From the Guidelines
End tidal waveform not returning to baseline zero in between waveforms is indicative of incomplete exhalation or air trapping, which can be a sign of obstructive lung diseases or issues with ventilation. This pattern is often associated with conditions that affect lung function and airway resistance, such as asthma or chronic obstructive pulmonary disease (COPD) 1. The "shark fin" appearance on capnography occurs when patients cannot fully exhale before the next breath begins, causing carbon dioxide to accumulate in the lungs and preventing the CO2 level from dropping to zero between breaths.
Key Points to Consider:
- Incomplete exhalation or air trapping can lead to increased airway resistance, bronchospasm, or decreased lung elasticity, prolonging the expiratory phase 1.
- Clinically, this finding suggests the need for interventions such as bronchodilator therapy, increased expiratory time, or reduced respiratory rate to allow more complete exhalation.
- In mechanical ventilation, adjustments might include decreasing respiratory rate, increasing inspiratory flow rate, or adding bronchodilators to mitigate air trapping and its complications.
- Recognizing this pattern is crucial as persistent air trapping can lead to dynamic hyperinflation, increased work of breathing, and potentially dangerous complications like auto-PEEP (intrinsic positive end-expiratory pressure) and hemodynamic compromise 1. Some key considerations in managing such patients include the use of continuous capnography to assess the quality of chest compressions and ventilation, as well as the correct placement of the endotracheal tube 1.
From the Research
End Tidal Waveform Analysis
- The end tidal waveform not returning to baseline zero in between waveforms is indicative of inadequate expiration or increased dead space, which can lead to hypercapnia 2.
- Hypercapnia is the elevation in the partial pressure of carbon dioxide (PaCO2) above 45 mmHg in the bloodstream, and it can be caused by a decrease in minute volume, an increase in dead space, or an increase in carbon dioxide (CO2) production per sec 2.
- In patients with severe acute respiratory distress syndrome (ARDS), permissive hypercapnia can occur, and the end tidal waveform can be affected by the ventilatory techniques used, such as expiratory washout or optimized mechanical ventilation 3.
Ventilatory Techniques and Hypercapnia
- Optimized mechanical ventilation, which includes increasing the respiratory frequency and reducing instrumental dead space, can be as efficient as expiratory washout in reducing PaCO2 in patients with severe ARDS and permissive hypercapnia 3.
- The combination of expiratory washout and optimized mechanical ventilation can have additive effects and result in PaCO2 levels close to normal values 3.
- In patients with ARDS and renal failure, rescue therapy for hypercapnia due to high PEEP mechanical ventilation can include extracorporeal removal of carbon dioxide, which can provide partial lung support 4.
Pathophysiological Mechanisms
- Hypercapnic respiratory failure can be associated with hypoxemic respiratory failure and places high demands on mechanical ventilation 5.
- The pathophysiological mechanisms of hypercapnia include the decrease in minute volume, an increase in dead space, or an increase in carbon dioxide (CO2) production per sec, which can generate a compromise at the cardiovascular, cerebral, metabolic, and respiratory levels 2.
- Acidosis and hypercapnia induced by tidal volume reduction and increase in PEEP at constant plateau pressure can be associated with impaired right ventricular function and hemodynamics despite positive effects on oxygenation and alveolar recruitment 6.