Mechanical Ventilation Flashcards
What is happening physiologically during inspiration?
- Contraction of diaphragm and intercostal mm +/- abd mm
- Enlargement of chest cavity > drop in pleural pressure = drop in alveolar pressure
- Air moves from atmosphere to alveoli
What is happening physiologically during expiration?
- Contraction of resp mm ceases
- Elastic recoil of chest wall and lungs incr alveolar pressure over the atm pressure
- Air moves from alveoli to atm
During spontaneous ventilation, as transpulmonary pressure and alveolar pressure drop, the alveolar volume ______
Increases
Describe the work of breathing
- Energy required by resp mm to produce an inspiration - under normal conditions expiration is passive (No WOB)
Work of breathing is needed to do what 3 things?
- To expand lungs against elastic forces
- To overcome the viscosity of the lung and chest wall structures
- To overcome airway resistance
Airway resistance has to do with what?
Atm P - Alveolar P
Volume of airflow
Define tidal volume
Volume of air inspired or expired with each normal breath; 10-20 ml/kg
Define minute ventilation (Vm)
Total amount of new air moved into the resp passages each minute
Vm = TV x RR
Define alveolar minute ventilation (Va)
Total volume of new air entering the alveoli each minute
Va = RR x (TV - anatomical dead space volume)
What are the normal FiO2 and FiN2 of atmospheric air?
FiO2 = 21% (159 mmHg)
FiN2 = 78% (590 mmHg)
What is the PaCO2 and PaO2 (arterial) during normal alveolar ventilation? PvCO2 and PvO2?
PaCO2 = 40 mmHg; PaO2 =100 mmHg
PvCO2 = 50 mmHg; PvO2 = 40 mmHg
What 4 things cause CNS depression and affect spontaneous ventilation? What is the effect of these on spontaneous ventilation?
general anesthesia, sedatives, opioids, CNS dz; decr alveolar ventilation and reduce central drive
What types of thoracic abnormalities affect spontaneous ventilation?
Open chest, pneumothorax, pleural effusion, external pressure on the chest, obesity
What things cause increased intra-abdominal pressure that affect spontaneous ventilation? What is the effect of these on spontaneous ventilation?
Pregnancy, GDV, abdominal fluids, large abd masses, pneumoperitoneum, obesity; decr alveolar ventilation, reduce compliance
Define compliance
Measure of lung’s ability to stretch and expand; is the change in the volume for any given applied pressure
What are the two main effects on spontaneous ventilation we are concerned about with hypoventilation?
Hypoxemia (unless high FiO2 is provided) and hypercapnea
What are four causes of hypoxemia?
- Hypoventilation - inability of resp system to maintain a normal alveolar ventilation = CO2 not eliminated adequately
- Diffusion limitation
- Shunt
- Venilation-perfusion inequality
*plus low FiO2
What are situations where ventilatory support is mandatory?
- Open chest sx
- Use of neuromuscular blocker agents
- Resp arrest
- Lung dz where normoxemia is not maintained by supplementing O2
- Hypercapnia
- Patients that cannot tolerate incr in CO2 (ie. brain tumors)
What are situations where ventilatory support is highly recommended?
- Low lung/chest compliance
- Obese
- GDV
- Pregnant
- Horses
- Laparoscopic sx
*these animals are prone to severe hypoventilation therefore ventilatory support can become mandatory!
What are situations where ventilatory support is beneficial?
Dorsal recumbency
Any patient under general anesthesia
During positive pressure ventilation, positive pressure is generated in the breathing system producing movement of air into the alveoli. Therefore, during inspiration the alveolar pressure is _______ compared to the atmospheric pressure (as opposed to spontaneous ventilation).
Positive
What is a demand valve?
Provides high flow of oxygen (up to 160 L/min); used in large animals before connecting to anesthetic machine or in the process of weaning from the ventilator
What is the name of the bag used for manual positive pressure ventilation in small animals?
Ambu bag
How does mechanical positive pressure ventilation differ from manual?
The bag is replaced by a bellow in a jar attached to a ventilator controller with high pressure oxygen flowing through
*often use O2 over room air in case there’s a hole in the bellow/jar
Describe the concept of assisted ventilation?
- To reduce the work of breathing
- Patient is able to initiate breath
- Tidal volume is enhanced by manual or mechanical support
What are 2 examples of assisted ventilation modalities?
Synchronized intermittent-mandatory ventilation (SIMV)
Pressure Support Ventilation
Describe controlled ventilation
- Complete control of ventilation
- Operator sets tidal volume and respiratory rate
- Also known as IPPV (intermittent positive pressure ventilation)
What are 2 controlled ventilation modalities?
Volume controlled ventilation
Pressure controlled ventilation
Describe the process of volume controlled ventilation
- The fixed delivered TV will generate a certain airway pressure (normal = 10-20 cmH2O) = peak inspiration pressure (PIP)
- Airway pressure will depend on compliance of resp system
- pressure is limited - safety feature of ventilators to avoid barotrauma (<30 cmH2O)
Describe the concept of pressure controlled ventilation
- The fixed PIP is set at 10-20 cmH2O and the ventilator will deliver the TV reqiuired to generate the pre-set PIP
- TV will depend on the compliance of the resp system
- If pulmonary compliance is reduced, the TV will be reduced, resulting in a reduced alveolar ventilation
Define positive end expiratory pressure (PEEP)
PEEP maintains a positive pressure during expiration to avoid collapse of the poorly ventilated alveoli
Usually kept at 5-10 cmH2O
Reduces venous return and can have a profound CV impact on hypovolemic BP
What are you monitoring for during controlled ventilation?
- Airway pressure:
- avoid barotrauma, ideally PIP < 20 cmH2O
- avoid volutrauma, ideally TV < 20 cmH2O, check PIP and compliance
- End tidal CO2:
* TV, PIP, RR and I:E adjusted to maintain normocapniahigh; CO2 = hypoventilation - low CO2 = hyperventilation - Compliance
Describe the dual chamber ventilator
- Driving gas compresses bellow
- Driving gas: compressed air or O2
- Most common in vet med
- Ascending or descending bellow
Describe a piston ventilator
- More sophisticated
- Advanced methods of ventilation
- Very precise
- Does not require driving gas
What are the physiological consequences of positive pressure ventilation on the CV system?
High and sustained intrathoracic pressure > decr venous return > hypotension - esp hypovolemic patients, close monitoring CV function after initiating IPPV
What are the physiological consequences of positive pressure ventilation on the resp system?
High intrathoracic pressure and volume > risk of barotrauma > always monitor airway pressure, if thoracic cavity is open, compliance is sig incr > TV and PIP should be adjusted to avoid overdistension
Risk of pneumothorax - worsening previous pneumothorax
Patient-Ventilatory Asynchrony occurs when?
The timing of the ventilator cycle is not simultaneous with the timing of the patient’s resp cycle
What are some causes of patient-ventilator asynchrony?
Light anesthetic plane
Nociception
Hypercapnia
Hypoxemia
Hyperthermia
Describe how positive pressure ventilation causes diaphragmatic hernia?
- In chronic cases > re-expanding the lung can lead to:
- Repercussion injury
- Acute resp distress syndrome
- Use low TV and high RR to try to maintain a normal minute ventilation
- Permissive hypercapnia (ETCO2 50-55mmHg)
Define atelectasis and what are some causes and consequences?
Collapse or closure of a lung resulting in reduced or absent gas exchange
Causes: compression, absorption, decr surfactant
Consequences: V/Q mismatch, hypoxemia, risk of post-operative pulm infection
How do you deal atelectasis?
- Recruiting maneuvers (artificial sigh)
- check CV fxn b/c CV impact may be profound esp in hypovolemic patients
- Add PEEP after recruiting - prevents re-collapse of alveoli maintaining a positive pressure during expiration
