Fluid management should be an integral part of our sport, yet most pilots only worry about either how much water they can pack inside their wings for ballast or better yet, what is the cleverest way to dispose of their urine output. But as an anesthesiologist, I have always found that I am worried about fluid INPUT (what and how much I drink) while in flight. So the Camelbak Flow Meter(R) caught my eye yesterday at REI since it offered a solution on how to properly maintain my brain and other organs well hydrated while I am going down to the Whites.

In fact, for the purists, this device will allow you to measure both fluid INTAKE and OUTPUT, thus allowing you to be in optimal physiologic homeostasis at any time during your flight - which is by the way what an anesthesiologist does for you while you are having any major medical procedure.

Fluid Management Basics

In anesthesia we like to think of fluids as follows: 1) The fluids you OWE before we begin your surgery, 2) your MAINTENANCE fluids, and 3) your LOSSES (sensible and insensible such as blood loss, urine output, humidity in the air you breathe out, sweat, etc.) Your anesthesiologist has many ways of deciding where you are fluid-wise during a procedure, including many monitors, but here we will review the basic calculations used for a simple surgical procedure.

Let's start with your MAINTENANCE fluids first, which will be our BASIC fluid requirements in flight. These requirements are what your body needs to keep running on idle, which is not really where we are at when flying close to 18,000 feet on our way to the Whites! For this we use what we call the 4:2:1 rule, which most residents learn on their first week of training. The rule allows for a simple approximation of fluid requirements to keep our bodies running on idle (sitting in your sailplane at 10am, playing with your new version of Winpilot). The 4:2:1 rule says that for the first 10 kgs of your weight, you multiply times 4. The second 10 kgs you multiply times 2. And for anything above 20 kgs, you multiply times 1 for your HOURLY requirements. So if you weigh 70 kgs (154 lbs) your HOURLY MAINTENANCE fluid requirements will be 40 + 20 + 50 = 110 mls per hour. Do this math for yourself, and place this number somewhere on your panel - we'll see why later!

The first thing that becomes obvious here is that if you are out to break any world record and do a 10 hour flight, you will have to take more than 1 liter of fluids (1.1 to be exact) just for your MAINTENANCE requirements.

Let's start adding to these your insensible LOSSES.  I suspect these are pretty major in a sailplane, since we tend to lose so much fluids from sweat while inside the cockpit on a hot summer day. Nobody has measured these, but as an anesthesiologist, my gut feeling is that you are losing some 3-5 mls/kg/hour.

Our fluid requirements have just gone up dramatically for our cross country flight. Let's use the lower 3mls/kg/hour for our example. (3 x 70) = 210 mls/hour. We now need to add our MAINTENANCE requirements, so we are up to 210 + 110 = 320 mls per hour, or just over 3 liters of fluids (3.2) for a 10 hour flight! Don't believe me? Weigh yourself BEFORE takeoff, and as soon as you land! I've lost up to 2 kgs (2 liters) on a given flight!

Did we forget anything? If for some reason you did not have a good breakfast, you have generated a FLUID DEFICIT. What does a good breakfast mean from the fluid viewpoint? We usually use midnight as our starting point, so if you take off at 10am, you should have 10 x 110 (10 hours x MAINTENANCE fluids) or 1.1 liters of fluids before takeoff. So if you did not drink a liter before takeoff, you are going to have to add this amount too.

Camelbak Flow Meter(s)

With these really basic concepts understood, it is now easy to see how this device will become the next purchase for your gliding season. The Camelbak Flow Meter allows you to 1) calculate the fluids you have ingested during a given flight or over a given time period, 2) let you know how much you have left in your pouch, and  3) set your personal hydration goal.

Make sure you use kgs and liters when you do your setup for the device. Medicine is a metric world.

If you are thinking like an intensive care doctor - another branch of anesthesia, you can buy 2 flowmeters, one for fluid INTAKE, and a second one for urine OUTPUT. The latter you can connect to your pee-tubing contraption so as to measure your output. How much should you be urinating in flight if you are properly hydrated? An easy number to look for is 1 ml/kg/hr. So if you weigh 70 kgs, think of having a urine output of 70 mls per hour. 

Fluid Management in Flight

If you want to get serious about breaking records, optimal fluid management is important.  Why so - you may ask? You have probably been flying fine for years - or so you think. But fluid management is really all about perfusion (how much blood your target organ receives) - and in this case, we are interested in perfusing our brain optimally so that we can make our best decisions during a long flight. Adequate perfusion, with our red blood cells carrying enough oxygen (do you fly with a pulse oxymeter to corroborate this?) will keep our brains functioning at their best during a prolonged flight.

So let's go flying... I check the weather, and Doug Armstrong is predicting a strong day with 1000K flights possible. I hydrate myself well for breakfast, and drink at least a liter of orange juice as I get ready to go out to the field. I calculate my fluid requirements for the flight with the above formulas, figuring that I may be in the air for some 10 hours (Fellow pilot Mitch Polinski does flights this long!) I add extra water to my requirements in case I have to land out and spend the night in the desert... (How much? You know the formulas for maintenance requirements). Before I get into my sailplane, I weigh myself (I brought my bathroom scale from home to the airfield.)

As my flight progresses, I monitor my fluid intake and output per hour. (If I weigh 70 kgs - I will be drinking some 320 mls per hour, and peeing some 70 mls/hour. When I land, after my 1000K, I weigh myself again. If I am off by more than 500 grams, I will need to adjust my fluid requirements for my next flight up or down (most likely, up!) 

What fluids to take?

What are the goals for fluid administration in flight? Pretty much the same as those you have during a surgical procedure: 1) Maintenance of proper O2 delivery and perfusion, 2) Maintenance of proper electrolyte balance, and 3) Maintenance of a normal blood glucose (sugar).

From the above, it appears that we also need to concern ourselves with WHAT fluids we take on-board. My gut feeling is that we need some 80% of our fluids as plain water, and 20% of a fluid such as gatorade which contains electrolytes and glucose. You could just drink 100% gatorade, with no adverse effects, but I am not sure that the extra expense will help.

Of interest, anesthesiologists have measured the electrolyte imbalances in marathon runners after a race, and the most common finding they have is hyponatremia (low sodium), probably from drinking too much water without electrolytes, along with excessive sodium losses during the race.

Epilogue

With the understanding that you now have of fluids, and the advent of an inexpensive monitor ($30) to manage your fluid intake in flight, you should be able to avoid complications that will impair your flight capabilities.

Dehydration and hypovolemia (low volume in your vessels) can have dire consequences for a pilot. If you are down a couple of liters in your flight, your heart's venous return will probably be decreased. Your heart will then try to increase its output, by increasing its rate. This means that if you fly around at 90 beats per minute (not an uncommon number at high altitude) your heart may increase its rate to for instance, 120 bpm. The higher your heart rate, the lower your coronaries have time to fill (they fill during diastole, which is the time between heart beats). What can this translate to? Coronary ischemia, (lack of oxygen delivered to your heart muscle), a subsequent arrhythmia, (ventricular fibrillation) followed by loss of consciousness in 5-10 seconds. 

I have no idea what percentage of the accidents that we have had at Minden where we attribute them to hypoxia or unknown causes were perhaps related to a cardiac arrhythmia that was generated by a combination of hypoxia and dehydration.

The bottom line is that in the operating room we take your fluids just as seriously as your oxygen, so the time has come for us to do the same while in flight - and now, we have the tools for it!

For more information on the Camelbak check out this device's instructions.

Fly safe! (and hydrated...)