1. Why ⇌ How
Exploring (some of) the equations,
physiology and mechanisms which
inform and underlie our practice.
Adam Bell, BS, CCEMT-P, CPTC,
Donor Network of Arizona, AZOB
NATCO Annual Meeting
Aug 8, 2015

2. “What senses do
we lack that we
cannot see or
hear another
world all around
us?”
-Frank Herbert, DUNE

3. Objectives
 Provide an example of an order for a bicarbonate drip
which will also lower sodium in the setting of
hypernatremia and an indication for the same and define
a % solution
 Calculate a mean airway pressure given PIP, PEEP and I:E
ratio
 Demonstrate how to predict CO2 with vent rate change
assuming apneic pt on vent with constant compliance and
volume
 Verbalize the two most significant factors in delivery of
O2 to tissues and two interventions to support each
factor
Which sounds hard, right?

4. “…A lot of what we do in the (ICU)…at
least the biology and the chemistry, is
really strongly founded on what I learned
in high school science…A lot of the science
really isn’t that highbrow…”
-Dr. Jeffery Guy,
ICU Rounds Podcast, “Colloids: Details and Myths” 2012

5. “All vague notions must fall before the
pupil can call himself a master”- Bruce Lee
• Why do we breathe?
• How many of us were told “CO2 Level?”
• Typical DKA ABG: 7.19/27/110/15/98%
• pH of CSF

6. DEFINITIONS & PRECISION
How does a tension pneumothorax kill you?
What does CVP Measure?
What are preload and afterload?
Where does vasoconstriction mostly occur?

7. Mean Airway Pressure (MAWP)
• What is the formula for Mean Arterial
Pressure and why does it make sense?
Hint: Time Averaged Pressure

8. Calculating MAWP
• Find I:E ratio, for each unit of I time add 1x
PIP, for each unit of E time: add 1x PEEP,
then divide by total units of time
• e.g. PIP 25 PEEP 5, I:E 4:1:
4 x 25 =100 ( for this 4 units of PIP)
+ 1 X 5 [ for single unit of PEEP] = 105 / 5
( the total units of PIP + units of PEEP) =
MAWP of 21 cm H2O

9. I:E Ratio 1:2 1:1 3:1 11:1 1:2
PIP 30 30 30 30 40
PEEP 5 5 5 0 15
Equation (30+5+5)/3 (30+5) /2 (30+30+
30+5) /4
(30x11)
+0/12
(40 +15+15)
/3
MAP 13.3 17.5 23.75 27.5 23.3

10. MV changes and PaCO2
Master Equations
• Minute Volume (MV) =
(tidal volume/ breath) X (breaths/ min)
• (MV X PaCO2) / desired PaCO2 = Desired
MV
&
• (MV X PaCO2) / New MV = Predicted
PaCO2

11. Predicting CO2 changes w/volume changes
• Vent volume assuming you keep same
rate:
• (Current PaCO2 x Current tidal volume)/ Desired
PaCO2 = Desired tidal volume to normalize CO2
• And so:
(Current PaCO2 x Current tidal volume)/
New Tidal Volume = new predicated
PaCO2

12. Rate Changes and PaCO2:
Vent Rate assuming constant tidal
volume:
(Current PaCO2 x Current Vent Rate)/
Desired PaCO2 = Desired Vent Rate
“Repetition of error is not experience”
-Dr. Jeffery Guy, ICU Rounds Podcast

13. Same Donor, top ABG is earlier,
No Bicarb given
BD Donors still compensate for hyperventilation

14. Anion Gap, AG (in one slide)
• The body must be electrically neutral. Charges must
balance.
• <12 (+/-2)
• Na+ - ( Cl- + HCO3-)
• The “gap” represents other normally unmeasured anions
such as phosphate, sulfates and proteins in serum plasma
• Low Albumin lowers Anion gap drop of 1gm Albumin yields
drop of 2.5 in gap calculation
• w/ Acidosis: Pos AG: treat cause, Neg AG: give bicarb

15. Bicarb Drips (back to chemistry class)
• mEq measure electrical charge
• mg measure mass
• % solution means:
X grams solute/100ml fluid where X=
the %
Mass Charge
23 mg Na + 1 mEq
39 mg K+ 1 mEq
36 mg Cl- 1 mEq
30 mg HCO3- 1 mEq

16. The solution to pollution is dilution?!
Serum Na
mEq/L
DONORS
3% NaCl
mEq/L
Na, Cl
0.9 %
NaCl
mEq/L
Na, Cl
(5% alb is
in NS)
3 “amps”
bicarb/L
Sterile
H2O
LR
mEq/L
Na:
0.45 % NS
& D5 ½ NS
mEq/L Na:
D5W
50 gm
Dextrose/L
mEq/L Na:
175 513,
(513 Cl)
154,
(154 Cl)
150
(0 Cl)
130
(110 Cl)
77
(77 Cl)
0
(0 Cl)
155 513 154 * 150
(0 Cl)
130 77 0
(0 Cl)
145 513 154 150
(0 Cl)
130 77 0
(0 Cl)
135 513 154 150
(0 Cl)
130 * 77 0
(0 Cl)

17. Bicarb Drips Cont.
• 0.45% NaCl has 450 mg NaCl/
100ml or 4,500 NaCl mg/L which =
• 77 mEq each of Na+ and Cl- /L
Thus 1771 mg Na+ electrically
balance 2772 mg Cl- for total mass of
4543 mg /L
Why do you care???

18. Bicarb Drips Cont.
• In hypernatremia we’d like to
lower Na+
• Thus if we want a bicarb gtts w/
same Na+ of 0.45% NaCl:
We can calculate how much bicarb
to use and not let our Na+ rise!

19. Replacement Formulae
• Bicarb Replacement:
• 0.15 x weight {in kg} x Base Deficit = mEq
NaHCO3 for replacement of 50% of Deficit
• K+ Replacement (assumes no continuing losses)
• Desired K - Current K x 20 = mEq to
replace
• Note: K-Phos has:
approx. 1.5 mEq K+/mmol K-Phos
(30 mmol k-Phos = 44- 45 mEq K+)

20. Delivered O2
Delivered O2 =
CO x {1.36 x Hbg x SaO2} + {0.0031 x PaO2}
BP 48/33 Good? Bad?
ECMO pt: Flow 4.5 L/min
If you double the diameter of a tube: flow
increases 16X, and speed & pressure drop.

21. Putting it all together
Consider a brain dead pt:
Cardiac/Fluid Status:
200 mcg/min Neo, 60
mcg/min Levo, Vasopressin
0.04u/min, Fluid -2L, 100/86,
CVP 8, CO 3, CI 1.2, SVV 38,
SVI 24
Respiratory/Pulmonary
Status:
Peep +15, , FiO2 1.0, SpO2 92%
Chemistry Status:
lactate 7, Alb 1.9, TP 3.8,
Hbg 8.6,

24. Sources Used
· Mosby’s Pocket Guide to Fluids and Electrolytes 4th Ed.
· Quick Reference to Critical Care 3rd Ed.
· Online Database of “Progress in Transplantation”
· The ICU Book 2nd Ed.
· ICU Rounds Podcast by Dr. J Guy
· Critical Care Medicine 3rd Ed.
· Egans- Fundamentals of Respiratory Care
· Oaks- Guides to: Hemodynamics and Ventilator Management
· Robbins- Pathologic Basis of Disease
· Pulmonary Physiology and Pathophysiology 2nd Ed.
· Jones and Bartlett – Critical care Transport
· Epocrates Free Version Apple app.
· Principles of Anatomy and Physiology 9th Ed.
· Sauders- Pharmacology for Nursing Care 4th Ed.
· Harrison’s Internal Medicine, e-book Ed.
· Lehninger- Principles of Biochemistry