Notes from the Video

What are you going to learn in this video?

Let's start with two really important questions!

1. How do you determine the fluid needs of your patient?

    2.  How do you determine how much energy your patient is going to need every day?

These are the two things we're going to focus on as we continue this deep dive into surgical nutrition!

Fluid and Energy

Now we're going to move on to the guts of this and that's fluid and energy!

When I talk to the medical students and the residents I like to say "Well, what is parenteral nutrition?"

All it is is a bag of fluid and energy! 

You have an amount of volume of fluid that the patient needs for the day and then you have a bunch of energy that you have to give to the patient.

There are also vitamins, minerals, micronutrients and sometimes some medications like insulin or a proton pump inhibitor but at the beginning I want you to think of TPN as a big bag of Fluid and Energy.

Determining Fluid Requirements

Let's look at the fluid first!

How do you determine the fluid requirements for a patient?

There are a lot of different rules and because I'm a pediatric provider and I have a lot of people that are less than 20 kilos I use the 4-2-1 rule!

That means that you use 4ml/kg for the first ten kilos add that to 2 mils ml/kg for the next 10 kg and then 1ml/kg for each kg over 20kg in weight.  That gives you the fluid requirements per hour for your patient. 

Remember this is ideal body weight!  How do you calculate ideal body weight?

For men, ideal body weight is 50kg +2.3x(height in inches - 60)

For women, ideal body weight is 45.5kg +2.3(heigh in inches - 60)

Here is a link to ideal and adjusted body weight calculations!

Let's do a sample calculation!

How we determine fluid levels for a 70 kilogram male? 

It's easy...4ml/kg for the first 10 is 40ml/h, 2ml/kg for the next 10kg is 20ml/h and the remainder is 50kg so 50ml/h...add that all up and you are giving your patient 110ml/h (40+20+50) of fluid or 2640ml in 24 hours.

Determining Energy Requirements

When we think of energy we have to think that your typical healthy male's requirement is 25 to 30 kilocalories/kg/day

Now there's a spectrum from low to high of what your energy needs are and we'll go through those here:

15 to 20 kcal/kg/day - Severe malnutrition and they're at risk of refeeding syndrome

What is re-feeding syndrome?

It is when your body is in starvation mode and you give a high glycolytic load or a high glucose load you're gonna shift that patient from fatty acid oxidation and ketogenesis to now going right through the glycolytic cycle.

They're going to produce in so much ATP through that glycolytic cycle that they're going to use up all their phosphate, become hypophosphatemic with confusion, respiratory depression and even cardiac arrest.

Can you think of another patient population that also gets hypophosphatemia after surgery?

That is going to be your big liver resections and they can have a profound drop in their phosphorous so you'll need to supplement when necessary!

Let's look at some other calorie requirements:

20 to 25 kcal/kg/day - Morbidly obese patients, as the BMI increases these patients typically need less calories.  Based on their weight, as their BMI increases they are going to need less calories and their total energy expenditure, their resting energy expenditure is actually quite low.  These patients might require, at least in the beginning, a little bit less calories per kg than a person who's not obese and has a normal BMI.

25 to 30 kcal/kg/day - Healthy adult, so if you can think about it we have a 2,000 calorie a diet you take a 70 kilo male and multiplied by 30 and there you go 2100 kilocalories. 

30 to 35 kcal/kg/day - Critically ill patients in the burn unit who may have kcal/kg requirements approaching 50!  Imagine after injury your body is in super inflammatory mode and beginning to repair itself, it needs these calories as a new baseline for recovery.  As we're going to talk about the caloric need will change depending on their state of recovery.

How do you know where you're at?  What is the caloric requirement of a particular patient after injury?

These parameters above will give you a really good idea of where to start but let's say you're a week into it, maybe you're 10 days into it and you're asking yourself "oh man are we feeding this person correctly?"

I mean if we're giving enteral and parental nutrition are we giving enough kilocalories per day or are we giving too much?

So how do you know, what do you calculate?

Indirect calorimetry is the answer!

Indirect calorimetry calculates a respiratory quotient (RQ), and it is based on the volume of gas that you're inspiring and expiring and the partial pressure of oxygen and CO2 within that gas.

This can only be used in patients that have less than 60 percent FiO2 or else the number becomes quite skewed. 

The respiratory quotient is the amount of oxygen consumed over carbon dioxide being produced = O2 consumed / CO2 produced

So what does that mean and how does it help you?

It allows you to determine if you're meeting the caloric needs of your patient, if you're overfeeding or if you're underfeeding!

There are a few numbers that you've probably memorized over the years but I'm gonna make it simpler.

What if your RQ is over 1?

I want you to think that if your RQ is over 1 that means you're being overfed or you're over feeding your patient.

You've given too many kilocalories and an increased risk of being hyperglycemic as well as hepatic steatosis.

What if your RQ is less than 1?

If your RQ is less than the one that typically means that you're underfeeding!

All right now let's take a closer look at a respiratory quotient so we know that based on the number for the respiratory quotient we know what fuel we're oxidizing.  I use oxidizing instead of the word burning because oxygen is what's important in the respiratory quotient I'm going to show you right here, for the biochemist and the chemists in the audience, you're gonna love this!

All right, I'm going to show you two things here, one is going to be the oxidation of glucose and second is going to be the oxidation of a fat!

Oxidizing Glucose

When you oxidize glucose you look at the reaction here, so glucose is C6H12O6 and that requires six oxygens to produce six carbon dioxides and six waters.

Now if you look at it this is a balanced equation, you get six oxygens to oxidize glucose to get six carbon dioxides and water so you have six over six.  So that quotient is equal to 1 and that is why when you're oxidizing carbohydrate your RQ = 1.

Oxidizing Fat

Let's look at palmitate, now palmitate is C16H3202 and you need 23 oxygens to oxidize palmitate and that produces 16 carbon dioxides and 16 waters.  So 16 over 23 is 0.696 which is approximately 0.7 and that's why you get an RQ at 0.7 when you're oxidizing fat!

Now when we oxidize protein it gets a little more complicated when looking at carbons, oxygen and hydrogens but that number falls right around 0.8 for an RQ.

As the RQ gets closer to 0.7 you're more and more underfed!