ECG Kindergarten

An electrocardiogram measures the electrical signals that travel through the heart while it beats.  Doctors are looking at how long it takes the electrical wave to pass through the heart, as well as how much signal there is.

The printout from an ECG is more than just a bunch of squiggly lines.  A closer look will reveal that not only are there squiggly lines, but there are also a few letters and numbers scattered about the page.  What does it all mean?

ecgStrip Leads

All Those Wires

If you have an ECG done, you’ll rest flat on your back and they’ll hook up a bunch of electrodes. It does not hurt.  These electrodes allow the doctor to look at the heart’s electrical activity from different angles.

To illustrate how this works, consider these pictures:

Nativity

These pictures are quite different but they’re all photos of the same thing, just from different angles.  Likewise, the different leads used in an ECG give doctors heart-activity pictures from a bunch of different angles.  It’s one heart, but the electrical waves look different depending on your perspective.

Chest Electrodes and Their Leads

Six electrodes are attached to the chest at specific places to provide a picture of how the heart is functioning. These six chest electrodes show the electrical signal looking from the horizontal plane.

  • V1 and V2 look at the heart from the center (septal)
  • V3 and V4 look at the heart from the front (anterior)
  • V5 and V6 look at the heart from the side (lateral)

Limb Electrodes and Their Leads

There will also be an electrode for each limb: right arm, right leg, left arm, left leg. One peculiar thing is that the leg electrodes can be positioned anywhere between your torso and ankle.  As long as they’re the same location on both sides of the body it doesn’t seem to matter (I’ve had them near my ankle and also had them on my lower abdomen, and was confused about the variation depending on who’s doing the test – but now it makes more sense).  These limb electrodes generate views from a vertical plane.

  • When measuring the flow of electricity toward the right arm, this is called augmented vector right (aVR)

  • When measuring the flow of electricity toward the left arm, this is called augmented vector left (aVL)
  • When measuring the flow of electricity toward the left foot, this is called augmented vector foot (aVF)
  • When measuring the flow of electricity toward the right foot – nope, that doesn’t happen. They only consider the left electrode; the one on the right is neutral.

 

Maybe you’re puzzled, as I was, about how it can be a 12-lead ECG when there are only ten electrodes (6 chest + 4 limbs).  It turns out that leads are not synonymous with electrodes.  The six electrodes for the chest are the same as the leads, but to understand the others, we have to recognize that those leads are made up of pairs of electrodes.  Think back to high school science class and what we learned about electricity.  Or maybe it might help to picture the terminals on your car battery, where we know that electricity flows from the negative to the positive.

An ECG also measures the flow between the limb electrodes (from negative to positive):

  • Lead I measures the flow from the right arm’s electrode to the left arm’s electrode
  • Lead II measures the flow from the right arm to the left leg
  • Lead III measures the flow from the left arm to the left leg

If you ever want to have even a fuzzy idea of what your ECG shows, it’s important to know about the leads, because the printout of your heart’s rhythm will reference those leads.  The letters and numbers on the ECG’s printout are referring to the lead used for that portion of the tracing.  The ten electrodes give twelve leads: six chest and six limb.

The Strip (Printout)

The printout from an ECG shows the heart’s electrical signal as waves.

  • If there is no electrical impulse, the line on the ECG is basically flat. This is called the baseline.
  • When an electrical impulse is travelling toward a lead, the line on the graph will move upward.
  • When an electrical impulse is travelling away from a lead, the line on the graph will move downward.

Just as the front and back of my nativity scene look very different, waves look different depending on which lead is being used.  For example, this shows the same heartbeats as seen from two different leads:

ecg updown

But what does it mean?  Notice that the squiggles seem to have a pattern.  Every distinct portion of the ECG means something specific. First the heart is at rest, and the horizontal line showing no electrical activity is the baseline.

ECGPartsPtoT in colorThe P-wave indicates the heart’s electrical signal traveling through the atria (top chambers of the heart).

Following the P-wave, the QRS complex shows the electrical signal traveling through the ventricles (bottom part of the heart).  When this happens, the ventricles contract and the atria re-set.

The T-wave indicates that the ventricles are resetting and getting ready for the next heartbeat.

ecg contractions

When we combine what we know about leads with what we know about these waves, we can look at the printout from an ECG and see how it all fits together.  This illustration is color-coded to show which leads view the heart from different angles, and where that information shows on the ECG’s printout.

Certainly this isn’t enough information to allow patients to read an ECG, but it does give us some background to have a hope of understanding what doctors are saying if they ever attempt to explain what they’re seeing on the printout of our electrocardiogram.

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About Pericarditis ECG’s:

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RA is a Risk Factor for Osteoporosis

Osteoporosis is silent. People’s bones can gradually become weaker and weaker with no outward hints that there is a problem — until suddenly bones break.  Spontaneous compression fractures of the spine can be quite painful and lead to deformity, chronic pain, and premature death.  Broken hips are another risk factor for early death — approximately 20% of people with a broken hip die within one year of the fracture.

In an ideal world, osteoporosis would not occur.  In theory, osteoporosis is entirely preventable.  Since we don’t live in an ideal world, it is crucial that osteoporosis be identified early and treated aggressively.

DEXA is the gold-standard in osteoporosis testing.  DEXA scanners (Dual-Energy X-ray Absorptiometry) (also called DXA) use two separate very low-radiation x-ray beams – about 1/10th the radiation of standard x-rays — to image the hips and spine to measure bone mineral density.  As we would expect from the “dual energy” portion of the name, these two x-ray beams have different energy levels.  Bone mineral density is calculated by measuring the difference between what is absorbed from the first beam and the second.

Test results will provide a variety of numbers.

  • Bone Mineral Density
  • T-Score
  • Z-Score

Bone Mineral Density (BMD) is a raw number indicating the average concentration of minerals in your bones. The higher the number, the higher the bone mineral density and the stronger the bones.  Lower numbers indicate weaker bones.

T-score and Z-score are based on statistics.  Compiling the results from many people has allowed scientists to determine what is normal bone mineral density, and what constitutes strong or weak bones.  Graphing the data forms a picture shaped somewhat like a bell.

A brief aside about statistics:  in statistics, the mean is the average — it tells us what is normal. The standard deviation tells us how far away something is from what is normal. 68% of all data will only deviate slightly from the average (will be within one standard deviation of the mean) — this makes sense because obviously most things should be close to what is normal. On a bell curve (pictured below), the mean does not deviate at all from what is normal, thus the center of the curve deviates zero (labeled 0), and most of the data clusters close to the middle — one standard deviation is labeled +1 (above zero) and -1 (below zero).  95% of the data will be within two standard deviations of the mean (labeled +2 and -2), and 99.7% of the data will be within three standard deviations of the mean. It is very rare for something to deviate significantly from what is normal.

T-score and Z-score numbers indicate standard deviations from the mean on a bell curve.  A T-score compares your BMD with healthy young adults who have good bone mineral density.  A Z-score compares your BMD with others of your age and ethnicity.  Doctors are most concerned with the T-score.

DEXATscores

A bone mineral density scan T-score that is more than one standard deviation below the mean is bad. Between one and 2.4 standard deviations below normal is osteopenia, while a T-score of 2.5 or more standard deviations below the mean is osteoporosis.

Who should get a bone density scan? The general rule is women at age 65 or men at age 70.  Before age 65, the test is only considered if you have risk factors, and if treatment would occur based on test results.  If you wouldn’t be treated, there’s no point in having the test done.  Bone density scans are rarely done on premenopausal women; until menopause, high estrogen levels seem to provide protection against broken bones even in people with low bone mineral density.  The question is, what are the risk factors?

Rheumatoid arthritis is just one of many risk factors for development of osteoporosis.  Other risk factors include:

  • history of taking 5mg or more of corticosteroids for more than three months
  • taking methotrexate (other meds, too)
  • family history of osteoporosis
  • history of an immediate family member with a fragility fracture
  • history of bone fracture as an adult
  • loss of height
  • weight of less than 127 pounds
  • being a smoker
  • menopause
  • eating a diet low in calcium
  • avoiding sunlight (indicative of low vitamin D production)

Given these risk factors, it is no surprise than rheumatologists refer patients for bone density scans.

Everything published about DEXA says that it is painless.  This information is obviously prepared by people who have never had the test.  Although it is technically true that the x-rays themselves do not inflict pain, before the scan is taken you’re strapped to a table in an uncomfortable position and required to stay tied down for the duration of the 15-20 minutes of the test. Although the average person might not be physically injured by the scan, it is inaccurate to say that the test is painless.  More accurately, the test is uncomfortable, but not unbearable.

After the test is complete, the referring doctor will receive a report showing your Bone Mineral Density, your T-score, and your Z-score.  Your report might also include use of the World Health Organization’s Fracture Risk Assessment Tool (FRAX) . This attempts to calculate a person’s probability of fracture within the next ten years with the goal of frightening patients into taking osteoporosis seriously.  A 28% risk of fracture within ten years is about 2.8% per year.  2.8 doesn’t sound nearly as scary as 28%, though, thus the use of ten-year risk estimates.  If you click on the link, select “calculation tool” and then select your continent/country and complete the questionnaire.  The calculation can be made either with or without results of a bone density scan.

Measuring Inflammation

Remember those old childhood cartoons where a guy hits his thumb with a hammer?  The thumb turns bright red, swells to three times its normal size, and hurts so much that painful rays emanate for all to see.

I remember the day I awoke in excruciating pain, my entire hand red and throbbing as if it was a model for one of those old hammer-to-thumb cartoonists.  In the cartoons, it’s funny – not so much in real life.

There have also been times my hands swelled to a lesser extent – not enough for anyone else to notice, but enough that it cut off the circulation in my ring finger.  When fingers swell so much that a perfectly sized wedding ring cuts off circulation and the finger turns blue, it’s a safe bet that there’s a fair amount of inflammation going on.

One would think that a doctor could write “significant swelling” as documentation of such events, but doctors seem to have a preference for numbers to quantify everything, including inflammation.  ESR (erythrocyte sedimentation rate) and CRP (C-Reactive Protein) are two lab tests that can be done to “measure” inflammation.  Unfortunately, those tests aren’t perfect.  People can have significant inflammation but perfectly “normal” lab results.  How ironic that someone’s feet can be too swollen to fit into shoes, hands can be too swollen to do anything, and a doctor can say, “I’m sorry, but according to your lab work, there is no inflammation.”  Many RA patients have “normal” inflammatory markers – accompanied by enough disease activity that joints are destroyed.

I’ve wondered why, if pharmaceutical companies can develop drugs to inhibit TNF-α, nobody can come up with a test to measure TNF-α in the bloodstream.  Same with interleukin-6 and the other proteins that are thought to be associated with disease activity in RA.  If numbers are so important, there should be a way to measure those things.

Scientists have been working on it, and have a new test.  This test doesn’t diagnose RA, but gives the doctor an objective number to indicate how active the RA monster is.  Announced late last year, the Vectra Disease Activity Test measures twelve proteins, applies a convoluted formula (take the logarithm of 1+CRP, multiply by .36, add the square root of this, multiply by that…) and produces a single number to quantify disease activity.

Yay!  With luck, this will be a better test.  Questions remain, though.

Given the number of RA patients with normal CRP, I don’t understand why that information is used in the new test’s calculations.  The testing company’s website compares the results to DAS28. I’d rather see some comparisons that show Vectra DA to be superior to existing tests.  What I’d really like to see is data from hundreds of RA patients with obvious disease activity but normal ESR & CRP, and see that this new test returns a high number, regardless of the normal ESR/CRP.  Show us the data.