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
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
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
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
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)
6
Q
What are the phases of respiration?
A
- Inspiration
- Inspiratory flow time
- Insiratory pause
- Expiration
- Expiratory flow time
- Expiratory pause
7
Q
What do resistance and compliance limit?
A
- Resistance limits flow
- = change in pressure / flow
- Compliance limits volume
- = volume / flow
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
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
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
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)
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
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
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
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
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
- Source of driving power
- Control variable
- Cycle variable
- Trigger variable
- 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
