Mechanical ventilation Flashcards by Lindsey Fore

How does anesthesia affect ventilation?

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

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How does ventilation affect anesthesia?

Utake of inhalational anesthetics depends on ventilation
Controlled ventilation facilitates reliable uptake and smooth plane of anesthesia

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

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

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

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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What are the phases of respiration?

Inspiration
- Inspiratory flow time
- Insiratory pause
Expiration
- Expiratory flow time
- Expiratory pause

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What do resistance and compliance limit?

Resistance limits flow
- = change in pressure / flow
Compliance limits volume
- = volume / flow

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

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

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

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

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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What is the risk of not ventilating properly?

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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Should I ventilate during anesthesia?

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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What are the 3 types of ventilation?

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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What are the 2 different ventilation modes?

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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

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

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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

What are the 5 classifications of ventilators?

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Source of driving power
Control variable
Cycle variable
Trigger variable
Limit variable

What are the 2 sources of driving power for ventilators?

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

Control variable ventilators

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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

Cycle variable ventilators

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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

Trigger variable ventilation

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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

Limit variable ventilation

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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

Pressure limiting valve

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Safety pressure limit for the drive gas pressure
- The patient will receive less pressure than this
RUSVM: set at 20

Defining tidal volume (Vt) using I:E ratio

* 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)

Defining Vt using inspiratory time

* 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

PIP and PEEP; What are the indications for PEEP?

* 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)

IMV SIMV PSV CPAP

* 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

Ventilating healthy lungs (values)

* 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

Ventilating sick lungs

* 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)

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

What are the 3 mechanisms of atelectasis formation?

* Compression * Absorption * Lack of surfactant

Alveolar recruitment maneuver (ARM)

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

What is the open lung concept?

* 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

What are the clinical applications of an ARM?

* **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

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

* 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

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

* 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

Mechanical ventilation Flashcards

(37 cards)