Flashcards in Mechanical Ventilation Deck (61):
How does anesthesia affect ventilation?
1. Alters CO2 sensitivity
2. Relaxes respiratory muscles
3. Develops atelectasis
4. Worsens V/Q match issues
How can ventilation affect anesthesia?
1. Uptake depends on ventilation
2. Controlled ventilation facilitates reliable uptake and smooth plane of anesthesia
What is ventilation defined by?
Patient should have normal resp rate, rhythm and effort
What is oxygenation?
Process of oxygenation of arterial blood
Defined by PaO2
What is oxygenation monitored by?
Arterial blood gas or pulse oximetry
What does oxygenation depend on?
100% O2 typically insures good oxygenation
What are the two phases of respiration?
Inspiration and expiration
What does resistance do?
What does compliance do?
Indications for mechanical ventilation?
1. Need to decrease PaCO2
2. Need to increase PaO2
3. Need to decrease respiratory effort (mostly ICU)
Indication for mechanical ventilation during anesthesia
1. Control of respiratory function
2. Prolonged anesthesia
3. Maintain stable anesthesia plane
4. Neuromuscular blockade
5. Thoracic surgery, chest wall, hernia
6. Obesity, increased abdomen pressure
7. head down positioning
9. Control ICP
Side effects of MV
1. Impairs venous return and cardiac output
2. May cause hypotension especially in hypovolemic patients
3. Pneumothorax and lung injury
Direct effects of hypercapnia
1. Peripheral vasodilation
2. Decreased myocardial contractility
3. Bradycardia and possible arrest
4. Increased ICP
Indirect effects of hypercapnia
1. Tachycardia, arrhythmias
2. Increased myocardial contractility
3. Increased blood pressure
CO2 narcosis levels
>95 mmHg progressive narcosis
>245 mmHg complete narcosis
What may happen if not ventilating properly?
CO2 accumulation, hypoxemia, sudden death
Should horses be ventilated?
Types of ventilation
Spontaneous- patient breathing
Assisted- patient timed, machine assists
Mandatory/Controlled- ventilator controls
Mechanical- machine driven
What can result from volume ventilation?
Pneumothorax if compliance is decreased
Which ventilation mode is preferred if lung volume changes during a procedure?
Which ventilation mode is preferred if trans-pulmonary pressure changes during a procedure?
Which ventilation mode works well for all patient sizes?
Classification of ventilators
1. Source of driving power
2. Control variable
3. Cycle variable
4. Trigger variable
5. Limit variable
Source of driving power
1. Electronically driven
2. Pneumatically driven (pressurized gas)
Which source of driving power is more common?
1. Flow- delivers constant flow
2. Pressure- delivers constant patient
Triggers expiration when a set value is reached
4. Flow- diminishing flow
What is flow variable useful for?
Pressure support ventilation- helps accommodate the patients breathing pattern
Triggers inspiration when a set value is reached
1. Pressure- negative pressure
2. Flow- inpiratory flow
Trigger variables are used during what ventilation modes?
When value is reached, inspiration will be terminated
1. Volume limit
2. Pressure limit
What does a pressure limit prevent?
Barotrauma if ventilator is set inappropriately
How can volume controlled ventilation be achieved?
Flow controlled, time cycled ventilator
Flow controlled, volume limited, time cycled ventilator
How can pressure controlled ventilation be achieved?
-Pressure controlled, time cycled ventilator
- Pressure controlled, pressure cycled ventilator
- Flow controlled, pressure cycled ventilator
Ratio of inspiratory/expiratory times
What does the I:E ratio define?
Tidal volume (Vt)
Does the RR affect inspiratory time and Vt?
Peak inspiratory pressure- inflates alveoli
Positive end expiratory pressure- keeps alveoli open
Indications for PEEP
1. Open thorax
2. Lung parenchymal disease
3. Following alveolar recruitment maneuver
Should PEEP be used during routine anesthesia?
Benefits are questionable
Intermittent mandatory ventilation- allowed to breath freely between mechanical breaths
Synchronized IMV- each spontaneous breath is assisted
Pressure Support Ventilation- patient is breathing freely but supported with pressure
Flow termination, better synchrony than SIMV
Continuous Positive Airway Pressure- assisted ventilation mode when both insp/exp pressures are positive
Ventilating healthy lung values
Tidal volume : 10-15mL/kg
RR: 10-15 bpm
Inspiratory time: 1-2sec
PIP: 10-20 cmH2O
PEEP: 0-2 cmH2O
Ventilating sick lungs
Tidal volume : 4-8mL/kg
RR: up to 60 bpm
Inspiratory time: may be increased but watch expiration
PIP: 35-60max cmH2O
PEEP as needed: 5-20 cmH2O
Which lung typically collapses during anesthetic procedures?
The most dependent one very rapidly after induction
Can a collapsed lung persist for hours/days after surgery?
What is cyclic recruitment?
Opening and collapse of alveoli with each breath, may lead to lung injury
Mechanisms of atelectasis formation
3. Lack of surfactant
Alveolar recruitment maneuver (ARM)
Therapeutic maneuver aiming to open lung atelectasis and improve oxygenation
Types of ARM
CPAP and Cycling
Should be followed by PEEP
Open Lung concept
Therapeutic approach aiming to reverse atelectasis, prevent cyclic recruitment, and ventilator inducted lung injury
ARM followed by optimal PEEP
Clinical application of ARM
Safe airway pressures are highly individual variation
Should only be performed if you have sufficient monitoring/equipment and clinical indication
Patient attempts to breath out of phase with the ventilator
Common causes of patient-ventilator asynchrony
Inadequate anesthetic depth
Inadequate lung volume or tidal volume
ICU: pneumothroax, atelectasis, hypotension, hyperthermia
Treatment for patient-ventilator asynchrony
Treat underlying cause; may cause rapid deterioration of oxygenation and ventilation
Weaning from ventilator after surgery
If a normal healthy animal it's simple
Decrease ventilator setting or continue ventilating until fully awake
May use opioid antagonists if necessary