Test 1- Mechanical Ventilation Flashcards
Importance of ventilation during anesthesia
Anesthesia can affect ventilation
Alter sensitivity to CO2
Relaxes respiratory muscles (FRC decreases) Atelectasis develops
Make V/Q matching worse
Ventilation can affect anesthesia
Uptake of inhalational anesthetics depends on ventilation
Controlled ventilation facilitates reliable uptake and smooth plane of anesthesia
Ventilation
Ventilation is ALL ABOUT CO2!!!
The process involved in the movement of air (gas) in and out of the alveoli
Defined by PaCO2
Normal PaCO2 ≈ 34 - 45 mmHg
Patient should also have normal resp. rate, rhythm and effort
Monitored with arterial blood gas (PaCO2) or capnography
Oxygenation
The process of oxygenation of arterial blood
Defined by PaO2
Hypoxemia:
PaO2 < 60 mmHg SaO2 < 90%
Monitored with arterial blood gas (PaO2) or pulse oximetry
Strongly depends on inspired O2%
When breathing air
Also depends on ventilation
When breathing 100% O2
Oxygenation cannot be improved by more ventilation
Could be improved by special respiratory manoeuvers (see recruitment later)
Apneic oxygenation is possible (ventilation may not be needed for oxygenation)!
Using 100% oxygen can insure good oxygenation in most circumstances!
Resistance limits
Resistance limits flow
Resistance = ∆Pressure / Flow
Compliance limits
Compliance limits volume
Compliance = Vol. / Flow
Indications for mechanical ventilation (MV)
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 the patient is intubated and breathing 100% O2
If the patient is already intubated and breathing 100% O2 increasing oxygenation will only be possible with special respiratory manoeuvers and not with conventional ventilation
There is a need to decrease respiratory effort
Mostly happens in the ICU as a treatment for respiratory failure
Indications for MV during anesthesia
Convenient control of respiratory function
Prolonged anesthesia
Maintain a more stable anesthesia plane Neuromouscluar blockade
Thoracic surgery
Chest wall or diaphragmatic trauma
Obesity, increased abdominal pressure Head down positioning (Trendellenburg) Laparoscopy
Control of intracranial pressure
Indications for MV in the ICU
Depression of the respiratory center in the brain Inadequate thoracic expansion
Inadequate lung expansion
Airway obstruction
Respiratory arrest (or Cardio Pulmonary Arrest) Pulmonary edema, ARDS
Side effects of MV
Impairs venous return and cardiac output
May cause hypotension especially in hypovolemic patients
Treatment
Volume loading Decreasing airway pressures (change ventilator settings) Switch off the ventilator Inotropic drugs (e.g. dobutamine)
Others: pneumothorax and lung injury
Side effects of hypercapnia
Direct effects of CO2 Peripheral vasodilation
Decreased myocardial contractility
Bradycardia, possible cardiac arrest (very extreme case!) Increased intracranial pressure
Indirect effects of CO2 via catecholamine release Tachycardia, arrhythmias
Increased myocardial contractility Increased blood pressure
CO2 narcosis
>95 mmHg progressive narcosis >245 mmHg complete narcosis
Risk of not ventilating properly#
If you don’t control ventilation during thoracic surgery and let the lung be collapsed for prolonged time, not only CO2 will accumulate but the patient may quickly turn hypoxemic and you may encounter sudden death of the patient!
Should I ventilate during anesthesia?
Debated issue especially in horses
The point is how to balance between compromising either
cardiovascular or respiratory function (and oxygenation)
Permissive hypercapnia may be acceptable up to 60-70 mmHg
2 types of ventilation?
Manual or Mechanical
Ventilation modes
Volume controlled ventilation
Device sets the volume, pressure is a dependent variable
If compliance decreased (pneumothorax) pressure would increase Difficult to control the tidal volume in very small patients
Pressure controlled ventilation
Device sets the pressure, volume is a dependent variable
If resistance increased (airway obstruction) volume would decrease Works well regardless of body size
Clinical recommendation
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
Source of driving power
Electrically driven: e.g. using a linear motor
Pneumatically driven: using pressurized gas source. More common
What are the different control variables for different ventilators?
- Flow: the ventilator delivers a constant flow to the patient
- Pressure: the ventilator delivers a constant pressure to the
patient
Analogy with electricity: flow is current, pressure is voltage. A battery may supply either constant current or constant voltage
What are the cycle variables?
Triggers expiration when set value is reached Volume: volume controlled ventilation
Pressure: pressure controlled ventilation
Time: both
Flow: diminishing flow triggers expiration
Useful for pressure support ventilation (PSV) because it helps accommodating to the patient’s breathing pattern
What are the limit variables?
When value is reached inspiration will be terminated
Volume limit: e.g. metal rod limits the expansion of the bellows.
Used e.g. in the North American Draeger
Pressure limit is used to prevent barotrauma as a consequence of inappropriate ventilator setting
Pressure limiting valve at RUSVM
This is a safety pressure limit for the drive gas pressure. The patient will receive less pressure than this.
Volume controlled ventilation can be achieved by using:
Volume controlled ventilation can be achieved by using: Flow controlled, time cycled ventilator (flow x time = volume)
Currently available ventilators at RUSVM belongs to this category
Flow controlled, volume limited, time cycled ventilator
Pressure controlled ventilation can be achieved by using:
Pressure controlled ventilation can be achieved by using: Pressure controlled, time cycled ventilator (e.g. many modern ventilators)
Pressure controlled, pressure cycled ventilator
Flow controlled, pressure cycled ventilator (e.g. Bird ventilator)
Volume controlled ventilation
Flow controlled, time cycled ventilator
Pressure controlled ventilation
Pressure controlled, time cycled ventilator
Defining tidal volume (Vt) using the I:E ratio
Remember: inspiratory time and flow together define Vt with flow controlled ventilators
I:E ratio = the ratio of inspiratory / expiratory times
I:E ratio and the resp. rate (RR) together define inspiratory time
Summary: use flow, RR and I:E ratio together to set the Vt
Defining Vt using the inspiratory time
Currently available ventilators at RUSVM has a control knob for inspiratory time (you are lucky!)
This along with the flow will define Vt
I:E ratio and expiratory time are a dependent variables
RR setting will not affect inspiratory time (and Vt)
But the desired RR will only be delivered if it’s possible with the inspiratory time you set
PIP
Peak Inspiratory Pressure inflates the alveoli
PEEP
Positive End Expiratory Pressure keeps the 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
IMV
Intermittent Mandatory Ventilation
Patient is allowed to breath freely between mechanical breaths
SIMV
SIMV (Synchronised 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 the expiratory pressures are positive
Ventilating healthy lungs
Tidal volume: 10 - 15 ml/kg
Respiratory rate: 10 - 15 breath/min
Inspiratory time: 1 - 2 sec
PIP: 10 - 20 cmH2O
PEEP: 0 - 2 cmH2O
(For ruminants maybe 6-10 ml/kg)
Ventilating sick lungs
Tidal volume is smaller (baby lung): 4 - 8 ml/kg
Respiratory rate can be increased up to 60 breath/min
Inspiratory time can be increased but watch out for completeness of expiration
PIP can be increased to 35 (max 60) cmH2O
PEEP as needed: 5 - 20 cmH2O
Atelectasis
Atelectasis (lung collapse)
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.
Alveolar recruitment maneuver (ARM)
Therapeutic maneuver aiming to open lung atelectasis and improve oxygenation
Types: CPAP and Cycling
Both should be followed by PEEP
Clinical application of ARM
CAUTION: 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!
