Mechanical Ventilation Flashcards
Define the trigger variable
Parameter that initiates inspiration; how the ventilator determines when to deliver a breath
What is the trigger variable in patients not trying to breathe? Trying to breathe?
Time; a change in airway pressure or gas flow as a result from the patients attempt to breathe
What is the cycle variable?
Parameter by which ventilation is terminated; typically time; determined by preset RR and I:E ratio. I of 1 commonly recommended
What is the limit variable? How does it differ from the time variable?
Parameter that the breath cannot exceed during inspiration; it does not terminate the breath
What is the baseline variable?
Controlled during exhalation; airway pressure most common baseline variable manipulated
What are the 3 possible breath patterns on a ventilator?
Continuous mandatory ventilation, continuous spontaneous ventilation, intermittent mandatory ventilation
In continuous mandatory ventilation the ventilator is responsible for__________________.
All components of the breath.
Assist controlled ventilation provides _____________ and is used in patients with ____________ or ____________.
Maximal support of the respiratory system; severe pulmonary disease or no respiratory drive.
Define continuous spontaneous ventilation
Ventilation is triggered and cycled by the patient
________________ is the breath pattern used in patients with a reliable, adequate respiratory drive
Continuous spontaneous ventilation
CPAP is most appropriately used when?
As part of a spontaneous breathing trial to determine a patients suitability for weaning
What is PSV?
Pressure support ventilation; the ventilator augments inspiration during spontaneous breaths by increasing airway pressure
_________ is designed for use in patients with normal respiratory drive but inadequate ventilator ability
PSV (usually with CPAP)
What is SIMV? When is it used?
Synchronized intermittent mandatory ventilation - machine tries to synchronize mandatory breaths with patient inspiratory efforts; unreliable respiratory drive or those that do not require maximal respiratory support
What is the basic definition of a ventilator mode?
Identification of a control variable and the breath pattern (ideally including any phase variables)
What factors affect the I:E ratio?
inspiratory time, % inspiratory time, inspiratory flow rate in conjunction with respiratory rate
Why is an I:E ratio of 1:2 ideal?
To ensure the patient has fully exhaled before the next breath
What is breath stacking?
Animal is not able to fully exhale before the next breath, some degree of pressure/air remains in the alveoli at the beginning of the next breath
CPAP does what to lung function parameters?
Increases FRC and compliance, enhances gas exchange and oxygenation (does not augment airflow during inspiration more than baseline pressure provides)
CPAP alone is only suitable when?
In patients with a strong respiratory drive and adequate ventilatory function
PEEP does what to FRC?
Increases it
What is intrinsic vs extrinsic PEEP?
Extrinsic set by machine, intrinsic develop as a consequence of inadequate exhalation time or small airway collapse during exhalation
What are some positive effects of PEEP?
Recruit collapsed alveoli, prevent collapse of unstable alveoli, improve V/Q, improve pulmonary compliance, reduce work of breathing, reduce VALI
How does PEEP reduce VALI?
Preventing injury associated with cyclic re opening and collapse of alveoli with each breath
What are some possible detrimental effects of PEEP?
Decrease compliance (if excessive), overdistention of healthy alveoli (have higher compliance already, increase risk of VALI), increased alveolar dead space (obstruction of alveolar capillary flow), increased PVA, decr LV compliance, reduced CO from impaired venous return during expiration
Causes of low airway pressure alarm
Leak in the circuit, patient disconnected from circuit
Causes of high airway pressure alarm
VC- sudden decrease in pulmonary compliance (endobronch intubation, pneumo), increased system resistance from obstruction in circuit or airways. Patient-ventilator asynchrony
What should high and low airway pressure alarms be set at?
Low - 5-10 cm H2O below peek airway pressureHigh - 10 cm above PAP is reasonable
What is lung protective ventilation?
Low tidal volume, moderate to high PEEP, +/- permissive hypercapnia
List some potential adverse side effects of lung protective ventilation
Increases in ICP, acidemia, PEEP associated cardiovascular compromise
List 3 tools to assess PEEP effectiveness
Pressure-volume curve, monitoring changes in gas exchange (increases in PaO2, decreases in PCO2), CT
What are recruitment maneuvers?
Used in the ARDS lung - transpulmonary pressure (distending pressure of the lung) increased transiently to recruit collapsed alveoli
What are the 4 main determinants of mean airway pressure
Pressure to overcome circuit and airway resistance, pressure to deform lung and expand alveoli, pressure throughout expiratory flow phase, and PEEP
In volume control what shape is the flow waveform?
Square or descending ramp
In spontaneous breathing the flow waveform will typically be what shape?
Sine
In pressure control what shape is the flow waveform typically?
Exponential decay
What is a drawback of the constant flow approach?
Modestly higher PIP compared to a decelerating ramp approach to deliver the same tidal volume
What information can you gain from the volume scaler?
Rapid qualitative picture of the relative size of spontaneous and mandatory breaths during SIMV or pt effort with CPAP,
In mechanically ventilated patients, expiration is usually an _____ (active or passive) process
Passive process. It is driven by the elastic recoil of the respiratory system
True or False: There is a significant difference in the duration of weaning, rates of intubation, tracheotomy, or mortality between SIMV and ACV
FALSE
There is no difference
True or false: If using ACV as compared with PSV to achieve a set peak airway pressure there is no difference in tidal volume or respiratory rate
FALSE
Tidal volume was significantly higher and machine respiratory rate significantly lower with PSV.
This indicates that peak airway pressure during ACV is not an appropriate variable to adjust PSV for similar levels of assistance.
True or false: The work of breathing and the respiratory rate vary inversely with the PSV level
TRUE
What are possible causes of a high pressure alarm?
patient coughing, increased airway resistance, decreased compliance, need for airway suctioning, patient biting the ET tube, or bronchospasm
What does the ventilator do when a pressure alarm goes off?
it ends inspiration and stops flow to the patient
what is the normal setting for a high pressure alarm in regards to PIP?
usually 10 cm H2O above PIP
what is the normal setting for the low pressure alarm in regard to PIP?
5-10 cm H2O below PIP
what does a low pressure alarm indicate? what are possible causes?
pressure has fallen significantly; usually associated with a leak in patient-ventilator circuit
which waveform is useful in deciding if a leak is present?
the volume-time waveform; if it resembles an igloo then the inspired Vt is greater than expired Vt
what is the recommendation for placement of connection tubes to prevent false pressure readings or alarms going off?
place the connection tubes above the ET connection to prevent entry of water and secretions; low flow can cause a false pressure reading on the ventilator
where do leaks most often occur?
around the humidifier, through humidifier water-feed lines, at water traps, anywhere tubing connections join
others: etCO2 monitor, closed suction catheter, in-line thermometers
Differentials for abrupt changes in HR during ventilation?
stress, pain, anxiety, infarction, hypoxia, drug reaction, hypoxemia, hypercapnea, disconnection from ventilator
what are 3 areas that measure core body temperature?
esophageal, rectal, pulmonary artery
what happens to the CVP measurement during a positive pressure breath?
it increases- in ventilated patients measure at the end of expiration when intrapleural pressure returns to normal or is lowest
physical examination of the chest should include what?
inspection, palpation, percussion, auscultation
what are differentials for absence of breath sounds?
pneumothorax, complete airway obtruction, complete lung collapse, improper placement of ET tube, pleural effusion
what number should intracuff pressures not exceed?
20-25 mm Hg/27-34 cm H2O
t/f- tracheal damage and necrosis can occur if the cuff pressure transmitted to the trachea is greater than the perfusion pressure to the trachea?
true
t/f- handheld cuff pressure monitors are typically very accurate?
false
a high volume/low pressure ET tube cuff should require less than how many ml of air to inflate the cuff?
5 ml
5 steps to minimize risk of tracheal necrosis associated with cuff overinflation?
- use minimal leak technique when possible
- allow 50-100 ml of Vt to be lost during inspiration
- Use
list 2 reasons why a higher cuff pressure may be required to maintain occlusion
the cuff and ET tube may be too proximal, closer to larynx
may need larger ET tube
patient’s trachea may be dilated from cuff damage
what is the most common cause of a low or nonexistent cuff pressure?
leak in ET cuff or pilot system
how often should the ET tube be repositioned in the mouth?
8-12 hrs
list causes of decreased Cs (static compliance)
air trapping, pulmonary edema, ,atelectasis, consolidation, pneumonia, pneumothorax, hemothorax, pleural effusion
during pressure controlled ventilation, increased tidal volume delivery with the same pressure indicates what?
improved compliance or decreased airway resistance
total resistance is the sum of what?
airway and tissue resistance
what can cause increased tissue resistance?
ascites, pleural effusion
what is the shape of the P-V curve for a patient with ARDS?
sigmoid
True or false, when the ventilator is patient triggered, the pressure or flow required to trigger the ventilator should be no more than -1 or -2 cm H20
TRUE
No more than this should be required to trigger the ventilator
If a ventilator is flow triggered, what sensitivity should it be set at?
Sensitivity should be around a flow change of 2 to 3 L/min
What changes can you make to reduce auto-PEEP?
Increasing the flow (reducing inspiratory time)
Reducing tidal volume
Reducing the rate (reducing Ve)
Suctioning the patient
Changing the mode to allow for more spontaneous breaths
What is compressible volume?
Volume loss within the patient circuit to the effects of tubing compliance
How do you calculate compressible volume?
PIP x Ct
Ct = tubing compliance
What two factors must be considered when determining alveolar ventilation?
Anatomic dead space
Mechanical dead space
A consistent Vt with an increasing Ppeak is suggestive of what possible things?
Either reduction in lung compliance (Cl) or increase in airway resistance (Raw)
What value should the Pplateau be kept below?
Below 30 cm H20 to avoid ventilator induced lung injury
What is the equation for transairway pressure (Pta)?
Pta = PIP - Pplateau
What is end expiratory pressure (EEP)?
The lowest pressure measured during the expiratory phase of a breath
Below what oxygen saturation are pulse oximeters inaccurate?
Below 80%
Name 4 things that can interfere with the reading on a pulse oximeter
Low perfusion Abnormal hemoglobin (methemoglobin or carboxyhemoglobin) Skin pigmentation Ambient light interference Motion Low oxygen saturation (
What is fractional hemoglobin saturation?
The amount of oxyhemoglobin divided by the sum of the concentrations of all four types of hemoglobin
Fractional O2Hb = O2Hb / [HHb + O2Hb + COHb = MetHb]
What is functional hemoglobin saturation?
The amount of oxyhemoglobin divided by the concentration of hemoglobin capable of carrying oxygen
Function O2Hb = O2Hb / [HHb = O2Hb]
Do pulse oximeters measure functional or fractional hemoglobin saturation?
They measure functional. They are only capable of using two wavelengths to quantify the amount of HHb and O2HB. CO-oximeters can measure all four types of hemoglobin so can measure fractional hemoglobin saturation.
True or False: The presence of COHb will lead to an underestimation of SpO2
FALSE
It will lead to an OVERestimation of SpO2
How does methemoglobinemia affect the SpO2 value?
MetHb can absorb both red and infrared light. If enough is present to absorb all pulsatile absorption, the pulse oximeter will measure a red-to-infrared ratio of 1:1, corresponding to an SpO2 of 85%
What effect does skin pigmentation have on SpO2?
Darker skin pigmentation will read a higher SpO2
True or False: Hyperbilirubinemia affects the SpO2 measurement
FALSE: it does not appear to affect the pulse oximeter reading
On the oxyhemoglobin dissociation curve, what things cause increased affinity of hemoglobin for oxygen and what effect does this have on the curve?
Acute alkalosis Decreased PCO2 Decreased temperature Low levels of 2, 3 DPG Carboxyhemoglobin Methemoglobin Abnormal hemoglobin
Shifts the curve to the LEFT
On the oxyhemoglobin dissociation curve, what things cause decreased affinity of hemoglobin for oxygen and what effect does this have on the curve?
Acute acidosis High CO2 Increased temperature High levels of 2, 3 DPG Abnormal hemoglobin
Shifts the curve to the RIGHT
How is etCO2 measured with an infrared capnometer?
The concentration of CO2 is estimated as its concentration is directly related to the amount of infrared light absorbed.
What is pressure broadening and what effect does it have on the CO2 reading?
The presence of other gases such as O2 and N2O because the peak absorbance of infrared light by CO2 lies between the peak absorbance for O2 and N20.
Pressure broadening results in falsely high CO2 reading
How do we minimize the effect of pressure broadening?
Removing water vapor from the case sample before it is analyzed, and by using electronic filters to subtract the infrared absorption by gases other than CO2
What factors influence the etCO2?
Alveolar PCO2
VCO2 (CO2 production which is dependent on metabolic rate)
Effectiveness of ventilation
What is the single-breath CO2 curve?
It is produced by the integration of airway flow and CO2 concentration and is presented on a breath to breath bases.
On a single-breath CO2 curve, what do the following areas represent: Area X, Area Y, Area Z
Area X: Actual amount of CO2 exhaled
Area Y: Amount of CO2 that is not eliminated because of alveolar dead space
Area Z: Amount of CO2 that is not eliminated because of anatomic dead space
What four factors influence the amount of CO2 exhaled?
CO2 production
Perfusion of the lungs
Diffusion
Ventilation
On the capnogram, what are the four phases?
Where on the curve is etCO2 measured?
Phase I: Initial gas is exhaled from the conducting airways
Phase 2: Alveolar gas containing CO2 mixes with gas from the anatomic airways and the CO2 concentration rises
Phase 3: The curve plateaus as alveolar gas is exhaled
Phase 4: Inspiration, concentration of CO2 falls to zero
etCO2 is measured at the end of the alveolar phase or phase 3
what conditions can lead to reduction of exhaled nitric oxide?
systemic hypertension, pulmonary hypertension, cystic fibrosis, sickle cell anemia, ciliary dyskinesis
what conditions lead to increased levels of exhaled nitric oxide?
asthma, bronchiectasis, airway viral infections, alveolitis, allergic rhinitis, pulmonary sarcoidosis, chronic bronchitis, pneumonia
what effect does NO have on the airways?
potent dilatory effects on pulmonary vessels and airways; facilitates coordinated beating of ciliated epithelial cells
how do transcutaneous monitoring systems work (i.e. how are they different from SpO2 and capnography, which use spectrophotometric analysis)
uses modified blood gas electrodes to measure the O2 and CO2 tensions at the skin surface
List examples of hypo and hypermetabolic states
Hypo: starvation, hypoT4, anesthesia, sedation, coma, hypothermia
Hyper: pancreatitis, hyperT4, drugs, pregnancy, hyperthermia, seizures, burns
what are the most common airway pressure measurements that most ventilators display?
peak inspiratory pressure, static/plateau pressure
List differentials for sudden increases in peak inspiratory pressure
bronchospasm, mucus plugging, partially blocked heat/moisture exchanger, incorrect ET size, water in ventilator tubing, malfunctioning expiratory valves, barotrauma, VILI
What is the name of the law that states that a twofold decrease in airway diameter will result in a 16x increase in airway resistance?
Poiseuille’s law
Describe and give examples of intrinsic vs extrinsic work of breathing
Intrinsic: work done to overcome normal elastic and resistive forces; work to overcome disorder or disease affecting lung/thorax (ex: fibrosis of lungs)
Extrinsic: work imposed by systems that are added to the patient (ex: ET tube, machine sensitivity, humidification device)
In a static pressure volume loop what does the upper inflection point on the inspiratory limb represent
the point at which no more alveoli are being recruited
in a static pressure volume loop what does the line from the lower to upper inflection point mean on the inspiratory limb?
it represents the respiratory system compliance
what is the current recommendation for using static pressure volume loops to set PEEP?
set peep 2-3 cm H2O above the upper inflection point detected during deflation of the lungs
What are 3 common indications for applying recruitement maneuvers?
ALI/ARDS, post-op atelectasis, after suctioning maneuvers, after circuit disconnect
How can you ensure the lungs are protected from overdistension when applying recruitment maneuvers?
keep PIP (peak inspiratory pressure) lower than the upper inflection point of the PV loop
At what pressure does lung derecruitment begin? At what pressure does the lung completely collapse?
derecruitment- 8 cm h2o; collapse- 4 cm h2o
the ARDSnet study suggested what level of PEEP very early in the management of ARDS in people?
15 cm h2o
the sigmoid shape of a pressure volume loop suggests what?
that different areas of the lung open at different pressures
What are some adverse effects associated with increased thoracic pressure for extended periods of time (>40 sec)?
decreased venous return, drop in cardiac output, drop in blood pressure; uneven pressure to the lungs causing shifting of blood to other areas of the lungs
what types of things affect chest wall compliance?
forces from overlying ribs and muscles, force from the diaphragm, pressures from abdominal contents, obesity
T/F- higher than normal pressures may be necessary in critically ill and/or obese patients in order to recruit collapsed alveoli?
True- they have decreased chest wall compliance and often increased intra-abdominal pressures
List potential complications associated with lung recruitment maneuvers
Barotrauma, pneumothorax, subcutaneous emphysema, pneumomediastinum, hypotension
Name some types of recruitment maneuvers
Sustained inflation, PC-CMV with high PEEP levels, PC CMV with increased PEEP, recruitment and decremental PEEP, sigh techniques
T/F- recruitment maneuvers are more effective in patients with primary ARDS than secondary ARDS?
False- more effective with secondary ARDS
Name and describe the two categories of ARDS according to Pilbeam
- Indirect: secondary/nonpulmonary, associated with acute systemic inflammation such as acute sepsis or pancreatitis
- Direct: primary/pulmonary, pneumonia, aspiration, near-drownings
List possible chemicals that mediate the inflammatory response in ARDS
reactive oxygen species, nitric oxide, leukotrienes, cytokines, proteases, platelet activating factor, cationic proteins, interleukins (1, 6, 8, 10), TNFalpha
Describe the general shape of a pressure-time scalar for pressure controlled ventilation vs volume controlled ventilation
Pressure controlled is a flat top
Volume controlled is a “shark fin” shape
What flow pattern is seen on the flow-time scalar for a volume controlled breath?
Square shaped
What does air-trapping or auto-peep look like on a flow-time scalar?
Expiratory flow does not return to baseline before the next breath begins
Describe the flow-time scalar for a pressure supported breath?
Decelerating flow pattern, flat-topped airway pressure wave
Describe the flow-time scalar for a synchronized breath?
Triangular shaped pressure wave
How can you tell the difference between a patient triggered controlled or supported breath on a flow-time scalar?
With supported breaths the patient determines the inspiratory time so it will be variable
With controlled breaths, the inspiratory time will be identical
How can you tell that asynchrony is present when evaluating the flow-time scalar?
If the number of triggering episodes is greater than the number of breaths then asynchrony is present
What does the slope of a pressure-volume loop represent?
Dynamic compliance
It should be around 45 degrees
How is compliance defined?
Change in volume in relation to the change in intra-pleural pressure
Name common causes of increase pulmonary resistance
Bronchospasm
Airway secretions
Small-diameter ET tube
Mucosal edema of airways
Name common causes of decreased pulmonary compliance
Pleural space disease Pulmonary parenchymal disease Single-lung intubation Abdominal distension Chest wall disease or deformity
What do the inflection points represent on a static P-V loop?
A sudden change in alveolar recruitment during inspiration and derecruitment during expiration
What does an increase in the width of a P-V loop represent?
Increased resistance
What is hysteresis?
Lag in the change in volume compared with the rate of change in pressure that results from resistance to deformation (elasticity) and resistance of the airways
How can the work of breathing by the patient during mechanical ventilation be indirectly measured?
By plotting esophageal pressures - esophageal pressures are a surrogate for measuring intrapleural pressure and intrapleural pressure changes due to patient work
