Perioperative Care: Fluids and Electrolytes

00:01 Welcome back.

00:02 Thanks for joining me on this discussion of fluids and electrolytes in the section of perioperative care.

00:09 Fluids and electrolytes are incredibly important to maintain daily physiologic function, but also it’s incredibly high yield for standardized examinations.

00:19 So, I hope you're paying close attention.

00:22 Let's start the discussion by an estimation of total body water.

00:27 Total body water when expressed in liters is approximately two thirds or 0.6 times your total body weight in kilograms.

00:36 For example, a standard 70 kilo patient would have approximately 42 liters of fluid.

00:42 That's a lot of fluid.

00:44 Now, the distribution of the fluid is equally important.

00:48 The vast majority of that 42 liters of fluid is actually intracellular to the tune of two-thirds.

00:56 One-third of your total body water is actually extracellular and only a very small percentage one-third of one-third is contained in your intravascular or circulating volume.

01:08 When you do all the math, you figure that 7% of your total body weight is actually intravascular.

01:14 That's how we get the value of approximately 5 liters of fluid in your total circulating volume.

01:23 Our body does an amazing job of maintaining intake and output homeostasis.

01:29 On a daily basis, we approximately ingest 500 in – or potentially excess depending on how much water you drink.

01:37 In our food, we probably take in about 800 cc of fluid.

01:41 And normal cellular processes contribute about 300 cc.

01:46 In our output, we make about 500 cc of urine a day.

01:50 Our skin and insensible losses account for approximately 500 cc.

01:55 Breathing alone, because of the humidity, loses about 400 cc.

01:59 And our stool, although mostly without fluid, still contains about 200 cc.

02:05 And that's an important point, because patients with diarrhea can lose fair amount of fluid through their stools.

02:14 Electrolytes are incredibly important and will be the focus of the majority of this lecture module.

02:20 Remember, potassium is mostly intracellular.

02:24 If you forget that, all you have to do is look at standard chemistry laboratory values.

02:29 The potassium is rarely ever greater than 4.

02:33 Aldosterone is a very important – has a very important mechanism in the distal collecting system to maintain not only fluid homeostasis, but affect the potassium channels.

02:45 Aldosterone increases sodium reabsorption at the expense of excreting potassium.

02:53 This allows it to maintain electrical neutrality because otherwise we would have entropy.

02:59 Potassium-sparing diuretics, however, antagonize the action of aldosterone on potassium.