Non-Invasive Monitoring of Ventilation Status

By Bill Lamb, BS, RRT, CPFT, RCP(MO)

NO CEU OFFERED for WEB Based Presentation

Clinicians constantly look for methods to better monitor their patients’ ventilation status. Arterial Blood gas analysis is widely considered the Gold standard to assess acid base and ventilatory status through measurement of arterial pH and carbon dioxide tension (PaCO2). This focus topic’s objectives are to:

1.      Identify Indications for non-invasive monitoring of ventilation status

2.      Define & Discuss Capnography & Capnometry

3.      Discuss the Physiological basis for CO2 Monitoring

4.      Introduce New Generation Cutaneous CO2 monitoring technology

5.      Discuss Clinical Applications

Who to monitor?  As acuity increases and staffing resources continue to decrease, monitoring of high risk patients in becoming more important and is often a risk management consideration. Traditional monitoring of the patient’s Vital signs is an absolute necessity (Breath Rate, Heart Rate, Blood Pressure, EKG, Temperature, SpO2).

Although we all learned this in our clinical training, we often overlook that monitoring a patient’s “ventilatory status” with pulse oximetry often does not recognize hypoventilation until respiratory acidosis is far advanced, especially when the patient is supported with supplemental oxygen. This may result in catastrophe.

       Indications for non-invasive monitoring of ventilatory status by monitoring CO2:

a.       Patients supported by invasive conventional mechanical ventilation

b.      Patients supported with non-invasive mechanical ventilation

c.       Patients supported with High Frequency Ventilation (HFV)

d.      Patients receiving procedural sedation

e.       Patients treated for pain management

f.        High risk operative and post operative patients (e.g. COPD, OSA)

g.       Patients being evaluated for sleep disorders

h.       Any patient where hypoventilation or hyperventilation is at risk or suspected

Capnometry is the measurement & numerical display of the carbon dioxide level appearing in the airway. Often represented as End Tidal CO2 (PetCO2) in mmHg.

Capnography is the measurement AND Graphic display of the carbon dioxide level appearing in the airway. Often represented as the End Tidal CO2 (PetCO2) and graphic waveform of the CO2 throughout the breath.

PaCO2: 1) A zero baseline 2) A rapid sharp up rise 3) An alveolar plateau 4) A well defined end tidal point 5) A rapid sharp down stroke

Commonly used End Tidal CO2 monitors use Infrared Spectroscopy, which compares the amount of light absorbed by a sample of patient gas to the amount of light absorbed in a chamber containing no CO2.  End tidal CO2 sampling methods include:

a.       Mainstream sampling/ where the optical bench is attached to an adapter at the airway. Advantages: No aspiration of water or mucus into the measuring chamber; no lag time; no mixing of gases in a sample tube. Disadvantages to mainstream sampling: often a bulky airway adapter; unable to monitor non-intubated patients without modification; May contribute to significant deadspace; moisture and or secretions can contaminate the airway adapter chamber and cause drift.

b.      Side Stream sampling/ where respiratory gases are continuously aspirated from an adapter placed at the airway and transported to the optical bench located inside the monitor via sampling tubing. Advantages: can utilize a zero reference cell to keep the monitor in calibration; optical bench is less likely to become contaminated; No or little added weight at the airway connection; usually much less deadspace than side stream adapters. Disadvantages of Side Stream technology: Water and or mucus may obstruct flow of gas to the sensor bench; if sampling flow rate is too high relative to the patients tidal volume, inhaled gas may be aspirated along with the exhaled gas and may give false low CO2 readings.

c.       Micro-Stream Technology: where respiratory gases in small sample sizes are aspirated to the optical bench.  Advantages: Offers advantages of mainstream & Side-stream by using smaller sample size, faster response time and less vulnerability to moisture and secretions; calibration and correction for environmental factors.

Key issues in using Capnography: Calibration; Moisture Control; Sample Flow rate; Transit Time; Response time.

Key issues in normal CO2 Physiology:

1.      Combustion of food by cellular metabolism to produce energy is the source of physiologic CO2 in the body.

2.      CO2 is transported in venous blood for elimination in the lungs in three principle forms:

a.       5-10% is carried in physical solution and reflected by the PCO2 tension

b.      20-30% is bound to blood proteins as carbamino compounds. The protein that has the greatest share of CO2 is Hemoglobin.

c.       60-70% of Co2 is carried as bicarbonate ion after conversion in the red blood cell using carbonic anhydrase

See figures below

Challenges to monitoring our patients’ ventilation status using End Tidal CO2 (PetCO2): when End Tidal CO2 monitors were initially released in the 1980’s, they were exciting but largely misunderstood. Unfortunately, this technology continues to be misunderstood and often discarded because end tidal CO2 values often do not correlate or trend with Arterial PaCO2 measurements. To understand the issues relative to end tidal CO2 and its relationship to arterial CO2 tension, it is important to understand the arterial to alveolar carbon dioxide difference (a-ADCO2).  In normal physiology (and ventilation and perfusion relationships), the arterial to Alveolar CO2 difference is 2-3 mmHg. In patients presenting with normal physiology and ventilation and perfusion relationships, End Tidal CO2 measurements generally correlate & trend with arterial measurements; however, if the patient is moderately or severely ill and has a ventilation and perfusion (V/Q) mismatch, End Tidal CO2 often measures much less than arterial CO2 due to the widening of the a-ADCO2 gradient.  COMMON CAUSES OF WIDENING OF THE

 a-DCO2 GRADIENT INCLUDE:     

1.      Incomplete alveolar emptying

2.      Poor sampling

3.      Ventilation-Perfusion (V/Q) Abnormalities

Ventilation/Perfusion abnormalities are frequently the cause of poor correlation of PetCO2 with PaCO2. Normally, the upper lung lobes receive more ventilation than perfusion; the lower lobes receive more perfusion than ventilation. The overall ratio of V/Q is 0.8 and is calculated based upon 4 parts ventilation to 5 parts perfusion. V/Q mismatch is often caused by Shunt or Deadspace conditions:

a.       Deadspace Ventilation: V/Q ratio is high and is resultant of ventilation in excess of or with less than normal perfusion. Deadspace ventilation results in a widening of the a-ADCO2 and may be caused by pulmonary embolism, systemic hypovolemia, cardiac insufficiency, and cardiac arrest.                               

 

All to often, the clinician discards End Tidal CO2 monitoring due to lack of correlation with the Arterial Blood Gas. With widening of the a-DCO2 gradient, it is not unusual to experience 20 mmHg or greater differences with PetCO2 significantly less than arterial values. In cases where V/Q is improving, an increase in PetCO2 may reflect a decrease in PaCO2.

TRANSCUTANEOUS CARBON DIOXIDE MONITORING:

Recent advances in technology have resulted in significant improvement in digital monitoring of CO2 transcutaneously. Newer technology provides clinicians with safe and reliable CO2 values in the majority of patient populations. Indications for Transcutaneous CO2 monitoring: 

a.       Non-Invasive Ventilation (very difficult to measure PetCO2)

b.      High Frequency Ventilation (very difficult to measure PetCO2)

c.       Sleep Diagnostics (very difficult to measure PetCO2)

d.      Procedural Sedation (very difficult to measure PetCO2)

e.       Pain Management (high risk patients -JCAHO Sentinel Alert,

f.         And very difficult to measure PetCO2)

g.       Patient supported with mechanical ventilation

h.       High-risk patients (Asthma, COPD, etc.)

The SenTec Digital Monitor is approved for Neonatal through Adult patients and measures SPO2 and PtcCO2 via an ear lobe sensor or PtcCo2 via a flat skin mount using the Severinghause principle. The sensor does not generate significant heat, therefore, the site is inspected and the sensor calibrated every eight hours. CO2 measurements via the SenTec Digital Monitor System (SDMS) have been independently shown to be within - 3 mmHg of the PaCO2 measurement. The ear lobe site is close to the heart, is a central site and is close to the brain. It is well accessible during surgery; it is a low motion site and provides a secure sensor fixation.

The SenTec Digital Monitoring system provides three parameters: PtcCO2, SpO2 and Pulse Rate. The advantages of using the SDMS are:

1.      Good correlation with ABG’s PaCO2

2.      Non-Invasive and easy to stabilize

3.      Fast response time to saturation and CO2 changes

4.      V/Q mismatch does not usually bias the reading unless severe blood pressure drop

5.      Low motion affected site

6.      Easily accessible during most medical procedures

For more information and or comments, please contact your Bemes Representative or Bill Lamb. There is also additional information regarding the SenTec elsewhere on the Bemes website.