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Basics of volumetric capnography - Part 3: Clinical relevance

Article

Author: Karjaghli Munir, Respiratory Therapist, Hamilton Medical Clinical Application Specialist; Matthias Himmelstoss, ICU Nurse, MSc Physics, Product Manager

Date of first publication: 16.11.2023

The CO2 curve can give an insight into multiple clinical scenarios, including ARDS, pulmonary embolism, and obstructive lung disease. It can also help manage the PEEP setting. Explore the clinical relevance.

Basics of volumetric capnography - Part 3: Clinical relevance

Improve ventilation quality and efficiency

You can use the insights from the CO2 curve to improve ventilation quality and efficiency for your patients. Below you will find examples of use of the CO2 curve for the following clinical scenarios:

  • Signs of ARDS
  • PEEP management
  • Recruitment maneuver
  • Expiratory resistance
  • Obstructive lung disease
  • Pulmonary embolism
  • Hemorrhagic shock
  • Optimize management of the weaning process
  • Monitor perfusion during patient transport
  • Detection of rebreathing

Signs of ARDS - Acute respiratory distress syndrome

In ARDS, the ventilation/perfusion ratio is disturbed and changes in the slope of the volumetric capnogram curve can be observed (Figure 1).

Good to know: Phase I is larger due to increased anatomical dead space caused by PEEP. The slope of Phase II is decreased due to lung perfusion abnormalities. The slope of Phase III is increased due to lung heterogeneity.

Illustration of the volumetric capnogram for an ARDS patient
Figure 1: Volumetric capnogram for an ARDS patient
Illustration of the volumetric capnogram for an ARDS patient
Figure 1: Volumetric capnogram for an ARDS patient

PEEP management

If PEEP is too high, the intrathoracic pressure rises, the venous return decreases, and pulmonal vascular resistance (PVR) increases. These changes can easily be observed on the volumetric capnogram (Figure 2). The clinician can use volumetric capnography to check and manage the PEEP setting.

Good to know: 

  • An increase in Phase I shows an increase in anatomical dead space.
  • A decrease in the Phase II slope indicates a decrease in perfusion.
  • An increase in the Phase III slope depicts a maldistribution of gas, which can be caused by an inappropriately low PEEP setting or an inappropriately high PEEP setting causing lung overdistension.
Illustration of the PEEP curve on the volumetric capnogram before and after PEEP reduction
Figure 2: PEEP curve on the volumetric capnogram before and after PEEP reduction
Illustration of the PEEP curve on the volumetric capnogram before and after PEEP reduction
Figure 2: PEEP curve on the volumetric capnogram before and after PEEP reduction

Recruitment maneuver

The volumetric capnogram can be used to assess the effectiveness of recruitment maneuvers and might give you an insight into the recruited lung volume (Figure 3).

Good to know: After a successful recruitment maneuver, you should see a transient increase in V‘CO2. Phase I may decrease a little. The slope of Phase II becomes steeper with improved lung perfusion. The slope of Phase III improves as a result of more homogeneous lung emptying.

Illustration of PetCO2 on the volumetric capnogram before and after recruitment
Figure 3: PetCO2 on the volumetric capnogram before and after recruitment
Illustration of PetCO2 on the volumetric capnogram before and after recruitment
Figure 3: PetCO2 on the volumetric capnogram before and after recruitment

Expiratory resistance

Concave Phase III volumetric capnograms have been seen with obese patients and patients with increased expiratory resistance. Obese patients (Figure 4) can have biphasic emptying and higher PetCO2 than PaCO2. That difference suggests varying mechanical and ventilation/perfusion properties. The increase in expiratory resistance (Figure 5) may reflect a slow expiratory phase with a slow accumulation of alveolar CO2. The alveoli that empty last may have more time for CO2 diffusion.

Illustration of a concave volumetric capnogram associated with obesity
Figure 4: Concave volumetric capnogram associated with obesity
Illustration of a concave volumetric capnogram associated with obesity
Figure 4: Concave volumetric capnogram associated with obesity
Illustration of a concave volumetric capnogram associated with increased airway resistance
Figure 5: Concave volumetric capnogram associated with increased airway resistance
Illustration of a concave volumetric capnogram associated with increased airway resistance
Figure 5: Concave volumetric capnogram associated with increased airway resistance

Obstructive lung disease

When spirometry cannot be performed reliably, volumetric capnography can be used as an alternative test to evaluate the degree of functional involvement in obstructive lung disease patients (COPD, asthma, cystic fibrosis, etc.). Obstructive lung disease is characterized by asynchronous emptying of compartments with different ventilation/perfusion ratios (Figure 6).

Patients with high airway resistance demonstrate a decrease in the Phase II slope and a steep slope in Phase III. The volumetric capnogram can give you insights into therapy efficiency (Figure 7).

Good to know: The volumetric capnogram in COPD patients shows a prolonged Phase II, an increase in PetCO2, and a continuously ascending slope without a plateau in Phase III (Figure 6).

A Phase II shift to the left indicates reduced resistance. The slope of Phase III shows a decrease in steepness indicating better gas distribution and reduced alveolar dead space (VDalv) (Figure 7).

Illustration of PetCo2 in COPD patients on a volumetric capnogram
Figure 6: PetCo2 in COPD patients
Illustration of PetCo2 in COPD patients on a volumetric capnogram
Figure 6: PetCo2 in COPD patients
Illustration of PetCo2 in COPD patients after therapy on a volumetric capnogram
Figure 7: PetCo2 in COPD patients after therapy
Illustration of PetCo2 in COPD patients after therapy on a volumetric capnogram
Figure 7: PetCo2 in COPD patients after therapy

Signs of pulmonary embolism

Pulmonary embolism (PE) leads to an abnormal alveolar dead space that is expired in synchrony with gas from normally perfused alveoli. This feature of PE separates it from pulmonary diseases affecting the airway, which are characterized by nonsynchronous emptying of compartments with an uneven ventilation/perfusion relationship. In the case of sudden pulmonary embolism, volumetric capnography has a typical unique shape (Figure 8).

Good to know: In patients with sudden pulmonary vascular occlusion due to pulmonary embolism, Phase I is increased due to increased anatomical dead space. The slope of Phase II is decreased due to poor lung perfusion. Phase III has a normal plateau with low PetCO2 because the number of functional alveoli is reduced. In this case, V‘CO2 drops suddenly.

Illustration of PetCO2 in pulmonary embolism (PE) on a volumetric capnogram
Figure 8: PetCO2 in pulmonary embolism (PE)
Illustration of PetCO2 in pulmonary embolism (PE) on a volumetric capnogram
Figure 8: PetCO2 in pulmonary embolism (PE)

Hemorrhagic shock

Hemorrhagic shock is a condition of reduced tissue perfusion, resulting in the inadequate delivery of oxygen and nutrients that are necessary for cellular function (Figure 9).

Good to know: The expired CO2 drops drastically. Phase I is unchanged and the slopes of Phase II and III are unchanged, but PetCO2 is decreased due to the increase in alveolar dead space.

Illustration of PetCo2 during hemorrhagic shock on a volumetric capnogram
Figure 9: PetCo2 during hemorrhagic shock
Illustration of PetCo2 during hemorrhagic shock on a volumetric capnogram
Figure 9: PetCo2 during hemorrhagic shock

Optimize management of the weaning process

The volumetric capnogram and trends show the patient‘s response to the weaning trial and allow for better management of the weaning process.

Indications for a successful weaning trial are:

  • Stable V‘alv and constant tidal volumes: As ventilatory support is being weaned, the patient assumes the additional work of breathing while V‘alv remains stable and spontaneous tidal volumes remain constant.
  • V‘CO2 remains stable and then slightly increases: The slight increase in V‘CO2 represents an increase in CO2 production as the patient's work of breathing increases in association with the decrease in ventilatory support. This suggests an increase in metabolic activity due to the additional task of breathing by the patient.

Indications for an unsuccessful weaning trial are:

  • Dramatic increase in V‘CO2: A more dramatic increase in V‘CO2 would suggest excessive work of breathing and the potential for impending respiratory decompensation. This scenario would be consistent with a visual assessment of increasing respiratory distress (for example, retraction, tachypnea, and agitation). The V‘CO2 will eventually decrease if the patient gets exhausted.
  • Decrease in V‘CO2: As the ventilator settings are decreased, the patient is no longer able to maintain an adequate degree of spontaneous ventilation, and total minute ventilation falls with a decrease in CO2 elimination.
  • Increased VDaw/VTE ratio: If reducing ventilatory support is followed by a decrease in tidal volume, the VDaw/VTE ratio increases. This reduces ventilatory efficiency and the patient’s ability to remove CO2.

Monitor perfusion during patient transport

If arterial access is not something you routinely perform when you transport a ventilated patient, PetCO2 can be used for monitoring perfusion and ventilation during transport.

Good to know: A decrease in PetCO2 accompanied by a decrease of VCO2 can signify:

  • ET-tube displacement
  • Decreased cardiac output
  • Pulmonary embolism
  • Atelectasis
  • Overdistension of alveoli (for example excessive PEEP)

Detection of rebreathing

An elevation of the baseline during Phase I indicates rebreathing of CO2, which may be due to mechanical problems or therapeutic use of mechanical dead space (Figure 10).

Good to know: In this case, consider recalibrating the CO2 sensor or reducing the airway accessories.

Illustration of a volumetric capnogram in case of CO2 rebreathing
Figure 10: Volumetric capnogram in the case of CO2 rebreathing
Illustration of a volumetric capnogram in case of CO2 rebreathing
Figure 10: Volumetric capnogram in the case of CO2 rebreathing

Volumetric capnography on Hamilton Medical ventilators

All Hamilton Medical ventilators offer volumetric capnography (All models except HAMILTON-MR1A​). It is available as an option on the HAMILTON‑C6, the HAMILTON‑G5, the HAMILTON‑C3, and the HAMILTON‑C1/T1, and as a standard feature on the HAMILTON‑S1.

 

Full citations below: (Anderson JT, Owings JT, Goodnight JE. Bedside noninvasive detection of acute pulmonary embolism in critically ill surgical patients. Arch Surg. 1999;134(8):869-875. doi:10.1001/archsurg.134.8.8691​), (Aström E, Niklason L, Drefeldt B, Bajc M, Jonson B. Partitioning of dead space--a method and reference values in the awake human. Eur Respir J. 2000;16(4):659-664. doi:10.1034/j.1399-3003.2000.16d16.x2​), (Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol. 2006;72(6):577-585. 3​), (Eriksson L, Wollmer P, Olsson CG, et al. Diagnosis of pulmonary embolism based upon alveolar dead space analysis. Chest. 1989;96(2):357-362. doi:10.1378/chest.96.2.3574​), (Fletcher R, Jonson B, Cumming G, Brew J. The concept of deadspace with special reference to the single breath test for carbon dioxide. Br J Anaesth. 1981;53(1):77-88. doi:10.1093/bja/53.1.775​), (Kallet RH, Daniel BM, Garcia O, Matthay MA. Accuracy of physiologic dead space measurements in patients with acute respiratory distress syndrome using volumetric capnography: comparison with the metabolic monitor method. Respir Care. 2005;50(4):462-467. 6​), (Kiiski R, Takala J. Hypermetabolism and efficiency of CO2 removal in acute respiratory failure. Chest. 1994;105(4):1198-1203. doi:10.1378/chest.105.4.11987​), (Kumar AY, Bhavani-Shankar K, Moseley HS, Delph Y. Inspiratory valve malfunction in a circle system: pitfalls in capnography. Can J Anaesth. 1992;39(9):997-999. doi:10.1007/BF030083538​) , (Nuckton TJ, Alonso JA, Kallet RH, et al. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome. N Engl J Med. 2002;346(17):1281-1286. doi:10.1056/NEJMoa0128359​), (Eriksson L, Wollmer P, Olsson CG, et al. Diagnosis of pulmonary embolism based upon alveolar dead space analysis. Chest. 1989;96(2):357-362. doi:10.1378/chest.96.2.35710​), (Pyles ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle anesthesia circuit--verification by analysis of carbon dioxide waveform. Anesth Analg. 1984;63(5):536-537. 11​), (RADFORD EP Jr. Ventilation standards for use in artificial respiration. J Appl Physiol. 1955;7(4):451-460. doi:10.1152/jappl.1955.7.4.45112​), (Rodger MA, Jones G, Rasuli P, et al. Steady-state end-tidal alveolar dead space fraction and D-dimer: bedside tests to exclude pulmonary embolism. Chest. 2001;120(1):115-119. doi:10.1378/chest.120.1.11513​) , (Yaron M, Padyk P, Hutsinpiller M, Cairns CB. Utility of the expiratory capnogram in the assessment of bronchospasm. Ann Emerg Med. 1996;28(4):403-407. doi:10.1016/s0196-0644(96)70005-714​), (Wolff G, Brunner JX, Grädel E. Gas exchange during mechanical ventilation and spontaneous breathing. Intermittent mandatory ventilation after open heart surgery. Chest. 1986;90(1):11-17. doi:10.1378/chest.90.1.1115​), (Wolff G, X. B. J. , Weibel W., Bowes C.L. , Muchenberger R., Bertschmann W. (1989). Anatomical and series dead space volume: concept and measurement in clinical practice. Applied cardiopulmonary pathophysiology, 2, 299-307.16)

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Footnotes

  • A. All models except HAMILTON-MR1

References

  1. 1. Anderson JT, Owings JT, Goodnight JE. Bedside noninvasive detection of acute pulmonary embolism in critically ill surgical patients. Arch Surg. 1999;134(8):869-875. doi:10.1001/archsurg.134.8.869
  2. 2. Aström E, Niklason L, Drefeldt B, Bajc M, Jonson B. Partitioning of dead space--a method and reference values in the awake human. Eur Respir J. 2000;16(4):659-664. doi:10.1034/j.1399-3003.2000.16d16.x
  3. 3. Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol. 2006;72(6):577-585.
  4. 4. Eriksson L, Wollmer P, Olsson CG, et al. Diagnosis of pulmonary embolism based upon alveolar dead space analysis. Chest. 1989;96(2):357-362. doi:10.1378/chest.96.2.357
  5. 5. Fletcher R, Jonson B, Cumming G, Brew J. The concept of deadspace with special reference to the single breath test for carbon dioxide. Br J Anaesth. 1981;53(1):77-88. doi:10.1093/bja/53.1.77
  6. 6. Kallet RH, Daniel BM, Garcia O, Matthay MA. Accuracy of physiologic dead space measurements in patients with acute respiratory distress syndrome using volumetric capnography: comparison with the metabolic monitor method. Respir Care. 2005;50(4):462-467.
  7. 7. Kiiski R, Takala J. Hypermetabolism and efficiency of CO2 removal in acute respiratory failure. Chest. 1994;105(4):1198-1203. doi:10.1378/chest.105.4.1198
  8. 8. Kumar AY, Bhavani-Shankar K, Moseley HS, Delph Y. Inspiratory valve malfunction in a circle system: pitfalls in capnography. Can J Anaesth. 1992;39(9):997-999. doi:10.1007/BF03008353
  9. 9. Nuckton TJ, Alonso JA, Kallet RH, et al. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome. N Engl J Med. 2002;346(17):1281-1286. doi:10.1056/NEJMoa012835
  10. 10. Olsson K, Jonson B, Olsson CG, Wollmer P. Diagnosis of pulmonary embolism by measurement of alveolar dead space. J Intern Med. 1998;244(3):199-207. doi:10.1046/j.1365-2796.1998.00356.x
  11. 11. Pyles ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle anesthesia circuit--verification by analysis of carbon dioxide waveform. Anesth Analg. 1984;63(5):536-537.
  12. 12. RADFORD EP Jr. Ventilation standards for use in artificial respiration. J Appl Physiol. 1955;7(4):451-460. doi:10.1152/jappl.1955.7.4.451
  13. 13. Rodger MA, Jones G, Rasuli P, et al. Steady-state end-tidal alveolar dead space fraction and D-dimer: bedside tests to exclude pulmonary embolism. Chest. 2001;120(1):115-119. doi:10.1378/chest.120.1.115
  14. 14. Yaron M, Padyk P, Hutsinpiller M, Cairns CB. Utility of the expiratory capnogram in the assessment of bronchospasm. Ann Emerg Med. 1996;28(4):403-407. doi:10.1016/s0196-0644(96)70005-7
  15. 15. Wolff G, Brunner JX, Grädel E. Gas exchange during mechanical ventilation and spontaneous breathing. Intermittent mandatory ventilation after open heart surgery. Chest. 1986;90(1):11-17. doi:10.1378/chest.90.1.11
  16. 16). Wolff G, X. B. J. , Weibel W., Bowes C.L. , Muchenberger R., Bertschmann W. (1989). Anatomical and series dead space volume: concept and measurement in clinical practice. Applied cardiopulmonary pathophysiology, 2, 299-307.

Bedside noninvasive detection of acute pulmonary embolism in critically ill surgical patients.

Anderson JT, Owings JT, Goodnight JE. Bedside noninvasive detection of acute pulmonary embolism in critically ill surgical patients. Arch Surg. 1999;134(8):869-875. doi:10.1001/archsurg.134.8.869



HYPOTHESIS

We hypothesized that late pulmonary dead space fraction (Fd(late)) would be a useful tool to screen for pulmonary embolism (PE) in a group of surgical patients, including patients who required mechanical ventilation and patients with adult respiratory distress syndrome.

DESIGN

We prospectively calculated Fd(late) in patients with suspected PE who underwent pulmonary angiography.

SETTING

University-based, level I trauma center.

MAIN OUTCOME MEASURE

Ability of Fd(late) to identify patients with PE.

RESULTS

Twelve patients had 14 angiograms for suspected PE. The Fd(late) was 0.12 or above in all 5 patients who had PE; 4 required mechanical ventilation. The Fd(late) values were below 0.12 in 8 of 9 patients without PE. Four patients had adult respiratory distress syndrome. The Fd(late) had 100% sensitivity and 89% specificity for the detection of PE.

CONCLUSIONS

The Fd(late) is a valuable tool for bedside screening of PE in surgical patients. We were able to accurately detect all PEs.

Partitioning of dead space--a method and reference values in the awake human.

Aström E, Niklason L, Drefeldt B, Bajc M, Jonson B. Partitioning of dead space--a method and reference values in the awake human. Eur Respir J. 2000;16(4):659-664. doi:10.1034/j.1399-3003.2000.16d16.x

Although dead space is often increased in disease, it is not frequently measured in the clinic. This may reflect that an adequate method as well as reference values are missing. Healthy males and females, n=38, age 20-61 yrs, were connected to a pneumotachograph and a fast CO2 analyser after radial artery catheterization. The physiological dead space was partitioned into airway and alveolar dead space using a delineation principle denoted the pre-interface expirate. Physiological dead space was 201+/-41 mL in males and 150+/-34 mL in females. Dead space values were depending upon parameters reflecting lung size (predicted total lung capacity), breathing pattern and age. After multiple correlation the variation decreased and differences between males and females disappeared. The residual SD was then for physiological dead space 18.9 mL. The clinical use of the new method for determination of dead space can be based upon reference values, with a more narrow range than previous data.

Volumetric capnography in the mechanically ventilated patient.

Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol. 2006;72(6):577-585.

Expiratory capnogram provides qualitative information on the waveform patterns associated with mechanical ventilation and quantitative estimation of expired CO2. Volumetric capnography simultaneously measures expired CO2 and tidal volume and allows identification of CO2 from 3 sequential lung compartments: apparatus and anatomic dead space, from progressive emptying of alveoli and alveolar gas. Lung heterogeneity creates regional differences in CO2 concentration and sequential emptying contributes to the rise of the alveolar plateau and to the steeper the expired CO2 slope. The concept of dead space accounts for those lung areas that are ventilated but not perfused. In patients with sudden pulmonary vascular occlusion due to pulmonary embolism, the resultant high V/Q mismatch produces an increase in alveolar dead space. Calculations derived from volumetric capnography are useful to suspect pulmonary embolism at the bedside. Alveolar dead space is large in acute lung injury and when the effect of positive end-expiratory pressure (PEEP) is to recruit collapsed lung units resulting in an improvement of oxygenation, alveolar dead space may decrease, whereas PEEP-induced overdistension tends to increase alveolar dead space. Finally, measurement of physiologic dead space and alveolar ejection volume at admission or the trend during the first 48 hours of mechanical ventilation might provide useful information on outcome of critically ill patients with acute lung injury or acute respiratory distress syndrome.

Diagnosis of pulmonary embolism based upon alveolar dead space analysis.

Eriksson L, Wollmer P, Olsson CG, et al. Diagnosis of pulmonary embolism based upon alveolar dead space analysis. Chest. 1989;96(2):357-362. doi:10.1378/chest.96.2.357

Pulmonary embolism (PE) leads to an abnormal alveolar deadspace that is expired in synchrony with gas from normally perfused alveoli. This feature of PE separates it from pulmonary diseases affecting the airways, which are characterized by nonsynchronous emptying of compartments with an uneven ventilation/perfusion relationship. An analysis of the single breath test (SBT) for CO2, SBT-CO2, focusing on the late tidal expirate, was made in order to evaluate the feasibility to use the SBT-CO2 for the diagnosis of PE. The test was evaluated in 38 patients with suspected PE where pulmonary angiography showed that nine had PE and 29 did not. It was also tested in a reference population consisting of patients with normal lung function, obstructive lung disease and interstitial lung disease. Previously suggested gas exchange measurements for the diagnosis of PE, ie, the physiologic deadspace fraction, VDphys/VT, and the arterial-to-end-tidal CO2 gradient, P(a-E')CO2, were also evaluated in the groups. SBT-CO2 achieved a nearly complete separation between the patients with PE and those without. The other measurements, however, showed a substantial overlap between patients with PE and those with obstructive or interstitial lung disease. The SBT-CO2 is simple and potentially widely available and warrants further study as a routine technique for the diagnosis of PE.

The concept of deadspace with special reference to the single breath test for carbon dioxide.

Fletcher R, Jonson B, Cumming G, Brew J. The concept of deadspace with special reference to the single breath test for carbon dioxide. Br J Anaesth. 1981;53(1):77-88. doi:10.1093/bja/53.1.77

We present a review and a theoretical analysis of factors determining airway deadspace (VDaw) and alveolar deadspace (VDalv), the two constituents of physiological deadspace (VDphys). VDaw if the volume of gas between the lips and the alveolar/fresh gas interface, the location of which is determined by inspiratory flow pattern and airway geometry. VDalv can be caused by incomplete alveolar gas mixing and associated V/Q mismatching within the terminal respiratory units, temporal V/Q mismatching within units, spatial V/Q mismatching between units, and venous admixture. Most causes of VDphys are influenced by inspiratory flow pattern and the time available for gas diffusion and distribution. Analysis can be made from the single breath test for carbon dioxide (SBT--CO2) which is the plot of fraction of carbon dioxide in expired gas against expired volume. The common causes of VDalv are associated with a sloping SBT-CO2 phase III. Combination of SBT-CO2 with PaCO2 yields VDphys and VDalv. A sloping phase III with a negative arterial-end-tidal PCO2 gradient implies compensation by perfusion for early emptying, overventilated alveoli.

Accuracy of physiologic dead space measurements in patients with acute respiratory distress syndrome using volumetric capnography: comparison with the metabolic monitor method.

Kallet RH, Daniel BM, Garcia O, Matthay MA. Accuracy of physiologic dead space measurements in patients with acute respiratory distress syndrome using volumetric capnography: comparison with the metabolic monitor method. Respir Care. 2005;50(4):462-467.



BACKGROUND

Volumetric capnography is an alternative method of measuring expired carbon dioxide partial pressure (P(eCO2)) and physiologic dead-space-to-tidal-volume ratio (V(D)/V(T)) during mechanical ventilation. In this method, P(eCO2) is measured at the Y-adapter of the ventilator circuit, thus eliminating the effects of compression volume contamination and the need to apply a correction factor. We investigated the accuracy of volumetric capnography in measuring V(D)/V(T), compared to both uncorrected and corrected measurements, using a metabolic monitor in patients with acute respiratory distress syndrome (ARDS).

METHODS

There were 90 measurements of V(D)/V(T) made in 23 patients with ARDS. The P(eCO2) was measured during a 5-min expired-gas collection period with a Delta-trac metabolic monitor, and was corrected for compression volume contamination using a standard formula. Simultaneous measurements of P(eCO2) and V(D)/V(T) were obtained using volumetric capnography.

RESULTS

V(D)/V(T) measured by volumetric capnography was strongly correlated with both the uncorrected (r2 = 0.93, p < 0.0001) and corrected (r2 = 0.89, p < 0.0001) measurements of V(D)/V(T) made using the metabolic monitor technique. Measurements of V(D)/V(T) made with volumetric capnography had a bias of 0.02 and a precision of 0.05 when compared to the V(D)/V(T) corrected for estimated compression volume contamination.

CONCLUSION

Volumetric capnography measurements of V(D)/V(T) in mechanically-ventilated patients with ARDS are as accurate as those obtained by metabolic monitor technique. .

Hypermetabolism and efficiency of CO2 removal in acute respiratory failure.

Kiiski R, Takala J. Hypermetabolism and efficiency of CO2 removal in acute respiratory failure. Chest. 1994;105(4):1198-1203. doi:10.1378/chest.105.4.1198



OBJECTIVE

To assess the effect of hypermetabolism, dead-space ventilation, and parenteral nutrition on the minute ventilation requirement in mechanically ventilated patients.

DESIGN

A retrospective analysis of data collected in study protocols unrelated to the present study.

SETTING

A medical-surgical intensive care unit in a tertiary care center.

PATIENTS

One hundred eleven mechanically ventilated patients were studied during volume-controlled ventilation.

MEASUREMENTS

Gas exchange measurement by indirect calorimetry and arterial blood gas analysis.

METHODS

Minute ventilation (VE), carbon dioxide production (VCO2), and respiratory exchange ratio (RER) were measured with indirect calorimetry. Arterial CO2 tension was sampled at the end of the measurement, and alveolar ventilation (VA), deadspace to tidal volume ratio (VD/VT) and predicted resting VCO2 were calculated. The VE demand at a standard PaCO2 was calculated and the contribution of the observed hypermetabolism and increased VD/VT was identified. In a subgroup of patients, the effect of initiating parenteral nutrition on the VE demand was assessed. There were four study groups: multiple trauma, sepsis, ARDS, and postoperative open-heart surgery patients.

MAIN RESULTS

A combination of hypermetabolism and increased dead-space was observed in 67 of the 111 patients. Increased VCO2 accounted for 69 percent of the excess VE demand in trauma, 67 percent in sepsis, 58 percent in postoperative patients, and 56 percent in ARDS. Parenteral nutrition with a caloric intake matching measured resting energy expenditure (REE) did not increase VCO2 or the demand for VE.

CONCLUSIONS

Increased VCO2 is the main cause of increased VE demand in the majority of mechanically ventilated ICU patients. Parenteral nutrition at energy intakes close to actual REE does not increase the ventilatory demand.

Inspiratory valve malfunction in a circle system: pitfalls in capnography.

Kumar AY, Bhavani-Shankar K, Moseley HS, Delph Y. Inspiratory valve malfunction in a circle system: pitfalls in capnography. Can J Anaesth. 1992;39(9):997-999. doi:10.1007/BF03008353

Capnography is a useful technique in monitoring the integrity of anaesthetic equipment such as the malfunctioning of unidirectional valves in circle system. However, the lack of a precise mechanism in existing capnographs to identify the start of inspiration and the beginning of expiration in the capnograms, makes the analysis of the carbon dioxide waveforms during inspiration difficult and thus results in inaccurate assessment of rebreathing. We report a case where, during the malfunction of the inspiratory unidirectional valve in the circle system, the capnograph failed to detect the presence of substantial rebreathing. Critical analysis of the capnogram recorded during the malfunction revealed that there was substantial rebreathing which was underestimated by the capnograph as it reports only the lowest CO2 concentration rebreathed during inspiration in such abnormal situations.

Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome.

Nuckton TJ, Alonso JA, Kallet RH, et al. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome. N Engl J Med. 2002;346(17):1281-1286. doi:10.1056/NEJMoa012835



BACKGROUND

No single pulmonary-specific variable, including the severity of hypoxemia, has been found to predict the risk of death independently when measured early in the course of the acute respiratory distress syndrome. Because an increase in the pulmonary dead-space fraction has been described in observational studies of the syndrome, we systematically measured the dead-space fraction early in the course of the illness and evaluated its potential association with the risk of death.

METHODS

The dead-space fraction was prospectively measured in 179 intubated patients, a mean (+/-SD) of 10.9+/-7.4 hours after the acute respiratory distress syndrome had developed. Additional clinical and physiological variables were analyzed with the use of multiple logistic regression. The study outcome was mortality before hospital discharge.

RESULTS

The mean dead-space fraction was markedly elevated (0.58+/-0.09) early in the course of the acute respiratory distress syndrome and was higher among patients who died than among those who survived (0.63+/-0.10 vs. 0.54+/-0.09, P<0.001). The dead-space fraction was an independent risk factor for death: for every 0.05 increase, the odds of death increased by 45 percent (odds ratio, 1.45; 95 percent confidence interval, 1.15 to 1.83; P=0.002). The only other independent predictors of an increased risk of death were the Simplified Acute Physiology Score II, an indicator of the severity of illness (odds ratio, 1.06; 95 percent confidence interval, 1.03 to 1.08; P<0.001) and quasistatic respiratory compliance (odds ratio, 1.06; 95 percent confidence interval, 1.01 to 1.10; P=0.01).

CONCLUSIONS

Increased dead-space fraction is a feature of the early phase of the acute respiratory distress syndrome. Elevated values are associated with an increased risk of death.

Diagnosis of pulmonary embolism by measurement of alveolar dead space.

Olsson K, Jonson B, Olsson CG, Wollmer P. Diagnosis of pulmonary embolism by measurement of alveolar dead space. J Intern Med. 1998;244(3):199-207. doi:10.1046/j.1365-2796.1998.00356.x



OBJECTIVE

Pulmonary embolism (PE) gives rise to alveolar dead space, which can be measured with a single breath test for CO2 (SBT-CO2). The characteristics of the SBT-CO2 are different in PE and other common conditions giving rise to alveolar dead space, notably airways disease. An analysis of alveolar dead space focusing on the late part of the breath (fDlate) has been suggested as a method for diagnosis of PE. Our aim was to evaluate this technique by comparison with lung scintigraphy.

METHODS

We randomly selected patients with clinical suspicion of PE. SBT-CO2 and lung scintigraphy were performed on the same day. The scintigraphies were reviewed and classified as high, intermediate and low probability of PE.

RESULTS

Out of 223 patients able to be evaluated, there were 20 of the high, 29 of the intermediate and 174 of the low probability category. There were large differences between the means of fDlate in the high and the intermediate and in the high and the low categories. We obtained a sensitivity of 85% and a specificity of 93% for diagnosis of PE, based on high and low probability categories. If a patient with previous PE, but no scintigraphic evidence of current PE, is excluded the sensitivity increases to 90%.

CONCLUSIONS

This study provides further support for the measurement of fDlate by the SBT-CO2 as a diagnostic test in patients with suspicion of PE. The test should be especially useful in small hospitals without access to pulmonary scintigraphy or pulmonary angiography.

Expiratory valve dysfunction in a semiclosed circle anesthesia circuit--verification by analysis of carbon dioxide waveform.

Pyles ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle anesthesia circuit--verification by analysis of carbon dioxide waveform. Anesth Analg. 1984;63(5):536-537.

Ventilation standards for use in artificial respiration.

RADFORD EP Jr. Ventilation standards for use in artificial respiration. J Appl Physiol. 1955;7(4):451-460. doi:10.1152/jappl.1955.7.4.451

Steady-state end-tidal alveolar dead space fraction and D-dimer: bedside tests to exclude pulmonary embolism.

Rodger MA, Jones G, Rasuli P, et al. Steady-state end-tidal alveolar dead space fraction and D-dimer: bedside tests to exclude pulmonary embolism. Chest. 2001;120(1):115-119. doi:10.1378/chest.120.1.115



STUDY OBJECTIVE

Less than 35% of patients suspected of having pulmonary embolism (PE) actually have PE. Safe bedside methods to exclude PE could save health-care resources and improve access to diagnostic testing for suspected PE. In patients with suspected PE, we sought to determine the sensitivity, specificity, and negative predictive value of (1) a steady-state end-tidal alveolar dead space fraction (AVDSf) of < 0.15, (2) a negative D-dimer result, and (3) the combination of a steady-state end-tidal AVDSf of < 0.15 and a negative D-dimer result.

STUDY DESIGN

Prospective cohort study.

SETTING

Tertiary-care center in Ottawa, Ontario, Canada.

PATIENTS

Consecutive inpatients, outpatients, and emergency department patients with suspected PE referred to the Departments of Nuclear Medicine or Radiology for investigation of suspected PE.

INTERVENTIONS AND MEASUREMENTS

All study patients had D-Dimer and alveolar dead space measurements prior to determining outcome (PE or no PE) with ventilation/perfusion scans and/or noninvasive leg vein imaging and/or pulmonary angiography.

RESULTS

Two hundred forty-six eligible and consenting patients underwent diagnostic imaging that excluded PE in 163 patients, diagnosed PE in 49 patients, and was indeterminant in 34 patients. A negative D-dimer result excluded PE with a sensitivity of 83.0% (95% confidence interval [CI], 69.2 to 92.4%), a negative predictive value of 91.2% (95% CI, 83.4 to 96.1%), and a specificity of 57.6%. A steady-state end-tidal AVDSf of < 0.15 excluded PE with a sensitivity of 79.5% (95% CI, 63.5 to 90.7%), a negative predictive value of 90.7% (95% CI, 82.5 to 95.9%), and a specificity of 70.3%. The combination of a negative D-dimer result and a steady-state end-tidal AVDSf of < 0.15 excluded PE with a sensitivity of 97.8% (95% CI, 88.5 to 99.9%), a negative predictive value of 98.0% (95% CI, 89.4 to 99.9%), and a specificity of 38.0%.

CONCLUSION

This simple combination of bedside tests may safely rule out PE without further diagnostic testing in large numbers of patients with suspected PE.

Utility of the expiratory capnogram in the assessment of bronchospasm.

Yaron M, Padyk P, Hutsinpiller M, Cairns CB. Utility of the expiratory capnogram in the assessment of bronchospasm. Ann Emerg Med. 1996;28(4):403-407. doi:10.1016/s0196-0644(96)70005-7



STUDY OBJECTIVE

To determine whether the plateau phase of the expiratory capnogram (dco2/dt) can detect bronchospasm in adult asthma patients in the emergency department and to assess the correlation between dco2/dt and the peak expiratory flow rate (PEFR) in spontaneously breathing patients with asthma and in normal, healthy volunteers.

METHODS

We carried out a prospective, blinded study in a university hospital ED. Twenty adults (12 women) with acute asthma and 28 normal adult volunteers (15 women) breathed through the sampling probe of an end-tidal CO2 monitor, and the expired CO2 waveform was recorded. The dco2/dt of the plateau (alveolar) phase for five consecutive regular expirations was measured and a mean value calculated for each patient. The best of three PEFRs was determined. The PEFR and dco2/dt were also recorded after treatment of the asthmatic patients with inhaled beta-agonists.

RESULTS

The mean +/- SD PEFR of the asthmatic subjects was 274 +/- 96 L/minute (57% of the predicted value), whereas that of the normal volunteers was 527 +/- 96 L/minute (103% of the predicted value) (P < .001). The mean dco2/dt of the asthmatic subjects (.26 +/- .06) was significantly steeper than that of the normal volunteers (.13 +/- .06) (P < .001). The dco2/dt was correlated with PEFR (r = .84, P < .001). In 18 asthmatic subjects the pretreatment and posttreatment percent predicted PEFRS were 58% +/- 17% and 74% +/- 17%, respectively (P < .001), whereas the dco2/dt values were .27 +/- .05 and .19 +/- .07, respectively (P < .005).

CONCLUSION

The dco2/dt is an effort-independent, rapid noninvasive measure that indicates significant bronchospasm in ED adult patients with asthma. The dco2/dt value is correlated with PEFR, an effort-dependent measure of airway obstruction. The change in dco2/dt with inhaled beta-agonists may be useful in monitoring the therapy of acute asthma.

Gas exchange during mechanical ventilation and spontaneous breathing. Intermittent mandatory ventilation after open heart surgery.

Wolff G, Brunner JX, Grädel E. Gas exchange during mechanical ventilation and spontaneous breathing. Intermittent mandatory ventilation after open heart surgery. Chest. 1986;90(1):11-17. doi:10.1378/chest.90.1.11

Pulmonary gas exchange rates in eight patients after open heart surgery were studied during weaning from the ventilator. We investigated continuous positive pressure ventilation (CPPV), intermittent mandatory ventilation (IMV) and spontaneous breathing with continuous positive airway pressure (CPAP). During each mode of ventilation we measured: CO2 production (VCO2), O2 consumption (VO2), cardiac output (CO), PaO2, Qs/QT and functional residual capacity (FRC). In addition, we analyzed in each single breath: tidal volume (VT), series dead space volume (Vds), alveolar ventilation, alveolar efficiency for CO2 elimination (alv eff CO2) and end-tidal CO2 concentration (FCO2et). We compared the results of CPPV, IMV and CPAP and the mandatory breaths (MB) with the spontaneous breaths (SB) measured during IMV. CO was low during CPPV, when the patient still deeply sedated; it was increased in IMV and remained constant in the following CPAP period. VCO2 and VO2 did not differ significantly when switching from IMV to CPAP; therefore, work due to breathing seemed not to be reduced by the mandatory breath during IMV. Oxygenation (PaO2, Qs/QT) did not change significantly when switching from one mode to the other. FRC was constant when changing from CPPV to IMV, did not alter within the IMV-cycle and was reduced significantly when switching from IMV to CPAP. Dead space ventilation was reduced in SB (compared to MB). The latter result is discussed on the basis of two mechanisms: Vds was reduced and alv eff CO2 was increased. We conclude that compared to CPPV, IMV decreases mean alveolar pressure and reduces dead space ventilation at constant FRC and with constant oxygenation. This may explain why, in the weaning process, IMV makes it possible to start spontaneous breathing very early.

Anatomical and series dead space volume: concept and measurement in clinical practice

Wolff G, X. B. J. , Weibel W., Bowes C.L. , Muchenberger R., Bertschmann W. (1989). Anatomical and series dead space volume: concept and measurement in clinical practice. Applied cardiopulmonary pathophysiology, 2, 299-307.

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