LifeAfterDx--Diabetes Uncensored

A internet journal from one of the first T1 Diabetics to use continuous glucose monitoring. Copyright 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014

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Location: New Mexico, United States

Hi! I’m William “Lee” Dubois (called either Wil or Lee, depending what part of the internet you’re on). I’m a diabetes columnist and the author of four books about diabetes that have collectively won 16 national and international book awards. (Hey, if you can’t brag about yourself on your own blog, where can you??) I have the great good fortune to pen the edgy Dear Abby-style advice column every Saturday at Diabetes Mine; write the Diabetes Simplified column for dLife; and am one of the ShareCare diabetes experts. My work also appears in Diabetic Living and Diabetes Self-Management magazines. In addition to writing, I’ve spent the last half-dozen years running the diabetes education program for a rural non-profit clinic in the mountains of New Mexico. Don’t worry, I’ll get some rest after the cure. LifeAfterDx is my personal home base, where I get to say what and how I feel about diabetes and… you know… life, free from the red pens of editors (all of whom I adore, of course!).

Wednesday, April 01, 2009

Apples and Oranges

Let’s talk about alarms, which are sort of the whole point of wearing a CGM. Oh, sure, all that data will help you fine tune your control; if you are not overwhelmed with the sheer volume of information you are about to be deluged with.

But when it comes to alarms, there are some major differences between Navigator and Guardian. Both have threshold alarms, the most primitive of the alarms. Threshold alarms just let you know that the horse has left the barn and that the barn has burnt to the ground. When you go below, or above, a set point you get an alarm. For instance if your low threshold is set at 65 mg/dl you get an alarm when you cross that threshold.

The problems with threshold alarms are many-fold. Interstitial fluid, like alternate site testing, is late to the party, lagging fingertip capillary glucose by 15 or 20 minutes. Also, sampling delay means an alarm could be several minutes old; a life time when you are dropping quickly. Users of the early CGM systems had to set their low thresholds higher than they wanted them to ensure that the systems “caught” lows. The side effect was, for some folks, a near constant plague of false alarms. If your BGL was stable low you might get threshold alarms every 15 minutes without being in real danger; while if you set your threshold lower you risked missing an actual low.

Both Navigator and Guardian let us choose both low and high threshold alarms. Both systems also have the wonderful black magic projected (Abbott) or predicative (Med-T) alarms. These project or predict, in advance of the event, when you will be low or high. This allows you to set your threshold closer to target and have advance warning. Both systems use some sort of algorithm to study rate of change over time and predict a low or high in advance.

I can attest to the fact the Guardian’s predictive alarms work very well. I’m excited to see how the two systems compare to each other in the real world in real time.

But while both systems feature predictive alarms in general, this is the first area where we begin to see some real (and serious) differences between the systems. Navigator lets us choose a low setting and a high setting. Guardian lets us choose lows and highs as well, but also lets us choose different settings at different times of the day. Navigator only lets us choose one low number and one high number for all day and all night. Navigator zero. Guardian one. I use this feature to keep tighter control during the day with less nuisance alarms waking me up at night.

That’s it for glucose alarms on Navigator. Of course it will warn you of low batteries and the like. Guardian has yet another level of alarms for rate of change that are user programmable. You can independently choose both fall rate alarms and rise rate alarms anywhere between 1.1 mg/dl per minute and 5.0 mg/dl per minute. This gives you one more level of alertness to the changing sugar environment in your body. Navigator simply does not have it. Navigator zero. Guardian one.

It is also worth noting that the Navigator alarms can only be set generically. For instance, in the arena of the early warning system projected alarms, the Navigator only lets us choose between High, Medium, and Low Sensitivity. These translate into a 30 minute, 20 minute, or 10 minute advanced warning of a high or low blood sugar. Guardian lets us choose from 5, 10, 15, 20, 25, and 30 minutes. Whether or not this makes any real world difference is hard to say; but I always like having more options than less. Navigator seems dumbed down a bit in this area. Navigator zero. Guardian one. Both systems use your chosen threshold settings and your chosen time (Guardian) or generic sensitivity (Navigator) to run the predictive alarm features. The only real difference here is that Guardian will let you choose to vary the threshold numbers throughout the day and night if you choose to, and with Navigator you only get one set of numbers to work with.

Now in keeping with our Wild West shootout theme, we need to talk about arrows. Abbott put on the war paint and made a BIG deal about their arrow, the TRU (registered trade mark!) Directional Arrow. It is not an alarm, but part of the dash board.

As a quick side note, Abbott reps were handing out little metal discs with red rubbery plastic magnetic-headed arrows to promo this feature. They make for fun target practice between patients. I gotta say, though, that the aerodynamics are pretty poor.

So the whole point of the TRU is that the arrow is supposed to put your sensor glucose reading into perspective. Now you can know at a glance that you are 128 and rising. Or 128 and falling.

An arrow pointing to 3 o’clock means a “gradual” change in Glucose. The Navigator defines this as less than 60 mg/dl per HOUR. When the TRU is pointing 45 degrees up from the horizon you are rising at between 60 and 120 mg/dl per hour. Straight up is simply greater than 120 mg/dl per hour. Of course we have 45 degrees down and straight down too.

The Abbott reps made this sound like the greatest innovation since Saran Wrap. Well, compared to the old original garage-door Guardian, that would have been a step up. But modern Guardian leaves the TRU arrow choking stage coach dust. Guardian has arrows too. And much, much more.

Their registered trade mark is REAL-Time Trend Arrows. One arrow up means you are changing by 1-2 mg/dl per minute over the last twenty minutes, and two arrows up means you are soaring at more than 40 mg/dl over the last 20 minutes. The same is true for downward changes. Trying to compare the rate of change arrows between the two systems is like working on one of those perverted word problems from third-grade math about trains, and smoke, and wind, and….

But let me take a crack at it, and I’m fully prepared to be flamed in comments for poor mathematics. Guardian’s two arrow rate is defined as more than 40 mg/dl over last 20 minutes. Twenty goes into 60 three times, so there are three twenty-minute segments in an hour. Forty times three equals 120. That tells me that the two up arrows on Guardian mean the exact same thing as a straight up arrow on Navigator. I’m guessing that rates are rates regardless of whether they are expressed in X per hour or X per 20 minutes, but maybe not. By the same math one Guardian arrow means 60-120 mg/dl per hour. So a 45 degree Navigator arrow equals one Guardian arrow and a vertical Navigator arrow equals two Guardian arrows. Bear in mind I flunked third-grade math.

But the Guardian display also has much more.

Guardian displays your current “sensor glucose” on the right hand side of the screen; while on the left it shows a “trace” of the glucose over the last 3, 6, 12, or 24 hours. The device allows you to set the screen time-out to “never” so that the display always stays on (note: this is NOT true of the sensor-enabled Paradigm pumps).

A trace is like a heart monitor in the Intensive Care Unit, a seismograph, or a lie detector--a recording of the data points in a smooth line. It lets you see where you’ve been and where you are going. It is great for quick perspective.

So Navigator gives us the most recent number, plus the large TRU arrow that rotates like a compass rose to show us the direction and speed of that number in the liquid flow of our blood glucose over recent time. Guardian gives us the same number, an aerial picture of the river we are riding in, and four arrows: one up, two up; one down and two down. When we are flying straight and level we get no arrows at all with Guardian.

The arrows + graph is a lot more information; but not overwhelming. A two-second glace gives you a world of data about your world.

Navigator has been shot with a quiver full of Guardian arrows. Navigator zero. Guardian one.

Next time: book work.


Blogger Jonah said...

I didn't know that the arrows were about the twenty minute average. I find the arrows on my Guardian absolutely useless. They seem to show up just when my trend is about to go in the opposite direction. Since I leave the graph on all of the time, and since I know what insulin and food I've taken, the arrows are a very poor prediction compared to my own predictive abilities.

If I were to get some extra programmed options on the Guardian, here are the ones I'd want: let me scroll back in the screen, to see the details of the 3 hour graph for the entire past 24 hours. Let me have a rate alarm that goes off only if the rate is sustained (otherwise it goes off in the night when I roll onto the sensor, giving me one false reading 200 points higher than the rest).
Let me access the notes I put in, or at least put a blip on the screen for when I last took insulin, so that I can make some better blood sugar predictions.
And let the carelink software show my target zone as whatever I set it to rather than 70-140, and put my food and insulin onto the same carelink graph that shows my blood sugar.

As a last thumbs up for the guardian, I haven't seen any delay between the displayed numbers and my actual blood sugar changes, except when the numbers take their time showing up.

10:52 AM  
Anonymous Caro said...

I'm not a fan of Medtronic's rate of change arrows. This is partly because, as a Paradigm 722 user, I don't get alarms when they're there. As I don't spend all day staring at my CGM data, I usually don't even get to see them.

But even when I do, I'm not a fan. I don't like the way that they work purely from 20 minute pairs of data with no consideration for the intervening 3 readings. This can, and does, give rise to the situation where an up arrow when the current sensor glucose is lower than the previous one. (Is this the same on the Guardian? I guess it may be something they changed.)

I also used an original Guardian RT, and got so adept at drawing a mental graph from the numbers to see in my head what was happening, that I still prefer to do this now. I scroll back through the recent numbers to get the best sense of what is going on, because the graph is so small on the pump screen I find it fairly useless. (Each graph point spans more than half a mmol, so what looks steady can be far from it), Of course, I've not used a Navigator, so I'm very interested in your comparison.

1:19 PM  
Blogger Scott K. Johnson said...

Not sure if you plan to cover this or not, but what of the rate of sample (or data transmission) between the two systems and how that affects the projected and predicted alerts?

10:01 PM  

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