, higher number (systolic), or the second, lower number (diastolic)? A recent study showed that the systolic number is at least as important as the diastolic number, a finding which is somewhat contrary to traditional belief. To get the word out, the National High Blood Pressure Education Program, coordinated by the National Heart, Lung and Blood Institute (NHLBI), will be issuing, later this fall, a clinical advisory letter to U.S. health professionals warning of the danger of not properly treating high systolic blood pressure.

The systolic reading measures pressure in the blood vessels during that split-second when the heart is pumping out blood; the diastolic reading measures pressure when the heart is at rest. Up until recently, many in the medical community have emphasized the diastolic reading, but an accumulation of recent findings shows that the systolic reading is as -- if not more -- important in diagnosing and treating high blood pressure, or hypertension.

"To put it in context, we have to have a bit of historical perspective," Donald Lloyd-Jones, MD, from Massachusetts General Hospital in Boston, tells WebMD. "There has been this historical bias assuming that diastolic blood pressure and diastolic hypertension were more important and more risky than systolic hypertension. It was initially thought that the baseline pressure that the heart and the vessels see is the diastolic pressure, and then they see this brief instantaneous pressure when the heart pumps, the systolic pressure. People thought that the baseline tone was more important than that brief pulse of systolic pressure in determining what the overall long-term risk would be, [but] that has not turned out to be true."

Lloyd-Jones says this led many physicians to effectively disregard the systolic reading. "Typically clinicians have focused on the bottom number, the diastolic number, and they really have only gotten aggressive with treatment if that bottom number started to get elevated. It used to be thought that it was a normal finding to see systolic blood pressure climb with age. Indeed in most people, systolic blood pressure does climb with age, but it is not necessarily a normal phenomenon, it carries with it substantial risk of stroke, heart disease, and kidney disease," he says.

Lloyd-Jones is the lead author of the study, published in the September issue of Hypertension, that is part of the growing body of evidence pointing to the importance of elevated systolic pressure. Using 3,656 adults from the Framingham Heart Study, he and his colleagues showed that by using only the subjects' systolic blood pressure, 96% of them were correctly classified into the right blood pressure stage. Using only the subjects' diastolic reading, only 68% were correctly classified.

Patients are classified into blood pressure stages depending on their reading and treated accordingly. The stages, as outlined by the sixth Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC-VI), range from normal and high normal to stage 1, 2, 3 hypertension. The guidelines were updated in 1997.

"Under JNC-VI, you get staged into a certain blood pressure stage based both on your systolic and diastolic pressure," says Lloyd-Jones. "So if the systolic stage is higher than that diastolic stage you are supposed to be classified to that higher stage. What we found was, far more often the systolic pressure determined what stage you should be in ... so this kind of runs contrary to that historical bias."

"Many clinicians still have that historical bias, that we can ignore systolic elevation [that is] out of proportion [with diastolic pressure] because it is a normal aging process," says Lloyd-Jones. "But as I said, while it is a common aging process, it is not normal, and it carries substantial risk. We are trying to raise awareness in clinicians, policy makers, and patients that we really need to be paying attention to the systolic elevation."

So, what's the big deal about being misclassified? "For tomorrow, for next week, for next month, it is unlikely that misclassification of blood pressure stage would have ramifications, unless it is markedly elevated. But it is important to get it right and understand just how aggressive one needs to be in treating patients, because even [when] treating patients ... and dropping their blood pressure back to normal ... you never quite return their risk back to baseline," explains Lloyd-Jones. "We know that before stroke, heart disease, and kidney disease become clinically manifest, there are changes occurring in some of these organs -- and the eyes-- that can have long-term implications for the health of the patient."

Lloyd-Jones says the most important thing a person can do is know his or her own blood pressure. "The current recommendations are that people should have their blood pressure screened every 2 years. If it is normal, they can stay on an every-2-year schedule. If there are elevations of either systolic or diastolic, they should be followed relatively soon with another measurement." He says that depending on how high it is, a patient may need to be seen again, or may get counseling on diet and exercise to lower the blood pressure; some may be put on medication right away.