Indications for Ventilation Flashcards
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.
PaCO2 in Hypoxemic Respirtory Failure
< 60 mmHg on room air
PaCO2 and pH in Hypercapnic Respirtory Failure
Respirtory Acidosis
PaCO2 > 45 mmHg
ph < 7.35
Hypercapnic can cause an acute or acute on chronic
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
Hypercapnic Respirtory Failure PaO2, PaCO2, P(A-a)O2
PaO2-Low
PaCO2-High
P(A-a)O2-Normal
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
Hypoxemic Respirtory Failure PaO2, PaCO2, P(A-a)O2
PaO2-Low
PaCO2-Normal to Low
P(A-a)O2-High or Normal
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
Indication for Mechanicl Ventilation-Apnea
Apnea-The patient needs something to breath for them
Arrest, sedation, OD, drugs, C-spine injury, head trauma
Indication for Mechanical Ventilation-Acute Ventilatory Failure
Acute Ventilatory Failure- Determined through look at the ventilation critical numbers
Documented hypercapnia
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)
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)
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
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
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
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
PaCO2
Measure of Inadequate Alveolar Ventilation
Normal is 35-45 mmHg
PaCO2 Critical Number
>55 mmHg
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
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)
pH Critical Number
Normal 7.35-7.45
Critical Number is <7.25
Inadequate Lung Expansion Critical Numbers
- Tidal Volume
- Respirtory Rate
- Vital Capacity
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
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
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)
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)
Inadequate Muscle Strength
- Maximum Inspiratory Pressure (MIP)
- Maximum Expirtory Pressure (MEP)
- Vital Capacity
- Maximum Voluntary Ventilation (MVV)
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
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
Maximum Expiratory Pressure (MEP)
Normal is >100 cmH2O
Critical Number: <40 cm H2O
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
Increased WOB
1) Minute Ventilation
Deadspace to Tidal Volume Ratio
For a given VT, as VD s the VD/VT s!
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)
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
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
PaCO2 and ṾCO2/ ṾA
PaCO2 is indirctely porportional to ṾCO2/ ṾA
where ṾA = RR x (VT-VDphys)
Hypoxemia
1) P(A-a)O2 on 100% Oxygen
2) PaO2/FiO2
3) PaO2/PAO2
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
PaO2/FiO2
= PF ratio
Normal: 350 - 450
Critical Number: < 200
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
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
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.
Ideal Body Weight Calculation
Males: Wt Kg = 50 + 2.3 (height inches – 60)
Females: Wt Kg = 45.5 + 2.3 (height inches – 60)
Mild to Moderate Hypoxia-Respirtory Findings
Tachypnea
Dyspnea
Paleness
Severe Hypoxia-Respirtory Findings
Slowed irregular breathing
Respirtory Arrest
Dyspnea
Cyanosis
Mild to Moderate Hypoxia-CVS Findings
Tachycardia
Mild hypertension
Peripheral Vascoconstriction
Severe Hypoxia-CVS Findings
Tachycardia
Eventual Bradycardia
Arrhythmias
Hypertension
Eventual Hypotension followed by cardiac arrest
Mild to Moderate Hypoxia-Neurological Findings
Restlessness Disorientation
Headache
Lassitude (lack of energy)
Severe Hypoxia-Neurological Findings
Somnolence Confusion
Blurred Vision
Tunnel Vision
Loss of cooridnation
Impaired judgment
Slow reaction time
Coma
Mild to Moderate Hypercapnia-Respiratory Findings
Tachypnea
Dyspnea
Severe Hypercapnia-Respiratory Findings
Tachypnea and Eventual bradypnea
Mild to Moderate Hypercapnia-CVS Findings
Tachycardia
Hypertension
Vasodilation
Severe Hypercapnia-CVS Findings
Tachypnea
Hypertension
Eventual Bradycardia and hypotension
Mild to Moderate Hypercapnia-Neurological Findings
Headaches
Drowsiness
Severe Hypercapnia-Neurological Findings
Hallucinations
Convulsions
Coma
Mild to Moderate Hypercapnia-Other Findings
Sweating
Redness of Skin
