Mechanical ventilation Flashcards
Ventilation (CO2)
ventilation status is defined by PaCO2 PaCO2 depends primarily on alveolar ventilation -minute ventilation Normal PaCO2= 35-45 mmHg >= hypoventilation EtCO2 used as an estimate of PaCO2 -will be lower than PaCO2 -difference between them is CO2 gradient Normal difference ~2-5mmHg
Pt RR, effort, and pattern should be continuously evaluation- in conj with EtCO2
Ventilation and oxygenation are separate process
CO2 abnormalities are much easier to resolve
CO2 diffuses across the respiratory membrane much faster than O2
anesthesia causes hypoventilation
Ventilation required for maintaining a smooth plane of inhalant anesthesia- periods of apnea (esp right after induction) -> inadequate inhalant delivery to alveoli -> patient arousal
Minute Ventilation
tidal volume x respiratory rate
Hypoventilation
Decreased respiratory center sensitivity to PaCO2
Respiratory muscle relaxation
Positioning (esp dorsal recumbency)
What happens when the patient hypoventilates
Not hypoxemia if breathing 100% O2
Direct effects= vasodilation, dec inotropy, inc ICP, narcosis (>90 mmHg)
indirect effects via catecholamine release= tachycardia, inc inotropy and blood pressure
possible acidemia
Permissive hypercapnia
allowing mild-moderate hypoventilation may improve cardiovascular function
-not appropriate for patients with inc ICP
-consider pre-existing acidemia or electrolyte abnormalities
A PaCO2 up to 60-70 mmHg (esp horses) may be desirable
Oxygenation
process of oxygenation of arterial blood
tissue oxygenation requires adequate PaO2 and cardiac output
defined by PaO2 or SO2
SpO2= peripheral O2 saturation (pulse ox)
SaO2= arterial O2 saturation (from blood analyzer)
Hypoxemia
PaO2 < 60 mmHg
SO2 < 90%
Depends on fraction of inspired air (FiO2)
hypoventilation when breathing 100% O2 does not cause hypoxemia
Oxygenation is not improved by more ventilation
Improving V/Q matching does improve oxygenation
Why positive pressure ventilation
To decrease PaCO2 (resolve hypoventilation) by increasing tidal volume, RR, or both
To perform the work of breathing
Patients requiring positive pressure ventilation
-neuromuscular blockade
-thoracic sx or inj
-inc ICP
-horses anesthetized with inhalants
Other indications
-inc intra-abdominal pressure
-obesity
-maintaining a stable plane of anesthesia
Spontaneous ventilation
patient decides when and how to breath
Mandatory/controlled ventilation
ventilator determines
IPPV
intermittent positive pressure ventilation
PIP
Peak inspiratory pressure
cmH2O
Maximum breathing system (same as pulmonary) pressure on inspiration
PEEP
positive end-expiratory pressure (cmH2O)
positive pressure maintained after expiration
used to prevent alveolar collapse
requires equipment- PEEP valve, programmable ventilator
PPV side effects
PPV is not physiologically normal
Normal resp initiated by neg intrathoracic pressure-aids in venous return
PPV causes positive intrathoracic pressure- dec venous return
PPV may result in dec CO and hypotension, esp in hypovolemic patients
tx= volume loading (fluids), decreased PIP, dec inspiratory time, decreased RR
Other side effects are unlikely during short term ventilation
-pneumothorax, volutrauma, barotrauma
How is PPV administered during anesthesia
Manually using anesthesia machine
-close pop-off (APL valve)
-squeeze bag
-watch pressure gauge, in most cases apply <20 cm H2O
-release bag pressure and open pop-off- do not hold pressure
-give as many breaths and as frequently as needed to maintain EtCO2 within desired range
Anesthesia ventilator
How is PPV administered without an anesthesia machine
Ambu bag
-Can provide air or 100% O2
Demand valve
-provides pressurized 100% O2 when triggered by anesthetist
-patient can also breathe spontaneously
-useful for equine, esp during recovery
Usually no way to monitor airway pressure or tidal volume
-must watch patient thorax to deliver appropriate volume x
General guidelines for healthy patients
adjust based upon goals (usually PaCO2)
tidal volume= 10-15 mL/kg
RR= 8-12 bpm
Inspiratory time 1-2 seconds
Inspiratory:expiratory ratio 1:3 or less
PIP= 10 cmH2O if <10kg
20 if > 10kg
Relationship b/t TV and PIP determined by lung complianceLung compliance
Change in volume/change in pressure
affects the amount of pressure required to administer a desired tidal volume
Ruminants have lover compliance than other species (more fibrous lung) –> require lower TV
Disease processes decreasing compliance
-bronchoconstriction
-pleural space disease
-abdominal pressure
May require high PIP to supply adequate TV
-treat underlying cause
Mechanical ventilators-volume controlled
flow and inspiratory time set on the ventilator determines the tidal volume delivered to the patient
PIP will depend on lung compliance
Mechanical ventilators-pressur econtrolled
PIP set on the ventilator
When that pressure is reached, flow stops and patient exhales
TV will depend on lung compliance
Driving power
electric
Pneumatic
uses pressurized gas
compressed air or O2
Pneumatic ventilator
Space inside bellows is patient gas circuit - needs to be scavenged
Space between bellows and housing is driving gas circuit - exhausts to room
Surgivet large animal ventilator
volume controlled
Pneumatic
Surgivat small animal ventilator
Volume controlled
pneumatic
How to set up a pneumatic ventilator
1 Connect ventilator electricity, pressurized gas source, and exhaust to scavenging system
2 Set ventilator controls- min flow, inspiratory time ~1s, RR 8-12 min
3 Remove rebreathing bag and connect ventilator to patient circuit
4 close pop off
5 turn ventilator on
6 slowly inc flow until desired tidal vol/PIP is reached
-monitor pressure gauge and EtCO2
Patient-ventilator asynchrony
When patient continues breathing around the ventilator
may result in high breathing system pressure
Evaluate the patient: if ventilator isnt providing adequate minute ventilation, patient may inc on their own– in this case, inc minute ventilation (TV or RR or both) from ventilator
Patient too light under anesthesia
-inc depth
-maybe patient doesnt need a ventilator
Weaning from the ventilator
depends on plan for patient
small animal- easy as animal can be recovered while connected to anesthesia machine
Horses/cattle- must be breathing on own before moving to recovery area unless there is a method for providing IPPV
-demand valve allows IPPV during recovery (inc patient safety)
Weaning from the ventilator - general technique
dec RR or initiate pauses in ventilation (~30s)
allow PaCO2 to inc
Patient will begin spontaneous respiration
Continue to monitor EtCO2 while intubated
Ways to improve oxygenation
mechanical ventilation will not resolve hypoxemia if patient is already breathing 100% O2
must improve ventilation/perfusion matching
Atelectasis is most commonly the problem
-collapsed alveoli
-perfusion is normal unless patient has very low CO or other disease
More lung involved -> lower PaO2:FiO2
Alveolar recruitment maneuver (ARM)
-indicated to open alveoli in a patient with hypoxemia
- involves holding a high PIP once (sigh) or repeating at increasing and dec pressure (cycling)
-PEEP must be applied afterwards to keep alveoli open
Can be dangerous when using very high PIP
-diseased lung
-cardiovascular compromise
Modified ARM used in donkey lab to improve PaO2
-manual IPPV: hold PIP at 25 cmH2O for 5-10 sec
-apply 10 cmH2O PEEP using PEEP valve
-clinically useful without applying dangerously high pressures
Oxygen by demand valve supplied during recovery
