Exam 1- Capnography (6/13/23) Flashcards
Effects of Hypercarbia
- Respiratory acidosis can develop over time
- Increases cerebral blood flow (CBF)
- Increases ICP in susceptible patients
- Increases pulmonary vascular resistance, vasoconstriction
- Potassium shifts from intracellular to intravascular
Effects of Hypocarbia
- Respiratory alkalosis
- Decreases CBF
- Decreases pulmonary vascular resistance, vasodilation
- Potassium shifts to the intracellular space
- Blunts normal urge to breathe
Capnography provides information primarily on ventilation but can give info on:
- Pulmonary blood flow
- Aerobic metabolism
- Placement of ETT/LMA (presence of ETCO2)
- Integrity of breathing circuit
- Estimates the adequacy of cardiac output
What is the Bohr equation used to calculate?
- Physiological dead space
Define dead space.
- Volume of each breath inhaled that does not participate in gas exchange
Differentiate anatomical and physiological dead space.
- Anatomic – conducting zones of the airway (nose, trachea, bronchi)
- Physiologic – airway dead space + alveolar dead space
Define alveolar dead space
- Portion of the physiologic dead space that does not take part in gas exchange but is within the alveolar space
Conditions that increase alveolar dead space (V/Q mismatching)
- Hypovolemia
- Pulmonary hypotension
- Pulmonary embolus
- Ventilation of nonvascular airspace
- Obstruction of precapillary pulmonary vessels
- Obstruction of the pulmonary circulation by external forces
- Overdistension of the alveoli
Measurement and quantification of inhaled or exhaled CO2 concentrations.
- Capnometry
What is capnography?
- Method of CO2 measurement and a graphic display of time
- Detection of CO2 breath-by-breath
- Best method to confirm endotracheal intubation
What is time capnography?
- Pressure vs time plot; most common representation
- CO2 concentrations digitally reported as ‘inspired’ and ‘end tidal’
High-speed vs. low-speed time capnography.
- High-speed – user can interpret information about each breath
- Slow-speed – appreciation of the expired and inspired trend
What is the most common gas sampling system?
- Side-stream gas analyzer (time delay)
What phase on a capnograph will an ETCO2 be measured at?
- ETCO2 measured at the end-point of phase 3.
What can increase ETCO2?
What can decrease ETCO2?
Difference between PaCO2 and ETCO2 is approx ____ mmHg.
- 5 mmHg
Ex: ETCO2 of 35 mm Hg = PaCO2 of approx. 40 mm Hg
Problems that increase the difference between PaCO2 and ETCO2.
- V/Q mismatching increases the difference between PaCO2 and PACO2 (PE, endobronchial intubation)
- Breathing patterns that fail to deliver alveolar gas at the sampling site, increases the difference between PACO2 and true ETCO2 (alveolar gas) - COPD
- Problems with the capnograph increase the difference between true ETCO2 (alveolar gas) and measured ETCO2 (capnograph)
CO2 measurement most commonly relies on _____ light absorption techniques.
- Infrared (IR)
The greater the CO2 in the sample, the less IR light that reaches the detector
Describe the color change with a CO2 chemical indicator.
- Purple – No CO2
- Yellow – CO2
Sensitive to low levels of CO2
ETT placement still needs verification by alternative means
What are the CO2 monitor requirements/ standards?
- CO2 reading within +/- 12% of actual value
- Manufacturers must disclose interference caused by ethanol, acetone, halogenated volatiles
- Must have a high CO2 alarm for inhaled and exhaled CO2
- Must have an alarm for low-exhaled CO2
Information that can be interpreted from a time capnograph?
- Interpreting CO2 values
- Approximate blood CO2 levels
- Pulmonary blood flow
- Alveolar ventilation
- Differential diagnosis of loss of exhaled CO2 (esophageal intubation, cardiac arrest)
What are the inspiratory and expiratory segments of a normal capnograph?
- Inspiratory – Phase 0
- Expiratory – Phases I, II, and III
Describe Phase I of a normal capnograph.
- Baseline
- Exhalation of anatomic dead space and the apparatus (ETT, LMA)
- Essentially no CO2
- 1/3 of tidal volume is exhaled
Describe Phase II of a normal capnograph.
- Expiratory upstroke begins (CO2-rich alveolar gas)
- Sampling of alveolar gases
- Normally steep uprise
Describe Phase III of a normal capnograph.
- Alveolar Plateau phase
- Normally representative of CO2 in alveolus
- Can be representative of ventilation heterogeneity, slightly increasing slope
Describe Phase 0 of a normal capnograph.
- Sometimes called phase IV
- Inspiration of fresh gas, remaining CO2 washed out
- Downstroke returns to baseline
Describe the Occasional Phase IV (Phase IV Prime) of a capnograph.
- A sharp upstroke in PCO2 at the very end of phase III
- Upstroke probably results from the closure of lung units with lower PCO2
- Allows for regions with higher CO2 to contribute to more of the exhaled gas sample
- Seen in pregnant and obese pts
- Decreased FRC and lung capacity
Describe the alpha angle of the capnograph.
- Separates phase II and phase III
- 100 – 110 degrees
- Angle increases with an expiratory airflow obstruction
- Ex: COPD, bronchospasm, or kinked ETT
Describe the beta angle of the capnograph.
- Separates phase III and phase 0
- 90 degrees
- Angle increases with malfunctioning inspiratory unidirectional valves, rebreathing, and low tidal volume with rapid respiratory rate
Describe the capnograph
- Normal Capnograph
- Mechanical Ventilation
Describe the capnograph
- Normal Capnograph
- Spontaneous Ventilation
What is the issue with this capnograph?
- Inadequate Seal around ETT
What is causing this capnograph?
- Faulty Inspiratory Valve (top capnograph)
- Rebreathing (bottom capnograph)
What is causing this capnograph?
- Sample line leak
Take note that this a small wave form and that ETCO2 does not even reach 40 mmHg
What is causing this capnograph?
- Hyperventilation
- Gradually decreasing waveforms
What is causing this capnograph?
- Hypoventilation
- Gradually increasing waveforms
What is causing this capnograph?
- Airway obstruction
What is causing this capnograph?
- Cardiac oscillation
- Often seen in pediatric patients, the heart is close to the trachea
What is causing this capnograph?
- Re-breathing soda lime exhaustion
Take note that rebreathing is occurring. The capnograph does not return to baseline.
What is causing this capnograph?
- NMBD’s wearing off
- Presence of a “curare cleft”
What is causing this capnograph?
- Over-breathing
- Notice the spontaneous breath between the mechanical breath
What is causing this capnograph?
- Esophageal intubation
