Volumetric or time-based capnography for excluding pulmonary embolism in outpatients?

Volume Capnography in the Intensive Care Unit: Potential Clinical Applications

Volume capnography provides a noninvasive, continuous display of the fractional concentration or partial pressure of carbon dioxide (Pco₂) versus exhaled volume. Derived measurements and calculations are influenced by changes in both ventilation and perfusion and are therefore useful for assessing both respiratory and cardiovascular function. This article provides an evidence-based review of several potential uses of volume capnography in the intensive care unit: 1) monitoring the effectiveness of ventilation by using end-tidal Pco₂ as a surrogate for arterial Pco₂, 2) assessing volume responsiveness, 3) measuring cardiac output, 4) determining prognosis in patients with the acute respiratory distress syndrome, 5) optimizing alveolar recruitment, and 6) excluding pulmonary embolism. Studies performed during the past few decades have clearly shown that volume capnography can provide important prognostic information in patients with acute respiratory distress syndrome and that end-tidal Pco₂ should not be used to estimate or even to monitor the direction of change in the arterial Pco₂ in mechanically ventilated intensive care unit patients. Unfortunately, few conclusions can be made from studies evaluating other potential applications. Of these, the most promising are the noninvasive measurement of cardiac output and optimization of alveolar recruitment in patients with acute respiratory distress syndrome and in mechanically ventilated, morbidly obese patients.

Retrospective validation of a new volumetric capnography parameter for the exclusion of pulmonary embolism at the emergency department

Volumetric capnography might be used to exclude pulmonary embolism (PE) without the need for computed tomography pulmonary angiography. In a pilot study, a new parameter (CapNoPE) combining the amount of carbon dioxide exhaled per breath (carbon dioxide production (V_CO2)), the slope of phase 3 of the volumetric capnogram (slope 3) and respiratory rate (RR) showed promising diagnostic accuracy (where CapNoPE=(V_CO2×slope 3)/RR).

To retrospectively validate CapNoPE for the exclusion of PE, the volumetric capnograms of 205 subjects (68 with PE) were analysed, based on a large multicentre dataset of volumetric capnograms from subjects with suspected PE at the emergency department. The area under the curve (AUC) of the receiver operating characteristic (ROC) curve and diagnostic accuracy of the in-pilot established threshold (1.90 Pa·min) were calculated. CapNoPE was 1.56±0.97 Pa·min in subjects with PE versus 2.51±1.67 Pa·min in those without PE (p<0.001). The AUC of the ROC curve was 0.714 (95% CI 0.64–0.79). For the cut-off of ≥1.90 Pa·min, sensitivity was 64.7%, specificity was 59.9%, the negative predictive value was 77.4% and the positive predictive value was 44.4%.

The CapNoPE parameter is decreased in patients with PE but its diagnostic accuracy seems too low to use in clinical practice.

Retrospective validation in a large multicentre dataset shows that a novel volumetric capnography derived parameter (CapNoPE) is decreased in patients with PE but its diagnostic accuracy seems too low to use in clinical practicehttp://ow.ly/Ogpg30m0HuU

De l’entrée à la sortie du service de réanimation adulte : une mise au point sur l’utilisation courante du monitoring du CO2 expiré

L’objectif de cette mise au point est d’effectuer une revue des indications de l’utilisation du monitorage du CO2 expiré en réanimation adulte. De par sa physiologie, sa mesure est un reflet de l’état hémodynamique, respiratoire et métabolique du patient. La spectrométrie infrarouge est la méthode de mesure la plus courante. La capnographie commune (CO2 expiré en fonction du temps) est divisée en plusieurs phases dont l’analyse visuelle peut faire évoquer de nombreuses anomalies ventilatoires. La capnographie volumétrique fournit une mesure de l’espace mort. La capnométrie est recommandée en réanimation pour contrôler l’intubation trachéale ou bien au cours d’un arrêt cardiorespiratoire comme facteur pronostique. Tout patient traité par ventilation mécanique invasive, surtout lors d’un transport, doit être équipé d’un capnomètre afin d’anticiper toute complication respiratoire (extubation, bronchospasme, hypoventilation). La pression de fin d’expiration en CO2 (PetCO2) est une évaluation de la pression artérielle en CO2 (PaCO2) utile pour limiter le nombre de prélèvements biologiques, par exemple en neuroréanimation, mais de nombreux facteurs font varier le gradient entre ces deux valeurs. Les études n’apportent pas de preuve pour l’utilisation de la capnographie volumétrique dans le diagnostic d’embolie pulmonaire en réanimation. Chez les patients souffrant de syndrome de détresse respiratoire aiguë, la littérature médicale n’apporte pas de preuve suffisante pour un intérêt en pratique clinique courante de la capnométrie volumétrique qui semble limitée dans ce cas à la recherche.

Capnography in the Emergency Department: A Review of Uses, Waveforms, and Limitations

BACKGROUND

Capnography has many uses in the emergency department (ED) and critical care setting, most commonly cardiac arrest and procedural sedation.

OBJECTIVE OF THE REVIEW

This review evaluates several indications concerning capnography beyond cardiac arrest and procedural sedation in the ED, as well as limitations and specific waveforms.

DISCUSSION

Capnography includes the noninvasive measurement of CO₂, providing information on ventilation, perfusion, and metabolism in intubated and spontaneously breathing patients. Since the 1990s, capnography has been utilized extensively for cardiac arrest and procedural sedation. Qualitative capnography includes a colorimetric device, changing color on the amount of CO₂ present. Quantitative capnography provides a numeric value (end-tidal CO₂), and capnography most commonly includes a waveform as a function of time. Conditions in which capnography is informative include cardiac arrest, procedural sedation, mechanically ventilated patients, and patients with metabolic acidemia. Patients with seizure, trauma, and respiratory conditions, such as pulmonary embolism and obstructive airway disease, can benefit from capnography, but further study is needed. Limitations include use of capnography in conditions with mixed pathophysiology, patients with low tidal volumes, and equipment malfunction. Capnography should be used in conjunction with clinical assessment.

CONCLUSIONS

Capnography demonstrates benefit in cardiac arrest, procedural sedation, mechanically ventilated patients, and patients with metabolic acidemia. Further study is required in patients with seizure, trauma, and respiratory conditions. It should only be used in conjunction with other patient factors and clinical assessment.

Pathophysiology of dyspnoea in acute pulmonary embolism: A cross-sectional evaluation

BACKGROUND AND OBJECTIVE

Dyspnoea in pulmonary embolism (PE) remains poorly characterized. Little is known about how to measure intensity or about the underlying mechanisms that may be related to ventilatory abnormalities, alveolar dead space ventilation or modulating factors such as psychological modulate. We hypothesized that dyspnoea would mainly be associated with pulmonary vascular obstruction and its pathophysiological consequences, while the sensory-affective domain of dyspnoea would be influenced by other factors.

METHODS

We undertook a prospective study of 90 consecutive non-obese patients (mean ± SD age: 49 ± 16 years, 41 women) without cardiorespiratory disease. All patients were hospitalized with symptoms for <15 days and a confirmed PE (multi-detector computed tomography (MDCT) scan, n = 87 and high-probability ventilation/perfusion scan, n = 3). Patients underwent assessment of dyspnoea using the Borg score, modified Medical Research Council (mMRC) scale, assessment of psychological trait, state of anxiety and depression and chest pain via the Visual Analogical Scale at the time of maximum dyspnoea. Functional evaluations such as the quantitative ventilation-perfusion lung scan, echocardiography, alveolar dead space fraction and tidal ventilation measurements were completed within 48 h of admission.

RESULTS

Multivariate analyses demonstrated that dyspnoea was mainly linked to pulmonary vascular obstruction and/or its consequences such as raised pulmonary arterial pressure and chest pain. The sensory-affective domain of dyspnoea showed additional determinants such as age, depression and breathing variability.

CONCLUSION

Dyspnoea is mainly related to vascular consequences of PE such as increased pulmonary arterial pressure or chest pain. The sensory-affective domain of dyspnoea also correlates with age, depression and breathing variability.

Volumetric capnography: lessons from the past and current clinical applications

Dead space is an important component of ventilation–perfusion abnormalities. Measurement of dead space has diagnostic, prognostic and therapeutic applications. In the intensive care unit (ICU) dead space measurement can be used to guide therapy for patients with acute respiratory distress syndrome (ARDS); in the emergency department it can guide thrombolytic therapy for pulmonary embolism; in peri-operative patients it can indicate the success of recruitment maneuvers. A newly available technique called volumetric capnography (Vcap) allows measurement of physiological and alveolar dead space on a regular basis at the bedside. We discuss the components of dead space, explain important differences between the Bohr and Enghoff approaches, discuss the clinical significance of arterial to end-tidal CO₂ gradient and finally summarize potential clinical indications for Vcap measurements in the emergency room, operating room and ICU.

Using the features of the time and volumetric capnogram for classification and prediction

Quantitative features derived from the time-based and volumetric capnogram such as respiratory rate, end-tidal PCO₂, dead space, carbon dioxide production, and qualitative features such as the shape of capnogram are clinical metrics recognized as important for assessing respiratory function. Researchers are increasingly exploring these and other known physiologically relevant quantitative features, as well as new features derived from the time and volumetric capnogram or transformations of these waveforms, for: (a) real-time waveform classification/anomaly detection, (b) classification of a candidate capnogram into one of several disease classes, (c) estimation of the value of an inaccessible or invasively determined physiologic parameter, (d) prediction of the presence or absence of disease condition, (e) guiding the administration of therapy, and (f) prediction of the likely future morbidity or mortality of a patient with a presenting condition. The work to date with respect to these applications will be reviewed, the underlying algorithms and performance highlighted, and opportunities for the future noted.

CO2 measurement for the early differential diagnosis of pulmonary embolism-related shock at the emergency department: A case series

Objective

An early differential diagnosis is mandatory when facing a patient with clinical shock of unclear aetiology, in order to guide proper treatment. We assessed if the expired CO₂ measurement and alveolar-arterial CO₂ calculation could improve the differential diagnosis of shock during its initial presentation, particularly in separating pulmonary embolism from other causes of shock.

Methods

We analysed the charts of 12 patients who presented with clinical shock and had end-tidal CO₂ (EtCO₂) and arterial CO₂ partial pressure (PaCO₂) measurements.

Results

In cases with pulmonary embolism-related shock (n = 3), the gradient between PaCO₂ and EtCO₂ was increased (37 vs 0.2 mmHg). There was a similar trend for a higher PaCO₂ value (60 vs 32.2 mmHg) and a lower EtCO₂ value (23 vs 32 mmHg).

Conclusion

An initial CO₂ measurement might be an easily available tool for the early diagnostic work-up of clinical shock.

Current applications of capnography in non-intubated patients

Current clinical guidelines recommend capnography as one of the best non-invasive methods to assess adequacy of ventilation in the non-intubated patient. Alveolar hypoventilation or respiratory depression is a serious event that occurs in a variety of clinical settings where patients receive sedatives and opioids. With the large number of procedures performed outside the operating room under the effects of sedatives and the increased use of patient-controlled analgesia, the need for capnography for monitoring has dramatically increased. Despite the succesful use of capnography to monitor ventilation in the operating room over several decades, other clinical areas have been very slow adapters of the technology and still rely heavily upon pulse oximetry to detect hypoventilation. This article reviews the most current evidence for using capnography in the non-intubated patient and summarizes the results of outcome measures reported in recent clinical trials. Capnography should be routinely used for non-intubated patients at risk for respiratory depression, in particular those receiving supplemental oxygen.

D-dimer and exhaled CO2/O2 to detect segmental pulmonary embolism in moderate-risk patients

RATIONALE

Pulmonary embolism (PE) decreases the exhaled end-tidal ratio of carbon dioxide to oxygen (etCO(2)/O(2)).

OBJECTIVES

To test if the etCO(2)/O(2) can produce clinically important changes in the probability of segmental or larger PE on computerized tomography multidetector-row pulmonary angiography (MDCTPA) in a moderate-risk population with a positive D-dimer.

METHODS

Emergency department and hospitalized patients with one or more predefined symptoms or signs, one or more risk factors for PE, and 64-slice MDCTPA enrolled from four hospitals. D-dimer greater than 499 ng/ml was test(+), and D-dimer less than 500 ng/ml was test(-). The median etCO(2)/O(2) less than 0.28 from seven or more breaths was test(+) and etCO(2)/O(2) greater than 0.45 was test(-). MDCTPA images were read by two independent radiologists and the criterion standard was the interpretation of acute PE by either reader. PE size was then graded.

MEASUREMENTS AND MAIN RESULTS

We enrolled 495 patients, including 60 (12%) with segmental or larger, and 29 (6%) with subsegmental PE. A total of 367 (74%) patients were D-dimer(+), including all 60 with segmental or larger PE (posterior probability 16%). The combination of D-dimer(+) and etCO(2)/O(2)(+) increased the posterior probability of segmental or larger PE to 28% (95% confidence interval [CI] for difference of 12%, 3.0-22%). The combination of D-dimer(+) and etCO(2)/O(2)(-) was observed in 40 patients (8%; 95% CI, 6-11%), and none (0/40; 95% CI, 0-9%) had segmental or larger PE on MDCTPA. No strategy changed the prevalence of subsegmental PE.

CONCLUSIONS

In moderate-risk patients with a positive D-dimer, the et etCO(2)/O(2) less than 0.28 significantly increases the probability of segmental or larger PE and the etCO(2)/O(2) greater than 0.45 predicts the absence of segmental or larger PE on MDCTPA.