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1

ABG Normals

pH- 7.35-7.45

PaCO2-35-45

HCO3-22-26

SaO2-95-100%

ABG allows for an objective criteria to evaluate, quantitate and classify respiratory failure.

2

PaCO2 in Hypoxemic Respirtory Failure

< 60 mmHg on room air

3

PaCO2 and pH in Hypercapnic Respirtory Failure

Respirtory Acidosis

PaCO2 > 45 mmHg

ph < 7.35

Hypercapnic can cause an acute or acute on chronic

4

Hypoxemia Respirtory Failure

Results in a normal A-agradient when it is due to a decreased PiO2 or hypoventilation. There can be an increased in A-a gradient when the hypoxemia is due to a true shunt, V/Q mismatch, or a diffusion defects

PaCO2 may be low because you might be breathing faster

5

Hypercapnic Respirtory Failure PaO2, PaCO2, P(A-a)O2

PaO2-Low

PaCO2-High

P(A-a)O2-Normal 

6

Combined Hypoxemic and Hypercapnic Respirtory Failure PaO2, PaCO2, P(A-a)O2

PaO2-Low 

PaCO2-High

 P(A-a)O2-High

It is common for hypercapnic respiratory failure to be combined with hypoxemic due to the mechanism of the disease 

7

Hypoxemic Respirtory Failure PaO2, PaCO2, P(A-a)O2

PaO2-Low

 PaCO2-Normal to Low

P(A-a)O2-High or Normal

8

Name the Classic Indications for Mechanical Ventilation

Apnea

Acute Ventilatory Failure

Impending Ventilatory Failure

Severe Refractory Hypoxemia- Look at oxygenation critical numbers

One sign may be you are giving tons of oxygenation but saturation levels remain low

9

Indication for Mechanicl Ventilation-Apnea

Apnea-The patient needs something to breath for them

Arrest, sedation, OD, drugs, C-spine injury, head trauma

10

Indication for Mechanical Ventilation-Acute Ventilatory Failure

Acute Ventilatory Failure- Determined through look at the ventilation critical numbers

Documented hypercapnia

11

Indication for Mechanical Ventilation-Impending Ventilatory Failure

Impending Ventilatory Failure-Look at patients work of breathing, muscle strength, and lung expansion critical numbers

Air Hunger, tachypnea, diaphoretic, neuromuscular (Guillain-Barre, MS)

12

Indication for Mechanical Ventilation-Impending Ventilatory Failure

Impending Ventilatory Failure-Look at patients work of breathing, muscle strength, and lung expansion critical numbers

Air Hunger, tachypnea, diaphoretic, neuromuscular (Guillain-Barre, MS)

13

Inidcation for Mechanical Ventilation-Severe Refractory Hypoxemia

Severe Refractory Hypoxemia- Look at oxygenation critical numbers

One sign may be you are giving tons of oxygenation but saturation levels remain low

14

Critical Numbers and The Whole Body

THE CLINICAL STATUS OF THE PATIENT IS ALWAYS THE MOST IMPORTANT FACTOR!!!!!!

Remember that the patient’s history will be a good indicator on whether or not they need to be mechanically ventilated

They may commonly have high or low critical numbers

Just because a patient has a normal ABG does not mean we can rule out the need for mechanical ventilation, as there are ALWAYS other parameters that need to be looked at 

15

What Critical Numbers Do You Need (Broad Categories)

Inadequate Alveolar Ventilation-Not moving enough air in and out of the lungs

Inadequate Lung Expansion-Collapse of lungs because they are unable to pull them open enough

Inadequate Muscle Strength-Not strong enough to take a breath

Increased Work of Breathing

Hypoxemia

16

Inadequate Alveolar Ventilation

Looks at PaCO2 and pH

Both the critical numbers for a high PaCO2 and a low pH in order to determine that mechanical ventilation is needed

17

PaCO2

Measure of Inadequate Alveolar Ventilation

Normal is 35-45 mmHg

 

18

PaCO2 Critical Number

>55 mmHg

19

Mechanism for increases in PaCO2

1) Increased Deadspace-Increased deadspace will cause an increased PaCO2 when minute ventilation (VE) cannot be increased enough to compensate

2) Increased CO2 Production-Increased CO2 production will cause an increase in PacO2 when min ventilation cannot be increased enough to compensate

 

20

Alveolar Ventilation Equation (VA)

VA=RR x (Vt-VDphys)

VA=Alveolar Ventilation

RR= Respirtory Rate 

Vt=Tidal Volume 

VDphys=Physiological Deadspace

Decreased alveolar ventilation is an indication for mechanical ventilation

Decreased VA can be due to a decreased RR, decreased Vt (with a constant Vd) or an increased Vd (RR and Va remains constant)

21

pH Critical Number

Normal 7.35-7.45

Critical Number is <7.25

 

22

Inadequate Lung Expansion Critical Numbers

  1. Tidal Volume
  2. Respirtory Rate
  3. Vital Capacity

23

Tidal Volume

 

Normal is 5-8 ml/kg

Critical Number is <5 mL/kg

Small tidal volumes result in a inadequate lung expansion which will contribute to atelectasis and impaired gas exchange

Usually results in an increased RR to maintain VE

Is a measure of lung expansion

24

Respirtory Rate

Normal is 12-20 bpm

Critical Numbers is >35 bpm

High RR tends to correspond to lower volumes and can lead to respiratory muscle fatigue 

Is a measure of lung expansion

25

Vital Capacity

Vital capacity is the voluntary amount of air you can maximally breath in one breathe 

The volume of air exhaled after a maximal inspiration

Normal is 65-75 mL/kg

Critical Number is < 10 mL/kg

Indicates that both muscle strength and lung expansion ability and thus the ability to cough and clear the air way

A VC of 2 x Vt is needed for an adequate cough (needed to protect your airway)

26

Measuring Vital Capacity

Typically done with a Wright’s (turbine) or a bedside spirometer

A Wright’s Spirometer is a cumbersome measurement that we only use on a special type of patient (we don’t need to know the part of a Wright’s Spirometer)

Commonly measured in patient’s with progressive muscle weakness (MG, GBS, ALS)

27

Inadequate Muscle Strength

  1. Maximum Inspiratory Pressure (MIP)
  2. Maximum Expirtory Pressure (MEP)
  3. Vital Capacity
  4. Maximum Voluntary Ventilation (MVV)

28

Maximum Inspirtory Pressure (MIP)

Normal is -80 and -100 cmH2O

Critical Number: Greater than or equal to -20 cmH2O (i.e. 0 to -20 cmH2O)- Remember that because it is a negative number less than means less negative

A measure of the patient muscle strength

Typically a consistent and accurate measurement

 

29

Maximum Inspiratory Pressure (MIP) and Maximum Expiratory Pressure (MEP) Measurements

Uses nose plugs, a pressure gauge and a one way valve

For MIP measurement the one way value will only allow for exhalation and for a MEP measurement the one way value will only allow the release of inspiration 

MIP measurements are commonly done for progressive neuromuscular disorders (MG, GBS, ALST), and are typically a consistent and accurate measurement. 

MEP are typically measured in PFT

30

Maximum Expiratory Pressure (MEP)

Normal is >100 cmH2O

Critical Number: <40 cm H2O

 

31

Maximum Voluntary Ventilation (MVV)

The maximum volume of air that a subject can breathe during a 12-15 second period

Units are liters per minute

Normal is 120-180 lpm

Critical Number < 2 x VE

Measuring

Rarely done (because it is hard on the patient) and only in PFTs

 

32

Increased WOB

1) Minute Ventilation

 

Deadspace to Tidal Volume Ratio

For a given VT, as VD Ÿs the VD/VT Ÿs!

And, for a given VD, when VT Ds, the VD/VT Ds!

Deadspace-does not take part in gas exchange so if your ratio is greater than 60% you have too much deadspace and not enough working areas

Normal is 0.25-0.40 (25-40%)

Critical Number

> 0.60 (60%)

It takes work to move the air in and out of the deadspace

As deadspace increase the work of breathing to maintain alveolar ventilation is increased

For any given PaCO2 as deadspace increases then minute ventilation must increase as well, in order to maintain PaCO2

PaCO2 µ ṾCO2/ ṾA

where ṾA = RR x (VT-VDphys)

33

Minute Ventilatin (VE)

Normal is 5-6 LPM

Critical Numbers >10 LPM

At 10 LPM there is increased probability of respiratory failure and developing second degree muscle fatigue

The minute ventilation needed to maintained stable PaCO2 may become so high that it cannot be attainted by the patient

34

Deadspace to Tidal Volume Ratio

For a given tidal volume, as deadspace increases so will the tidal volume to deadspace ratio. And for a given deadspace as tidal volume increase so will the tidal volume to deadspace ratio.

Deadspace-does not take part in gas exchange so if your ratio is greater than 60% you have too much deadspace and not enough working areas

Normal is 0.25-0.40 (25-40%)

Critical Number > 0.60 (60%)

As deadspace increase the work of breathing to maintain alveolar ventilation is increased. For any given PaCO2 as deadspace increases then minute ventilation must increase as well, in order to maintain PaCO2

 

35

PaCO2 and ṾCO2/ ṾA

 

PaCO2 is indirctely porportional to ṾCO2/ ṾA

where ṾA = RR x (VT-VDphys)

36

Hypoxemia

1) P(A-a)O2 on 100% Oxygen

2) PaO2/FiO2

3) PaO2/PAO2

 

37

P(A-a)O2 on 100% Oxygen

 

= alveolar-arterial gradient

Normal on room air:  2 - 30 mmHg

Normal on 100% O2:  25 - 65 mmHg

Critical Number:  > 350 mmHg

When the P(A-a)O2 is increased it is due to shunt, diffusion defect, or V/Q mismatch (ie. Hypoventilation and low inspired FiO2 have a normal P(A-a)O2.

A-a rule of thumb every 10 years another 4 mmHg difference

Anything that increases mean airway pressure will increase oxygenation and PEEP makes the biggest difference

 

38

PaO2/FiO2

= PF ratio

Normal:  350 - 450

Critical Number:  < 200

 

39

PaO2/PAO2

 

= arterial to alveolar ratio

Shows the percent of oxygen that is in the alveolus that gets thru to the arterial blood

Normal:  0.75 - 0.95

Critical Number:  < 0.15

40

Physiological Goals of Ventilatory Support

To Support or Manipulate Gas Exchange- Ventilation and Oxygenation

To Increase Lung Volume-End Inspiration, End Expiration, and FRC (increased through recruitment)

To Reduce or Manipulate the WOB

Minimize Cardiovascular Impairment-Mechanical ventilation can reduce myocardial demand secondary to hypoxemia and increased WOB

41

Specific Clinical Objectives of Ventilatory Support

To reverse hypoxemia

To reverse acute respiratory acidosis

To prevent or reverse atelectasis

To relieve respiratory distress

To reverse ventilatory muscle fatigue

To decrease systemic or myocardial oxygen consumption

To maintain or improve cardiac output

To reduce ICP

To stabilize the chest

Air will follow the path of least resistance and one lung might be easier to fill than the other and this can cause damage

To permit sedation +/- paralysis.

42

Ideal Body Weight Calculation

Males: Wt Kg = 50 + 2.3 (height inches – 60)

Females: Wt Kg = 45.5 + 2.3 (height inches – 60)

 

43

Mild to Moderate Hypoxia-Respirtory Findings

Tachypnea

Dyspnea

Paleness

44

Severe Hypoxia-Respirtory Findings

Slowed irregular breathing

Respirtory Arrest

Dyspnea

Cyanosis

45

Mild to Moderate Hypoxia-CVS Findings

Tachycardia

Mild hypertension

Peripheral Vascoconstriction

46

Severe Hypoxia-CVS Findings

Tachycardia

Eventual Bradycardia

Arrhythmias

Hypertension

Eventual Hypotension followed by cardiac arrest

47

Mild to Moderate Hypoxia-Neurological Findings

Restlessness Disorientation

Headache

Lassitude (lack of energy)

48

Severe Hypoxia-Neurological Findings

Somnolence Confusion

Blurred Vision

Tunnel Vision

Loss of cooridnation

Impaired judgment

Slow reaction time

Coma

49

Mild to Moderate Hypercapnia-Respiratory Findings

Tachypnea

Dyspnea

50

Severe Hypercapnia-Respiratory Findings

Tachypnea and Eventual bradypnea

51

Mild to Moderate Hypercapnia-CVS Findings

Tachycardia

Hypertension

Vasodilation

52

Severe Hypercapnia-CVS Findings

Tachypnea

Hypertension

Eventual Bradycardia and hypotension

53

Mild to Moderate Hypercapnia-Neurological Findings

Headaches

Drowsiness

54

Severe Hypercapnia-Neurological Findings

Hallucinations

Convulsions

Coma

55

Mild to Moderate Hypercapnia-Other Findings

Sweating

Redness of Skin