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Anesthesiology > Mechanical ventilation > Flashcards

Mechanical ventilation Flashcards

1
Q

How does anesthesia affect ventilation?

A
  • Alter sensitivity to CO2
  • Relax respiratory muscles (FRC decreases)
  • Atelectasis develops
  • Makes V/Q mismatch worse
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2
Q

How does ventilation affect anesthesia?

A
  • Utake of inhalational anesthetics depends on ventilation
  • Controlled ventilation facilitates reliable uptake and smooth plane of anesthesia
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3
Q

Ventilation–what is it? What is it defined by (and what is the normal value)? How is it monitored?

A
  • Process involved in the movement of air (gas) in and out of alveoli
  • Defined by PaCO2 (inversely proportional)
  • Normal PaCO2 ~ 35-45 mmHg
  • Monitored with arterial blood gas (PaCO2) or capnography
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4
Q

What is oxygenation? What is it defined by? What is considered hypoxemic (#’s)? How is oxygenation measured?

A
  • Process of oxygenation of arterial blood
  • Defined by PaO2
  • Hypoxemia
    • PaO2 < 60 mmHg
    • SaO2 < 90%
  • Monitored with arterial blood gas (PaO2) or pulse oximetry
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5
Q

How can oxygenation improve while breathing 100% oxygen? Is apneic oxygenation possible?

A
  • Cannot be improved by more ventilation
  • Can be improved by special respiratory maneuvers
  • Apneic oxygenation is possible (ventilation may not be needed for oxygenation)
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6
Q

What are the phases of respiration?

A
  • Inspiration
    • Inspiratory flow time
    • Insiratory pause
  • Expiration
    • Expiratory flow time
    • Expiratory pause
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7
Q

What do resistance and compliance limit?

A
  • Resistance limits flow
    • = change in pressure / flow
  • Compliance limits volume
    • = volume / flow
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8
Q

Indications for mechanical ventilation (MV)?

A
  • There is a need to decrease PaCO2
    • #1 indication under anesthesia
  • There is a need to increase PaO2
    • It’s easier to provide high FiO2 if patient is intubated and breathing 100% O2
    • If patient is already intubated and breathing 100% O2 increasing oxygenation will only be possible with special respiratory maneuvers and not with conventional ventilation
  • There is a need to decrease respiratory effort
    • Mostly happens in ICU as treatment for resp failure
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9
Q

What are the indications for MV during anesthesia (10)?

A
  • Conventional control of resp function
  • Prolonged anesthesia
  • Maintain more stable anesthesia plane
  • Neuromuscular blockade
  • Thoracic surgery
  • Chest wall or diaphragmatic trauma
  • Obesity, inc. abdominal pressure
  • Head down positioning (Trendellenburg)
  • Laparoscopy
  • Control of intracranial pressure
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10
Q

What are the indications for MV in the ICU (6)?

A
  • Depression of resp center in the brain
  • Inadequate thoracic expansion
  • Inadequate lung expansion
  • Obstructed airway
  • Resp arrest (or cardio pulmonary arrest)
  • Pulmonary edema, ARDS
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11
Q

What are some side effects of MV? What is the treatment?

A
  • Impairs venous return and CO
  • May cause hypotension, especially in hypovolemic patients
  • Pneumothorax, lung injury
  • Treatment
    • Volume loading
    • Decreasing airway pressures (change ventilator settings)
    • Switch off the ventilator
    • Inotropic drugs (i.e. dobutamine)
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12
Q

What are the side effects of hypercapnia (direct, indirect, narcosis)?

A
  • Direct effects of CO2
    • Peripheral vasodilation
    • Decreased myocardial contractility
    • Bradycardia, poss. cardiac arrest (very extreme case)
    • Increased intracranial pressure
  • Indirect effects via catecholamine release
    • Tachycardia, arrhythmias
    • Increased myocardial contractility
    • Increased BP
  • CO2 narcosis
    • > 95 mmHg progressive narcosis
    • > 245 mmHg complete narcosis
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13
Q

What is the risk of not ventilating properly?

A

If you don’t control ventilation during thoracic surgery and let the lung be collapsed for a prolonged time, not only will CO2 accumulate but the patient will quickly turn hypoxemic and you may encounter sudden death of the patient

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14
Q

Should I ventilate during anesthesia?

A
  • Debated issue, esp. in horses
  • Point is how to balance between either comprimising cardiovascular or resp function (and oxygenation)
  • Permissive hypercapnia may be acceptable up to 60-70 mmHg
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15
Q

What are the 3 types of ventilation?

A
  • Spontaneous
    • Patient determines when and how
  • Assisted
    • Patient determines when and ventilator determines how
  • Mandatory (or controlled)
    • Ventilator determines when and how
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16
Q

What are the 2 different ventilation modes?

A
  • Volume controlled
    • Device sets the volume, pressure is a dependent variable
    • If compliance decreases (pneumothorax), pressure would increase
    • Difficult to control the tidal volume in very small patients
  • Pressure controlled
    • Device sets the pressure, volume is a dependent variable
    • If resistance increases (airway obstruction), volume would decrease
    • Works well regardless of body size
17
Q

What are the clinical recommendations for using pressure or volume controlled ventilation?

A
  • If lung volume changes during procedure (e.g. thoracotomy), use pressure controlled ventilation
  • If trans-pulmonary pressure changes (e.g. laparoscopy), use volume controlled ventilation
18
Q

What are the 5 classifications of ventilators?

A
  1. Source of driving power
  2. Control variable
  3. Cycle variable
  4. Trigger variable
  5. Limit variable
19
Q

What are the 2 sources of driving power for ventilators?

A
  • Electrically driven (i.e. using a linear motor)
  • Pneumatically driven: using pressurized gas source (more common)
20
Q

Control variable ventilators

A
  • Flow–ventilator delivers constant flow to the patient
  • Pressure–ventilator delivers constant pressure to the patient
  • Analogy with electricity: Flow is current, pressure is voltage. A battery may supply either constant current or constant voltage
21
Q

Cycle variable ventilators

A
  • Triggers expiration when set value is reached
  • Volume: volume controlled ventilation
  • Pressure: pressure controlled
  • Time: both
  • Flow: diminishing flow triggers expiration
    • Useful for pressure support ventilation (PSV) because it helps accommodating to the patients breathing pattern
22
Q

Trigger variable ventilation

A
  • Triggers inspiration when set value is reached
  • Used during assisted ventilation modes
  • Pressure: negative pressure triggers a breath
  • Flow: inspiratory flow is a trigger.
    • Better, more sensitive method
23
Q

Limit variable ventilation

A
  • When value is reached inspiration will be terminated
  • Volume limit: e.g. metal rod limits the expansion of the bellows
    • Used in the North American Draeger
  • Pressure limit is used to prevent barotrauma as a consequence of inappropriate ventilator setting
24
Q

Pressure limiting valve

A
  • Safety pressure limit for the drive gas pressure
    • The patient will receive less pressure than this
  • RUSVM: set at 20
25
Q

Defining tidal volume (Vt) using I:E ratio

A
  • Inspiratory time and flow together define Vt with flow controlled ventilators
  • I:E ratio = ratio of inspiratory / expiratory times
  • I:E ratio and RR together define inspiratory time
  • Summary: use flow, I:E ratio, and RR together to set the Vt
  • (Normal I:E ratio is about 1.2-1.3)
26
Q

Defining Vt using inspiratory time

A
  • Ventilators at RUSVM have control knob for inspiratory time
  • This along with the flow will determine Vt
  • I:E ratio and expiratory time are independent variables
  • RR setting will not affect inspiratory time (and Vt)
  • The desired RR will only be delivered if it’s possible with the inspiratory time you set
27
Q

PIP and PEEP; What are the indications for PEEP?

A
  • PIP = peak inspiratory pressure–inflates alveoli
  • PEEP = positive end expiratory pressure–keeps alveoli open
  • Indications for PEEP
    • Open thorax
    • Lung parenchymal disease
    • Following alveolar recruitment maneuver
    • (The benefit of PEEP is questionable during routine anesthesia case management)
28
Q

IMV

SIMV

PSV

CPAP

A
  • IMV = Intermittent mandatory ventilation
    • Patient is allowed to breathe freely between mechanical breaths
  • SIMV = synchronized IMV
    • Each spontaneous breath of the patient is assisted
  • PSV = pressure support ventilation
    • The patient is breathing freely, but each breath is supported with pressure
    • Mechanical inspiration is terminated when flow stops (flow cycled)
    • Better patient-ventilator synchrony than SIMV
  • CPAP = continuous positive airway pressure
    • Assisted ventilation mode when both the inspiratory and expiratory pressures are positive
29
Q

Ventilating healthy lungs (values)

A
  • Vt = 10-15 ml/kg
    • Ruminants maybe 6-10
  • RR = 10-15 breath/min
  • Inspiratory time = 1-2 sec
  • PIP = 10-20 cmH20
  • PEEP = 0-2 cmH20
30
Q

Ventilating sick lungs

A
  • Vt is smaller (baby lung): 4-8 ml/kg
  • RR can be inc. up to 60 breath/min
  • Inspiratory time can be increased but watch out for completeness off expiration
  • PIP can be increased to 35 (max 60) cmH20
  • PEEP as needed (5-20 cmH20)
31
Q

Atelectasis (lung collapse)

A
  • General anesthesia causes collapse of the most dependent parts of the lungs in almost all patients
  • The lung collapses very rapidly after induction of anesthesia and persists for hours or days after surgery
  • Lung collapse impairs gas exchange and may contribute to development of pneumonia or lung injury
  • Cyclic recruitment: alveoli opens and collapses with each breath; may lead to lung injury
32
Q

What are the 3 mechanisms of atelectasis formation?

A
  • Compression
  • Absorption
  • Lack of surfactant
33
Q

Alveolar recruitment maneuver (ARM)

A
  • Therapeutic maneuver aiming to open lung atelectasis and improve oxygenation
  • Types: CPAP and Cycling
    • Both should be followed by PEEP
34
Q

What is the open lung concept?

A
  • Therapeutic approach aiming to reverse atelectasis, prevent cyclic recruitment and ventilator induced lung injury
  • Consists of an ARM followed by an individually defined optimal PEEP
35
Q

What are the clinical applications of an ARM?

A
  • ARM is not yet a standard clinical procedure
  • Safe highest airway pressures may depend on species, body size, clinical condition, etc.
    • ​Such safe pressure limits are not yet established
  • Other safety issues are concerning the cardiovascular system
  • Perform ARM ONLY if you have:
    • Appropriate clinical indication
    • Reliable mechanical ventilator able to supply PEEP
    • Sufficient monitoring
    • Sufficient clinical experience
36
Q

What is patient-ventilator asynchrony? What are the common causes during anesthesia? In the ICU? Treatment?

A
  • Patient attempts to breathe out of phase with the ventilator
  • Anesthesia
    • Inadequate anesthetic depth
    • Inadequate lung volume (FRC) or tidal volume
  • ICU
    • Inadequate anesthetic depth
    • Pneumothorax, atelectasis, hypotension, hyperthermia
  • Treat the underlying cause
  • Asynchrony may cause rapid deterioration of oxygenation and ventilation
37
Q

How do you wean a patient off the ventilator after surgery? In the ICU?

A
  • Surgery
    • If the lungs are healthy it is normally an easy procedure
    • Can decrease ventilator setting and cause hypercapnia
    • Or continue ventilating until fully awake
    • May use opioid antagonist if necessary
  • In the ICU
    • Weaning is more difficult because ventilation was long-lasting and lungs might not be healthy
    • Spontaneous ventilation trials may be used

Anesthesiology (16 decks)

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