Analysis of CO2 waveforms: 3

Descending limb:  
The descending limb can be prolonged or slanted, when the inspiratory valve of a closed circuit system is incompetent.1-3  or because of the use of side-stream sensor capnographs, with a prolonged response time.4 The capnograph with a prolonged response time also results in the prolongation of phase II and is observed commonly in children.4,5   

Ripple effect: At slow respiratory rates it is common to see a ripple effect, referred to as cardiogenic oscillations, superimposed on the expiratory plateau and the descending limb of the capnogram. This arises from small gas movements created largely by the pulsations of the aorta and heart.6-8 

Inspiratory valve malfunction resulting in prolongation of descending inspiratory limb in addition to prolongation of alveolar plateau

Normal	Prolonged inspiratory descending limb due to dispersion gases in the  sampling line or as well as prolonged response time of the analyzer. Seen in children who have faster respiratory rates.

Cardiogenic oscillations. Ripple effect, superimposed on the plateau and the descending limb, resulting from small gas movements produced by pulsations of the aorta and heart.

1.    Berman LS, Pyles ST.  Capnographic detection of anaesthesia circle valve malfunctions. Can J Anaesh 1988;35:473-5.

2.    Van Ganderingen HR, Gravenstein N, Van der Aa JJ, Gravenstein JS.  Computer-assisted capnogram analysis.  J Clin Monit 1987;3:194-200.

3.    Kumar AY, Bhavani-Shankar K, Moseley HSL, Delph Y.  Inspiratory valve malfunction in a circle system: Pitfalls in capnography.  Can J Anaesth 1992;39:997-9.

4.    Pascucci RC, Schena JA, Thompson JE.  Comparison of a sidestream and mainstream capnometers in infants. Crit Care Med 1989;17:560-2

5.    Schena J, Thompson J, Crone RK.  Mechanical influences on the capnogram.  Crit Care Med 1984;12:672-4.

6.    Adams AP. Capnography and pulse oximetery. In: Atkins RS, Adams AP (Eds.). Recent Advances in Anaesthesia and Analgesia.  London: Churchill Livingston, 1989;155-75.

7.    Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y. Anaesthesia and capnometry. (Review article).  Can J Anaesth 1992;39:617-32.

8.    Kalenda Z. Mastering Infrared Capnography. The Netherlands: Kerckebosch-Zeist, 1989

     

Analysis of CO2 waveforms:

Elevated inspiratory base line:            

Normal	Rebreathing -Exhausted soda line - Elevation of the base line

Rebreathing as a result of soda lime exhaustion -Trend and wave capnograms showing gradual elevation of the base line

 

Trend	Waveform

Contamination of CO2 monitor (sudden elevation of base line and top line)	Contamination of CO2 monitor - trend

Inspiratory valve malfunction - Elevation of the base line, prolongation of down stroke, prolongation of phase III

Capnogram during the use of Bain anesthetic circuit. Inspiratory base line and phase I are elevated above the zero due to rebreathing. Note the rebreathing wave during inspiration.  Inspiration =

References:

1.    Pyles ST, Berman LS, Modell JH.  Expiratory valve dysfunction in a semiclosed circle anesthesia circuits - verification by analysis of carbon dioxide waveforms.  Anesth Analg 1984;63:536-7.

2.    Berman LS, Pyles ST.  Capnographic detection of anaesthesia circle valve malfunctions. Can J Anaesh 1988;35:473-5.

3.    Van Ganderingen HR, Gravenstein N, Van der Aa JJ, Gravenstein JS.  Computer-assisted capnogram analysis.  J Clin Monit 1987;3:194-200.

4.    Weingarten M. Respiratory monitoring of carbon dioxide and oxygen:  a ten-year perspective. J Clin Monit 1990;6:217-25.

5.    Kumar AY, Bhavani-Shankar K, Moseley HSL, Delph Y.  Inspiratory valve malfunction in a circle system: Pitfalls in capnography.  Can J Anaesth 1992;39:997-9.

6.    Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y. Anaesthesia and capnometry. (Review article).  Can J Anaesth 1992;39:617-32.

7.    Adams AP. Capnography and pulse oximetery. In: Atkins RS, Adams AP (Eds.). Recent Advances in Anaesthesia and Analgesia.  London: Churchill Livingston, 1989;155-75.

8.    Sweadlow DB, Irving SM.  Monitoring and patient safety.  In: Blitt CD (Ed.). Monitoring in Anesthesia and Critical Care Medicine. 2nd ed., New York: Churchill Livingstone, 1990;33:64.

      

Analysis of CO2 waveforms: 2

Prolongation phase II and III:             
Prolongation or slanting of the expiratory upstroke phase II occurs when there is obstruction to expiratory gas flow (e.g., asthma, bronchospasm, obstructive pulmonary disease, and kinked endotracheal tube,1-9 or in the presence of leaks in the breathing system.10   A sidetream capnograph may allow gas mixing within the sampling tube (dispersion) if sampling rate is slow (50 ml.min-l) or if the tubing is too long or has too wide a bore, or both. Such dispersion of gases can also result in prolongation of the expiratory upstroke.11-13  The slope of the expiratory plateau (phase III) can be increased during pregnancy as a normal physiological variation.5,14  Besides, it can also result from factors that produce obstruction to expiratory gas flow which may also result in a prolonged phaseII.1-9  

Non Pregnant	Capnogram during cesarean section (The slope the expiratory plateau is increased as a normal physiological variation in pregnancy)

Normal	Airway obstruction (eg., bronchospasm). Phase II and phase III are prolonged and alpha angle (angle between phase II and phase III) is increased

Normal	Capnograms recorded with prolonged response time (Base line is elevated, prolongation of phase II and III, prolongation of inspiratory descending limb)

Curare Cleft: 
A dip in the plateau (curare cleft) indicates a spontaneous respiratory effort during mechanical ventilation.5,8,10

Curare cleft A dip in the plateau indicates spontaneous respiratory effort It can also result from surgical manipulations in abdomen

Terminal dip of alveolar plateau:  Dilution of PETCO2 by fresh gas flow (FGF)  in circuits and ventilators using a continuous flow may result in the dilution of expired gases by the FGF’s producing a terminal dip in alveolar plateau. This results in falsely low PETCO2 values.

Normal	Dilution of PETCO2 by fresh gas flow (use of PEEP or CPAP in IMV bird ventilators, can also occur as a result of dilution of expired gases by FGF in rebreathing circuits)

References

1.    Weingarten M. Anesthetic and ventilator mishaps: prevention and detection. Crit Care Med 1986;14:1084-6.

2.    Paloheimo M, Valli M, Ahjopalo H. A guide to CO2 monitoring. Finland: Datex Instrumentarium, 1988.

3.    Van Ganderingen HR, Gravenstein N, Van der Aa JJ, Gravenstein JS.  Computer-assisted capnogram analysis.  J Clin Monit 1987;3:194-200.

4.    Weingarten M. Respiratory monitoring of carbon dioxide and oxygen:  a ten-year perspective. J Clin Monit 1990;6:217-25.

5.    Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y. Anaesthesia and capnometry. (Review article).  Can J Anaesth 1992;39:617-32.

6.     Smallhout B, Kalenda Z. An Atlas of Capnography. 2nd e. Utrecht:Kerckebosch-Zeist, 1981.

7.    Nuzzo PF.  Capnography in infants and children.  Pediatric Nursing 1978;May-June:30-8.

8.   Cote CJ, Liu LMP, Szyfelbein SK, et al.  Intraoperative events diagnosed by expired carbon dioxide monitoring in children.  Can Aaesth Soc J 1986;33:315-20.

 9.   Leigh MD, Jones JC, Mottley HL.  The expired carbon dioxide as continuous guide of the pulmonary and circulatory systems during anaesthesia and surgery.  J Thoracic and Cardiovasc Surg 1961;41:597-610.

10.    Adams AP. Capnography and pulse oximetery. In: Atkins RS, Adams AP (Eds.). Recent Advances in Anaesthesia and Analgesia.  London: Churchill Livingston, 1989;155-75.

11.    Pascucci RC, Schena JA, Thompson JE.  Comparison of a sidestream and mainstream capnometers in infants. Crit Care Med 1989;17:560-2

12      Badgwell JM, Kleinman SE, Heavner JE.  Respiratory frequency and artifact affect the capnographic baseline in infants.  Anesth Analg 1993;77:708-11

13.    Schena J, Thompson J, Crone RK.  Mechanical influences on the capnogram.  Crit Care Med 1984;12:672-4.

14.    Shankar KB, Moseley H, Kumar Y, Vemula V, Krishnan A. The arterial to end-tidal carbon dioxide tension difference during anaesthesia for tubal ligations. Anaesthesia 1987;42:482-6.

     

From (a-ET)PCO2 gradients or differences- Alveolar dead space

Clinical Aspects

Monitoring PaCO₂

Measurements of PETCO₂ constitute a useful non-invasive tool to monitor PaC02 and hence the ventilatory status of patients during anesthesia or in the intensive care unit. In normal individuals, the (a-ET)PC02 may vary from 2-5 mmHg.1-5 The PETCO₂ is even more useful if its relationship to PaC02 can be established initially by blood gas analysis. Thereafter, changes in PaC02 may be assumed to occur in parallel with those in PETCO₂ thus avoiding repeated arterial puncture provided there are no major hemodynamic changes or respiratory abnormalities that may alter alveolar dead space and hence, (a-ET)PC02. (For details -Physiology section)

Monitoring alveolar dead space

The (a-ET)PCO₂ is a measure of alveolar dead space, and changes in alveolar dead space correlate well with changes in (a-ET)PCO₂.1 An increase in (a-ET)PCO₂ suggests an increase in dead space ventilation. Hence (a-ET)PCO₂ is an indirect estimate of V/Q mismatching of the lung.

Monitoring clinical progress of a critical patient

In patients with severe lung disease or hemodynamic instability, the PETCO₂ may not be good predictor of PaCO₂ because (a-ET)PCO₂ gradients vary with the changing V/Q relationship of the lungs, thus making PETCO₂ measurements less reliable.6 The emphasis here is on more ABG's until the V/Q mismatch improves and a more constant (a-ET)PCO₂ relationship is established. Establishment of constant (a-ET)CO₂ implies a good improvement in the V/Q status of the patient.

References:

1. Nunn JF, Hill DW. Respiratory dead space and arterial to end-tidal CO₂ tension difference in anesthetized man. J Appl Physiol 1960;15:383-9.

2. Fletcher R, Jonson B. Deadspace and the single breath test for carbon dioxide during anaesthesia and artificial ventilation. Br J Anaeasth 1984;56:109-19.

3. Shankar KB, Moseley H, Kumar Y, Vemula V. Arterial to end-tidal carbon dioxide tension difference during cesarean section anaesthesia. Anaesthesia 1986;41:698-702.

4. Askrog V. Changes in (a-A)CO₂ difference and pulmonary artery pressure in anesthetized man. J Appl Physiol 1966;21:1299-1305.

5. Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and Anaesthesia. Canadian J Anaesth 1992;39:6:617-32.

6. Phan CQ, Tremper KK, Lee SE, Barker SJ. Noninvasive monitoring of carbon dioxide: A comparison of the partial pressure of transcutaneous and end-tidal carbon dioxide with the partial pressure of arterial carbon dioxide. J Clin Monit 1987;3:149-54.