Exam 3: Lecture 20 - Mechanical Ventilation Flashcards
What is normal ventilation?
-Movement of gas in & out of alveoli & defined as the maintenance of normal arterial blood carbon dioxide concentration (PaCO2) of 35-45mmHg
Besides normal ventilation, patient should also have a normal
-Respiratory effort, rate & rhythm
What is indicated by the blue star?
-Inspiratory reserve volume
What is the inspiratory reserve volume?
-Extra bit you can breathe past the tidal volume
What is indicated by the blue star?
-Expiratory reserve volume
What is the expiratory reserve volume?
-Volume when you push all the air out
What is indicated by the blue star?
-Residual volume
What is the residual volume?
-The volume left over after exhaling everything you can
What is indicated by the blue star?
-Functional residual capacity
What is the functional residual capacity?
-Expiratory reserve volume + residual volume
What is indicated by the blue star?
-Vital capacity
What is the vital capacity?
-What your body can do
-Expiratory reserve volume + tidal volume + inspiratory reserve volume
What is indicated by the blue star?
-Inspiratory capacity
What is the inspiratory capacity?
-Tidal volume + inspiratory reserve volume
What is indicated by the blue star?
-Total lung capacity
What is indicated by the blue star?
-Tidal volume
What is the tidal volume?
-Small amount of the total amount that the patient is breathing
The air in the lungs can be divided into:
4 different volumes & 4 different capacities:
-Tidal volume (Vt)
-Inspiratory reserve volume (IRV)
-Expiratory reserve volume (ERV)
-Residual volume (RV)
-Inspiratory capacity (IC) = TV + IRV
-Functional residual capacity (FRC) = ERV + RV
-Vital capacity (VC) = IRV + TV + ERV + RV
-Total lung capacity (TLC) = IRV + TV + ERV + RV
Tidal volume and minute ventilation can be measured with
-Spirometer
What is minute ventilation?
-VE
-Tidal volume (Vt) x respiratory frequency (f)
What is the VE of a patient that has a tidal volume of 250 mL & a respiratory rate of 12 bpm?
250 mL x 12 bpm = 3,000 mL/min
Why do we care of ventilation?
-Anesthetic drugs can alter patient’s ability to normally ventilate
-Ventilation is required for inhalant anesthetics to be properly taken up & eliminated
How can anesthetic drugs alter the patient’s ability to normally ventilate?
-Could lead to inadequate gas exchange, hypoventilation, & eventually respiratory arrest -> cardiac arrest!
What is hypercapnia?
-CO2 level above 45 mmHg
What are the direct effects of hypercapnia?
-Causes vasodilation of peripheral arterioles & myocardial depression
-Associated w/ increased vagal tone -> slows heart rate, & could lead to cardiac arrest
-Increased intracranial pressure due to vasodilation
What are the indirect effects of hypercapnia?
-Increases circulating catecholamines -> cardiac arrhythmias, tachycardia, increased myocardial contractility, & BP elevation
Narcoses progresses with PaCO2 values above ____ mmHg and induces complete anesthesia at ____ mmHg
95 mmHg, 245 mmHg
What is IPPV?
-Intermittent positive pressure ventilation
-Positive pressure maintained only during inspiration
What is IMV?
-Intermittent mandatory ventilation
-Operator sets a predetermined number of positive breaths, but patient can also breathe spontaneously
What is PEEP?
-Positive end-expiratory pressure
-Airway pressure at end expiration is maintained above ambient pressure
-Applied when positive pressure is maintained between inspirations that are delivered by a ventilator
What is CPAP?
-Continuous positive airway pressure
-Spontaneous breathing with positive pressure during both inspiratory & expiratory cycles
What is HF(N)OT?
-High flow (nasal) oxygen therapy
-Administration of warm, humidified oxygen via nasal prongs, using a commercially available unit to deliver higher flow rates of oxygen & and FiO2 up to 100%
How can IPPV be performed?
-Closing/occluding pop-off valve & squeezing reservoir bag until 10-20 cm H2O reached, then pop-off valve is reopened so patient can passively expire (called “Manual IPPV”)
-A machine (called mechanical ventilator)
What is the preferred method of performing a “manual IPPV”?
-To utilize the safety occlusion valve instead of actually closing APL valve (aka pop off valve)
What is the advantage of using a mechanical ventilator IPPV, but also what are downsides?
-It frees your hands to do other things, but can do harm to your patient if incorrectly used
-Most general practices do not own this piece of equipment so don’t want to become dependent on having one
What are 6 reasons a patient may require mechanical ventilation?
- Respiratory center depression (drugs, toxins, acidosis, head trauma)
- Inability to adequately expand thorax (drugs, pain, chest wall trauma, etc.)
- Airway obstruction (foreign object or body fluid, nerve damage, trauma, etc.)
- Inability to adequately expand lungs (pneumothorax, pleural fluid, diaphragmatic hernia, etc.)
- Cardiopulmonary arrest
- Pulmonary edema or pulmonary insufficiency
What are some specific indications for IPPV during anesthesia?
-Thoracic sx (lungs can’t inflate when chest is open, IPPV minimizes spontaneous chest wall movements)
-Neuromuscular blocking drugs
-Prolonged anesthesia (> 60 min) (especially in horses)
-Chest wall or diaphragmatic trauma (“flail chest” results in paradoxical breathing)
-Maintain more stable anesthesia plane
-Obesity & special patient positioning
-Control of intracranial pressure
-Convenience (i.e. treatment of hypoventilation to free hands)
In otherwise healthy SA patients, when would you start IPPV?
-When the ETCO2 reaches the mid 50s
Controversy exists about the routine use of IPPV in anesthetized patients, especially horses, just to keep the PaCO2 near _____
40 mmHg
What are the negative effects of mechanical ventilation?
-Neg. pressure not generated during inspiration, so venous return to heart is not enhanced
-IPPV may actually physically impede venous return to right side of heart -> decreased SV, CO, & arterial BP
-Exacerbated by prolonged inspiratory time, PEEP/CPAP, obstruction to exhalation, & excessively rapid respiratory rate
-CV effects can be overcome often w/ expansion of extracellualar fluid volume & admin. of inotropic drugs
With IPPV, excessive or sustained pressure can lead to _____
over expansion & volutrauma -> alveolar membrane disruption, development of interstitial air, & eventual transfer to air to mediastinum, pleural space, or abdomen
IPPV can alter what neurohormonal systems?
-ADH release
-Sympathetic outflow
-Renin-angiotensin axis
-Atrial natriuretic peptide production
What 2 things do mechanical ventilators need?
-Power source & driving force
What is the “driving force” of mechanical ventilators?
-Driving gas cyclically introduced into cylinder, but outside bellows, causing pressure to increase within cylinder
-Inspiratory phase occurs when bellows are compressed & air contained is delivered to the patient
How do mechanical ventilators work?
-Consist of compliant pleated compressible bellows connected to anesthesia breathing circuit
-Bellows are contained within airtight rigid plastic cylinder the can be pressurized to compress bellows
-Driving gas introduced into cylinder, but outside bellows, causing pressure to increase in cylinder
-When pressure is released, process reverses & elastic recoil of lungs causes bellows to expand during expiratory phase
What is a double-circuit ventilator?
Refers to two gas sources:
-Driving gas circuit outside bellows which compresses bellows
-Patient gas circuit inside bellows that originates in anesthesia machine & provides O2 & anesthetic gasses to patient
Why is the VT for IPPV usually increased above normal spontaneous VT?
-To compensate for pressure-mediated increases in volume of breathing system & airway
What patients on IPPV may need to have an increased respiratory rate to maintain VE without creating excessive inspiratory pressures?
-Patients w/ lung trauma, diaphragmatic hernia, or gastrointestinal distention
What s the Tidal Volume (VT) guideline for IPPV in small & large animals?
-SA: 10-20 mL/kg
-LA: 15 mL/kg
What is the inspiratory time guideline for IPPV in small & large animals?
-SA: 1 to 1.5 seconds
-LA: 1.5 to 3 seconds
What is the I:E ratio IPPV guidelines for small & large animals?
-SA: 1:2 to 1:3
-LA: 1:2 to 1:4.5
(Basically want to spend twice as long in expiration vs. inspiratory to allow blood return to heart)
What is the Peak Inspiratory Pressure (PIP) guideline for IPPV in small & large animals?
-SA: 15 to 20 cm H2O
-LA: 20 to 30 cm H2O
What is the Respiratory Frequency (f) IPPV guideline for small & large animals?
-SA: dogs= 8-14 bpm, cats= 10-14 bpm
-LA: horses & cows= 6-10 bpm, pigs & small rumin.=8-12 bpm
Q: You have a 70 kg MN Boer that is hypoventilating under inhalant anesthesia. You would like to set up a mechanical ventilator. What settings should you select?
A. VT= 700 mL; I:E ratio = 1:2; PIP = 15 cmH2O; f=6 bpm
B. VT = 900 mL; I:E ratio = 1:2; PIP = 15 cmH2O; f= 24 bpm
C. VT = 1050 mL; I:E ratio = 1:3; PIP = 20 cmH2O; f=10bpm
D. VT = 1400 mL; I:E ratio = 1:2; PIP = 35 cmH2O; f = 12 bpm
C.
-Can get rid of choice B. b/c f=24 bpm is very fast for goat
-Can get rid of D. b/c PIP= 35 cmH2O id s lot of pressure giving into the lung
What are the 8 steps for general IPPV setup?
- Plug ventilator into oxygen & power source (ideally before induction) & leak test ventilator
- Determine VT resp. rate & PIP needed to achieve effective ventilation. Preset dials if possible
- Empty reservoir bag into scavenging system & replace w/ hose that connects ventilator to anesthesia machine
- Close pop off valve
- Increase O2 flow rate so bellows fill completely, then return O2 flow rate to maintenance level
- Turn on ventilator
- Closely monitor PIP, ETCO2 & chest wall excursions
- Adjust inspiratory flow, resp. rate, & I:E ratio to achieve effective ventilation w/out causing barotrauma
What are the normal values of ETCO2, PaCO2, & PIP?
-ETCO2 ~35-45 mmHg
-PaCO2 ~40 mmHg
-PIP < 20 cm H2O
The amount of gas delivered to a patient during ventilation depends on
-Resistance & compliance of breathing system & patient’s respiratory system
Although inspiratory pressure may not vary over time, the ____ may change as the compliance of the respiratory system changes
VT
What happens after IPPV is discontinued?
-If PaCO2 low, spontaneous ventilation may not return b/c certain level of PaCO2 required to stimulate ventilation
-Opioids, anesthetics, neuromuscular blocking drugs, hypothermia, or hypovolemia may delay return of consciousness & therefore spontaneous ventilation
-Patient should continue to receive supplemental O2 & can be manually ventilated at 1-4 bpm until spontaneous ventilation has returned & stabilized (i.e. normal VT & f)
How are ventilators classified?
-The power source, drive mechanism, cycling mechanism & type of bellows are used to classify anesthesia ventilators
What are the power sources of ventilators?
-Electricity, compressed gas, or both
What is the drive mechanism of ventilators?
-Compressed gas
What is the ventilator classification “volume cycled”?
-Inflate lungs to predetermined volume
What is the disadvantage of volume cycled ventilation?
-Inspiratory pressure may increase if compliance decreases during ventilation
What is the ventilator classification “pressure cycled”?
-Inflate lungs to predetermined pressure
What is the disadvantage of pressure cycled ventilation?
-VT delivered may decrease if respiratory compliance decreases in patient
What is the “time cycled” ventilator classification?
-Inflate lungs for a preset time at a predetermined gas flow rate
What are most anesthesia ventilators classification?
-Time cycled
What is used for IPPV of small animals with room or supplemental O2 components?
-Self-inflating resuscitation systems (e.g. Ambu bag)
____ is inserted on the proximal end of ET tube & delivers IPPV (O2 only) & works on demand from a patient-initiated breath or from operator assistance
-Demand valve
What are recruitment maneuvers?
-Used to reinflate collapsed alveoli by applying sustained pressure above normal PIP & using PEEP to prevent derecruitment
-Induces temp. improvement in lung function in healthy dogs under general anesthesia
