Ventilators and ABG*s

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Ventilators and ABG’s
PHILIP L. KALARICKAL, M.D., M.P.H.
Objectives
1.
2.
3.
4.
5.
Review and understand Intubation criteria
Understand basic ventilator settings and
mechanisms
Understand how to monitor a patient on a
ventilator
A systematic review of ABG analysis
Understand the physiology of ventilation and
oxygenation.
Patient in ER
 55 y.o. Male
 CC: SOB and cough
 Vitals: Ht: 5’10”, Wt: 70kg
 T: 38.7
RR: 35 HR: 105 BP: 140/95
 Pox: 90% on 100% NRBM
 What do you do?
Intubation Criteria
1.
2.
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Intubation Criteria
1. Hypoxemia: PaO2 < 60mmHg on >.6 FiO2
2.Hypercarbia: PaCO2 > 60mmHg
3. RR > 30
4. GCS <8
5. Hemodynamic instability - Pressors
says, “What vent settings do you want
doc?”
What settings need to be programmed into the
ventilator?
1.
2.
3.
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5.
The respiratory tech turns to you and
says, “What vent settings do you want?”
 What settings need to be programmed into the
ventilator?
1. Mode
2. Tidal volume
3. RR
4. FiO2
5. PEEP
Initial Vent Settings
Mode:
TV:
RR:
FiO2:
PEEP:
Initial Vent Settings
Mode:
2 basic modes – pressure control & volume control. TV depends on
lung compliance. Usually pick SIMV or PRVC
TV:
5-7 cc/kg IBW
RR:
start at 20. Can adjust based off of ABG
FiO2:
start at 1.0. Can adjust based off of ABG
PEEP:
start at 5cm H2O
Ok, the patient is on the vent. Now what?
How do we assess patients on vents?
1.
2.
ABG’s
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2.
3.
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ABG’s
1. pH: 7.35 – 7.45
2. PaCO2: 35-45 mmHg
3. PaO2: 80-100 mmHg
4. HCO3-: 22-28 mEq/dL
5.BE: -2 - +2
6. Sat: 97 – 100%
You get your first gas….
 7.30/55/200/24/0/100
 What do you think?
 What do you want to do?
1.
Acid/Base
2. Ventilation
3. Oxygentation
Acid/Base
1.
Look at pH to determine primary process
2. Then look at PaCO2 and HCO3- to determine
relative contribution of Respiratory and Metabolic
components to acid/base disturbance and the
degree of compenation.
Ventilation
 Measured by PaCO2
 ***Oxygentation and Ventilation are independent
processes***
 Ventilation is a function of MV


MV= TV x RR
Usually by RR rather than TV
 This patient has an elevated PaCO2. He is
hypoventilating (retaining CO2).
 We should increase RR
Ventilation
 You ask the Resp Tech to increase RR to 24 and
check an ABG in an hour
 The repeat gas is:
7.40/40/200/24/0/100
 Now what?
 Have we addressed oxygenation yet?
 What do you think about the PaO2 of 200mmHg?
Oxygenation
 How do we assess oxygenation?
1.
2.
 What is PaO2? What does PaO2 mean?
 It is the pressure of oxygen that is dissolved in plasma
 It contributes very little to oxygen delivery
 CaO2 = 1.34 x Hb x sat + .003(PaO2)
 Helps assess how well oxygen exchange occurs at the
alveolus
 You need to compare it to PAO2.
P AO 2
 PAO2 = [(Patm – PH2O)FiO2] –
(PaCO2/0.8)
 Because oxygen and carbon
dioxide are small molecules,
there should be almost
perfect gas exchange across
the alvelous to pulm capp.
P AO 2
 Example: on 100% oxygen,
 PAO2 = [(760-47)x 1] – (40/0.8)
= 710 – 50
= 650mmHg
 Rule of Thumb: PaO2 should be about 5x O2%

Ex: on 100% O2, PaO2 should be approx 500mmHg
 Now what do you think about the PaO2 of 200 mmHg on
our ABG?
 If we have an unexpected result, we should correlate it
clinically.
 Why would this patient have problems with oxygen
exchange across the alveolus?
 In other patients, different causes may be higher on
the differential


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CHF exacerbation and pulm edema
Pneumothorax
Mucous plug
ETT in main stem bronchus
Etc…
Ok, we know we have a problem with
oxygenation, now what?
 How do we improve oxygenation?
 2 ways:
 Increase FiO2
 Increase PEEP
 Are there any drawbacks to PEEP?
 Yes Barotrauma
 Inhibits venous return
Effect of Increased PEEP- example
•“Best” PEEP – the level of PEEP at which you
improve oxygenation the most without
significant effects on venous return
PEEP (cm H2O ) Sat
PaO2
BP
5
100
200
135/90
7
100
250
130/80
9
100
350
115/70
11
100
450
85/55
 In this example, 9cm H2O is the “Best” PEEP
Oxygenation
 Ok, we’ve improved oxygenation via PEEP and hope
to improve it further with anitbiotics for his
pneumonia.
 Now what?
 Are we happy with his FiO2?
 Are there problems with high FiO2?
1.
2.
Oxygenation
 You should wean FiO2 to minimum to maintain sats
> 95% (PaO2 > 80)
 Why?
Oxygenation
 Problems with high FiO2
 “oxygen toxicity” – due to oxygen free radicals that may cause
alveolar damage
 Risk factor – FiO2 > .6 for greater than 24 hours
 High FiO2 is not safe for patients
 Saturation is a relatively insensitive measure of oxygenation
(sats won’t drop until PaO2 is less than 80-100)
Low FiO2 allows you to know your PaO2 within a
narrow range without drawing an ABG.
1.

Ex:
Pt. on 100% O2 with sats 100%
 The lowest PaO2 can be is 80 mmHg
 The highest PaO2 can be is 500-600mmHg
 Pt on 30% O2 with sats 100%
 The lowest PaO2 can be is 80 mmHg
 The highest PaO2 can be is 150 mmHg

2. Low FiO2 will allow you to identify problems
earlier
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
A patient on 30% O2 will desat sooner than a patient on
100% O2
In other words, 100% O2 will “mask” a problem
3. If a patient is on a low FiO2 you can increase to
100% to buy time to make a diagnosis and treat.
Now you know almost everything you need to
know about Vents and ABG’s
Intubation Criteria
2. Basic ventilator settings
3. Assessing patients on vents
1.
1.
2.
Pulse oximeter
ABG’s
4. Analyzing ABG’s
1.
Acid/Base
2.
Ventilation
3.
Oxygenation

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