Welcome to PICU Doc On Call, a podcast dedicated to current and aspiring intensivists. My name is Pradip Kamat
My name is Rahul Damania, a current 2nd year pediatric critical care fellow. We come to you from Emory University,School of Medicine, Children’s Healthcare of Atlanta, Atlanta, GA.
Today's episode is dedicated to O2 delivery in the PICU. We would like to highlight in this episode Stanford University School of Medicine Pediatric Critical Care's LearnPICU website. The LearnPICU.com website Is dedicated to reviewing clinical topics related to pediatric critical care, and is an open access resources which Is widely accessed worldwide. The website has over 10,000 visits each month, and is managed by Dr. Kevin Kuo - Clinical associate professor of pediatrics pediatric critical care at Stanford University. Dr. Kuo has Been featured on our prior episode entitled seven habits of highly effective Picu fellows, and we are very excited to collaborate with his educational resources to provide you the listener a comprehensive educational experience.
Rahul, let's go ahead and get into today's case.
A 17-year old boy is admitted after he was struck by a car at slow speed while crossing the street.
He is has SPO2 of 98%, HR 98 bpm with a normal capillary refill and perfusion.
His blood gas at admission to the PICU reveals a ph of 7.3/PCO2 35/PaO2 196 mm Hg on 50% NRB with 100% O2 flowing at 12LPM.
His admission hgb is 10.5 gm%.
4 hours post admission, the nurses noticed that the patient is tachycardic to 150s, with a drop in his BP, delayed capillary refill, with cool extremities and increased output from the chest tube.
His SpO2 has decreased to 86% and PaO2 on his blood gas is now 65mm HG. He is found to have a POC Hgb of 6.8 mg/dL.
Let’s take this case and highlight key components of O2 delivery and O2 consumption.
Lets focus on O2 delivery first. Rahul What are the components of O2 delivery ?
Pradip, O2 delivery is made of O2 content X Cardiac output
Simply put, O2 content is the amount of blood present in 100ml of arterial or venous blood. Its is denoted by CaO2 or CvO2 and its unit is mL O2 / dL blood or mL O2 per 100 mL of blood.
Before we introduce the complicated formula, let's just appreciate the variables within the equation.
Oxygen content is going to be a function of three variables:
This is going to be Hgb, Saturations on the hemoglobin also known as SaO2, and the amount of oxygen that is dissolved within the blood also known as your PaO2.
Pradip, Can you elucidate further about O2 content?
O2 content is given by the formula: CaO2 = (1.34X Hgb gm/dl X SaO2) + (0.003X PaO2)
Important points to remember about above formula is that the constant 1.34 (or 1.36 as given by some textbooks) is the amount of O2 in mL bound by one gm of Hgb and is called as the O2 carrying capacity of Hgb. In a healthy person say with 15gm% of Hgb, the O2 carrying capacity is about 15X1.34 = 20gm%.
Now many times amount of O2 bound to Hgb may not always reflect 100% saturation So we need to factor the % oxygen saturation into the oxygen carrying capacity of the Hgb.
The final element is to understand that some oxygen is dissolved in the plasma and is calculated using a constant 0.003 X PaO2. Typically 100ml of arterial blood with a saturation of 100 will contain 100 X 0.003 = 0.30ml of dissolved oxygen.
Rahul can you calculate the pre-decompensation oxygen content in the above case?
The above patients hgb pre-decompensation = 10.5gm%. His room air saturation 98% and his PaO2 is 196.
CaO2 = (1.34X10.5X0.98) + 0.003 X 196 = 13.7 + 0.58 = 14.2ml O2/dL blood.
Great - what is the post decompensation CaO2.?
The post decompensation CaO2 can be estimated using same formula as above: CaO2 = (1.34 X 6.8 X0.86) + (0.003 X 65) = 7.8 + 0.195 = 7.9 O2/dL blood.
Exactly So if you see the pre and post bleed O2 content just with a drop in Hgb from 10.5 to 7.5gm/dL: There is almost a 38% decrease in patients O2 content (8.83/14.2 = 62%,)
What is the best strategy to increase the patients O2 content?
First we can increase the patients FiO2 from 50% to 100% (immediate bedside action). We can get consent from family to order blood for transfusion.
Increasing FiO2 will result in an CaO2 = (1.34 X 6.8 X1 ) + (0.003 X 65) = 7.8 + 0.195 of about 9. O2/dL blood.
If we transfuse to a hgb of 10gm% with no increase in FiO2: we will get an CaO2 of (1.34X10X0.86) + (0.003 X 65) = 11.52 + 0.195 = 11.71 ml O2/dL.
The summary of this is to understand that modulating the patients hemoglobin via transfusion gives greatest bang for your buck in terms of optimizing O2 content
Exactly. Now, there is some value of increasing PaO2 in patients with acute severe hgb (say a Hgb of < 3gm/dL). Placing a child on 100% FIO2 NRB or placing child in hyperbaric chamber (diving & increasing PaO2) can increase CaO2 significantly. This is rarely used however may be indicated in patients who present with severe anemia with difficulty finding blood for transfusion due to antibody development etc.
Except for acute severe symptomatic anemia,Hgb should not be the sole criteria to transfuse to improve O2 content. In fact recent studies report that liberal policy of transfusion may be associated with increased mortality compared to a more restricted (transfused only if Hgb < 7gm/dL). So you want to assess the clinical picture fully and identify, intervene, and reassess.
Rahul, can you create a mental model related to O2 content in the blood for our listeners?
Absolutely, I would like to create 2 mental models:
As we reviewed, the variables in the oxygen content equation are:
Thus, your total oxygen content can be thought of as CBC, pulse ox, an ABG.
This is great - What’s another way to think about CaO2?
You can think about CAO2 by visualizing a car.
Many of us have heard that hemoglobin is the car which carries oxygen throughout our body. So the car and its frame represents hemoglobin. The wheels on the car represents the saturations. Four wheels on a car, for binding sites on hemoglobin. And finally thinking about a car needing to travel on a fluid road, helps you remember that PaO2 is the dissolved O2 in the plasma.
Pradip, we also talked about Venous oxygen content. How is that calculated?
CvO2 is similar to CaO2 and it is calculated using the formula: 1.34XHgbXSvO2 + 0.003 X PvO2
Typically, mixed venous O2 sat is used instead of SaO2 and PVO2 is used instead of PaO2. The blood gas is typically obtained from a central venous line with tip at SVC-RA...
