Physiological Principles of Capnography
- Capnography is a continuous, non-invasive bedside monitoring modality utilized to qualitatively and quantitatively assess a patient's ventilation by measuring the end-tidal carbon dioxide (EtCO2) in exhaled breaths.
- It provides vital real-time information regarding alveolar ventilation and is primarily indicated for critically ill patients requiring invasive mechanical ventilation.
- Physiologically, the measurement relies heavily on adequate pulmonary blood flow, as carbon dioxide must be transported via the systemic venous circulation to the pulmonary capillaries to diffuse into the alveoli and be exhaled.
- Therefore, a sudden drop in EtCO2 often signifies a catastrophic drop in cardiac output or pulmonary perfusion, rather than strictly a primary ventilatory failure.
Normal Capnography
Phases Of Time-Based Capnography
| Phase | Nomenclature | Respiratory Cycle | Physiological Correlate & Characteristics |
|---|---|---|---|
| Phase I | Inspiratory Baseline | Early Expiration | Represents clearance of anatomical dead space. Sampled gas contains no CO2; baseline remains at zero. |
| Phase II | Expiratory Upstroke | Mid Expiration | Transition phase. Reflects rapid mixing of CO2-free dead space gas with CO2-rich alveolar gas. |
| Phase III | Alveolar Plateau | Late Expiration | Exhalation of predominantly alveolar gas. The slight upward slope is due to continuous CO2 diffusion. The peak represents End-Tidal CO2 (PETCO2). |
| Phase 0 | Inspiratory Downstroke | Inspiration | Initiation of inspiration. Rapid fall in CO2 concentration to zero as fresh gas washes over the sensor. |
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Key Diagnostic Angles
| Angle | Anatomical Position on Graph | Clinical Significance | Interpretation of Abnormality |
|---|---|---|---|
| Alpha (α) | Junction of Phase II and Phase III | Evaluates airway patency and expiratory flow. | An increased (obtuse) angle indicates uneven alveolar emptying, classically seen in airway obstruction (e.g., asthma, COPD, bronchospasm). |
| Beta (β) | Junction of Phase III and Phase 0 | Evaluates inspiratory circuit integrity and gas clearance. | An increased (obtuse) angle indicates rebreathing of expired CO2 (e.g., exhausted soda lime, faulty expiratory valve). |
Clinical Utility and Primary Indications
Airway and Ventilator Management
- Capnography is a highly reliable tool for the immediate verification of endotracheal tube (ETT) placement and position following emergency intubation.
- Continuous EtCO2 monitoring reliably detects accidental ETT displacement, acute ETT obstruction (e.g., by mucosal secretions), and the presence of patient-ventilator asynchrony.
- It serves as a primary monitor for continuously evaluating the respiratory rate and the underlying respiratory rhythm in mechanically ventilated children.
Role in Cardiopulmonary Resuscitation (CPR)
- During cardiopulmonary resuscitation, EtCO2 monitoring is utilized to evaluate the quality and effectiveness of ongoing chest compressions.
- If an advanced airway is already in place, continuous end-tidal CO2 monitoring acts as a real-time surrogate for cardiac output generated by manual compressions, although specific target values for pediatric patients are currently lacking.
- A sudden, sustained rise in EtCO2 during CPR is a primary physiological indicator of the Return of Spontaneous Circulation (ROSC), reflecting the sudden restoration of pulmonary blood flow.
Neurocritical Care and Raised Intracranial Pressure (ICP)
- In the management of non-traumatic coma and raised intracranial pressure, continuous monitoring of EtCO2 is mandatory alongside ICP, cerebral perfusion pressure (CPP), and SpO2.
- Because arterial carbon dioxide (PaCO2) is a potent cerebral vasodilator, EtCO2 acts as a continuous non-invasive surrogate to prevent hypercapnia (which acutely increases cerebral blood volume and ICP) or excessive hypocapnia (which can cause severe cerebral ischemia).
Common Capnography traces
Advanced Ventilator Management and ARDS
Dead Space and Ventilation Indices
- In Pediatric Acute Respiratory Distress Syndrome (PARDS), end-tidal CO2 monitoring is recommended to calculate derived parameters that indicate the severity of the lung injury.
- These advanced indices include the dead space to tidal volume ratio (VD/VT), the End-Tidal Alveolar Dead Space Fraction (AVDSF), and the Ventilation Index.
- These parameters serve as essential bedside markers to differentiate between effective alveolar recruitment versus harmful alveolar overdistension, and higher values are directly linked to an increased mortality risk in ARDS.
Calculation of Dead Space
- The proportion of the delivered tidal volume that does not participate in gas exchange (which comprises both anatomical airway dead space and alveolar dead space) is calculated utilizing the Enghoff modification of the Bohr equation.
- The formula utilized is: VD/VT = [PaCO2 - PeCO2] / PaCO2.
- In this physiological calculation, PeCO2 represents the mean expired carbon dioxide measured continuously by volumetric capnometry.
Role in Shock and Tissue Perfusion Monitoring
- Beyond traditional respiratory monitoring, advanced tissue capnometry can be utilized in the assessment of a child in shock.
- Gastric and ileal tissue capnometry serve as specialized, advanced optical monitoring tools to evaluate regional and peripheral tissue perfusion, reflecting the severity of microcirculatory failure and splanchnic ischemia.
Limitations of Capnography
- The most significant limitation of capnography is that its mathematical and clinical reliability is highly dependent on the presence of adequate pulmonary blood flow.
- Consequently, its diagnostic use for confirming airway placement and monitoring ventilation is limited to patients exhibiting a perfusing cardiac rhythm.
- Additionally, the standard use of capnography is technically limited to pediatric patients weighing more than 2 kg.