Ventilators and ABG’s

PHILIP L . KALARICKAL, M.D. , M.P.H.

Ventilators and ABG’s

Objectives

1. Review and understand Intubation criteria2. Understand basic ventilator settings and

mechanisms3. Understand how to monitor a patient on a

ventilator4. A systematic review of ABG analysis5. Understand the physiology of ventilation

and oxygenation.

Patient in ER

55 y.o. MaleCC: SOB and coughVitals: 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.

3.

4.

5.

Intubation Criteria

1. Hypoxemia: PaO2 < 60mmHg on >.6 FiO2

2.Hypercarbia: PaCO2 > 60mmHg

3. RR > 30

4. GCS <8

5. Hemodynamic instability - Pressors

You intubate the patient successfully. The respiratory tech turns to you and says, “What vent

settings do you want doc?”

What settings need to be programmed into the ventilator?

1.2.3.4.5.

You intubate the patient successfully. The respiratory tech turns to you and says, “What vent

settings do you want?”

What settings need to be programmed into the ventilator?

1. Mode2. Tidal volume3. RR4. 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

1.2.3.4.5.6.

ABG’s

1. pH: 7.35 – 7.452. PaCO2: 35-45 mmHg3. PaO2: 80-100 mmHg4. HCO3

-: 22-28 mEq/dL5. BE: -2 - +26. Sat: 97 – 100%

You get your first gas….

7.30/55/200/24/0/100What 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

plasmaIt contributes very little to oxygen deliveryCaO2 = 1.34 x Hb x sat + .003(PaO2)Helps assess how well oxygen exchange occurs at

the alveolusYou need to compare it to PAO2.

PAO2

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.

PAO2

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 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

PEEP (cm H2O )

Sat PaO2 BP

In this example, 9cm H2O is the “Best” PEEP

5 100 200 135/90

7 100 250 130/80

9 100 350 115/70

11 100 450 85/55

•“Best” PEEP – the level of PEEP at which you improve oxygenation the most without significant effects on venous return

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)

1. Low FiO2 allows you to know your PaO2 within a narrow range without drawing an ABG.

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

A patient on 30% O2 will desat sooner than a patient on 100% O2

In other words, 100% O2 will “mask” a problem3. 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

1. Intubation Criteria2. Basic ventilator settings3. Assessing patients on vents

1. Pulse oximeter2. ABG’s

4. Analyzing ABG’s1. Acid/Base2. Ventilation3. Oxygenation