C02 & Ventilation Lecture 7

• Ventilation is the movement of gas in and out of the alveoli
• Respiration is the process by which oxygen is supplied to and used by the tissues, also carbon dioxide is eliminated from the tissues

Indicators of ventilation: Respiratory Rate (RR)

• Number of respirations (breaths) per minute (rpm)

Monitoring RR

• Visually by watching chest wall or reservoir bag movements
• Mechanically with apnea monitor or capnograph
• A decrease during anesthesia can cause complications

Indicators of ventilation: Tidal Volume (VT)

• The normal amount of air inhaled during a breath equals Tidal Volume
• Tidal Volume = 15 mL/kg
• Bag choice is important as it factors in Vt
• Bag choice = (15mL/Kg) x 6

Monitoring Tidal Volume

• Visually by watching chest wall or movements of reservoir bag
• Mechanically with a device called a respirometer or apnea monitor

Indicators of ventilation: Respiratory Character

• Includes the depth, effort, and pattern required to breathe
• Can be monitored visually by watching the chest wall movements
• Time relationship is measured between inspiration and expiration
• Listen to the chest for harsh noises, whistles, or squeaks

Key Terms:

• Dyspnea: labored or difficult breathing
• Apneustic respiratory pattern: prolonged pause between inspiration and expiration
• Effort in the abdomen, or paradoxical movement of chest with abdomen

Indicators of Ventilation: Capnography

• Measures the carbon dioxide in inhaled and exhaled gases
• Value closely mirrors arterial CO2 (PaCO2)
• The capnometer provides the number value of CO2
• The capnograph provides the waveform

Uses of Capnography

• Provides RR - rpm
• Can be used to confirm proper tracheal intubation

Mainstream capnograph

• Sensor is placed directly between the endotracheal tube and breathing circuit

Sidestream capnograph

• Sensor is located in a computerized monitor away from the patient end
• Air is pulled in through a tube attached to the fitting between the endotracheal tube and breathing circuit

Physiological Processes

• Oxygen is inhaled and diffuses out of alveoli then into blood
• Oxygen attaches to arterial RBC Hb and travels around body/organs
• Aerobic cellular metabolism occurs and CO2 attaches to venous blood RBC
• CO2 diffuses into lug, is exhaled

CO2 Levels and Factors

• CO2 levels are determined by 3 factors:
  • Rate of cellular metabolism
  • Rate of transport to the lungs, think blood
  • Rate of elimination from the lungs, think breathing

ETCO2

Therefore ETCO2 is influenced by metabolism, perfusion, & ventilation

• Any change in metabolism, perfusion, or ventilation, as well as equipment malfunction, affects ETCO2 levels and/or the waveform configuration

Why is CO2 Monitoring Important?

• Adequacy of gas exchange (how well the patient is moving gas) and ventilation
• Insight to blood pressure and pulmonary perfusion (blood flow at lung capillaries where O2/CO₂ exchange occurs)
• Information on heart function and cardiac output
• Proof/verifies metabolism (b/c aerobic cellular metabolism's byproduct is CO₂)
• Immediate awareness of apnea/airway obstruction (capnogram shape/trace)
• Verifies endotracheal intubation (no trace or place)
• Verifies equipment set-up with a leaky ETT cuff, deadspace, defects in hoses, exhausted CO2 absorbent, low 0₂ flow, flutter valves stuck

Effective Capnogram Interpretation

• Requires evaluation of 4 aspects:
  • The capnometer baseline value: Fi/INCO₂ number
  • The capnometer value: ETCO2 number
  • The capnogram: waveform shape
  • The rate at which changes occur (suddenly, rapidly, or gradually)

Capnograph Waveform

• Waveform Shape is related to:
  • Beginning of inhalation
  • Inspiratory downstroke (rapid replacement of expired gas by inspired fresh gas at the level of the sampling part)
  • Inspiratory baseline (transition from end of inhalation to beginning of exhalation during which only fresh gas is sampled)
  • Expiratory upstroke (rapid replacement of dead-space fresh gas by alveolar gas)
  • Expiratory plateau (exhalation of pure alveolar gas and expiratory pause)
  • End of expiratory pause (End-tidal CO2 is measured here)

Capnogram Phases

• ETCO2 on the capnometer number comes from the ß angle (phase III expiratory plateau)
  • The end of expiration provides the highest accurate representation of the end-tidal CO2 level

Things To Consider Regarding Monitors

• Response time of real breath vs displayed capnography is slightly delayed on the monitor display
  • Delay in showing the measurement is due to two separate delays: Transit Time and Rise Time
• Transit time of sampled CO, is another delay due to distance it must travel
• ADS Choices to reduce delays by:
  • Using most appropriate ETT tube to limit deadspace (shorter)
  • Using a narrow CO₂ sampling line

Calibrating the capnograph monitor

• Calibrating increases accuracy and helps with repair
• This can be tested by making sure the waveform is at baseline & INCO2 is 0mmHg
• Can exhale into the adaptor to ensure the user gets a normal reading of their expired CO2 (but its not precise, dilution vs variation in normal ranges, technical error)
• Most machines can perform a “zeroing” or calibration process to ensure their accuracy
  • A zeroing will calibrate to room air (like pressing tare on a scale)

Ventilation Normals

• Minute Volume = Tidal Vol (Vt) x Respiratory Rate
  • Vt is 15 mL/kg (10-15 actually)
  • Respiratory Rate is species dependent

Adequacy of Ventilation

• Normal End Tidal Carbon Dioxide has an ETCO2 = 35-45 mmHg

CO2 Monitoring and Implications

• Low expired CO₂ <35mmHg indicates Hypocapnea
  • Can be cause by not intubating in the trachea, tachypnea or increased Vt, too much IPPV, a fall in body temp, poor cardiac output, arrest, decreased brain perfusion, vasoconstriction, decrease in metabolic rate, increased pH in blood (alkalosis)
• High CO₂ >45mmHg indicates Hypercapnea
  • Can be caused by inadequate ventilation/gas exchange, respiratory depression (low tidal volume, low rate), deep anesthetic plane (opioids too), decreased perfusion, contributes to tachycardia, raise in body temperature, increased metabolic rate, decreases pH in blood(acidosis)

Strategies for Managing Hypoventilation

• aka hypercapnia >35mmHg ETCO2

Hypoventilation Troubleshooting

• Check/try to lighten plane by reducing inhalant to give to the minimal % required for what is happening (clipping vs intense surgical stimulus/pain)
• Consider that the patient could be rebreathing CO2
  • Check oxygen flow, increase to rule in/out (esp. on NRB)
  • Check dead space, reduce as much as possible (preparation)
  • Check soda lime/CO2 absorbent
  • Check unidirectional flutter valve is stuck or not moving well, or any condensation
• If >45 mmHg, give IPPV to improve gas exchange (as needed)
• Consider the respiratory depressive effects of drugs (e.g. opioids) and support your patient with IPPV (to improve gas exchange by improving tidal volume & rate), titrate doses to effect or start low, & reduce inhalant when possible
• Correct any abnormals, hypotension will lead to ventilation issues (everything is connected)
• Physiological processes/diseases can produce excessive CO2 such as fevers, thyroid issues or malignant hyperthermia

Cautions for Lowering CO2

• What drives the body (brain) to take a breath is low oxygen or high C02 levels
• If IPPV is used and CO2 is lowered too much ,apnea may occur
• Must find out what CO2 level (ETCO2 number) keeps them breathing on their own
  • It may be safe to allow ETCO2s to approach 55mmHg while maintaining permissive hypercapnia

Permissive Hypercapnia

• Used to tolerate a hypercapnic state of 45 - 55 mm Hg
• For dogs, can be permissive at 60mmHg
• May have beneficial effects with improved cardiac output, arterial vasoconstriction
• If >60 mmHg this is not okay!

Situations for Using Capnography

• Hypercapnea and Hypocapnea
• Rebreathing of CO2
• Esophageal intubation
• Partial Obstruction or bronchospasm
• Full Obstruction / Apnea / Resp arrest

Indications for Capnography

• Minute volume, Ventilation and ETCO2

Other Parameters Related to Ventilation

• Rate and depth related to arterial blood gases, mucus membrane color or pulse oximetry