Waveform capnography is a commonly used monitor in the operating room, and is increasingly seen in non-operating room environments too! The capnographic waveform can be described as having several phases:
- Phase 0 (inspiratory baseline) represents the inspiratory phase of the respiratory cycle.
- Phase 1 is the initial part of expiration, when dead space gases are being exhaled. Since the exhaled gas in this phase did not take part in gas exchange, the PCO2 is 0.
- Phase 2 (expiratory upstroke) involves exhaled gases from alveoli reaching the detector. There is a sharp rise in PCO2 during this phase.
- Phase 3 is a (more or less) flat plateau showing continued exhalation of alveolar gas. The last, maximal part of this phase is the end-tidal point (ETCO2), which is usually 35-40 mmHg. ETCO2 tends to be 2-5 mmHg lower than PaCO2, though this difference can be increased/decreased under a variety of conditions, such as ventilation-perfusion mismatch.
The shape of the capnograph waveform can tell you a lot!
- A slanting upslope can represent airway obstruction (e.g., chronic obstructed pulmonary disease, bronchospasm, blocked endotracheal tube).
- In patients paralyzed with a neuromuscular blocker, as the paralytic wears off they may try to breathe asynchronously against the ventilator, producing a notch called a ‘curare cleft.’
- Quantitative capnography during resuscitation can be very useful. During CPR, there should be a visible waveform during high quality chest compressions; its absence may indicate accidental esophageal intubation
- A sudden loss is bad, as it means that the tube is fully obstructed or disconnected or that there has been a sudden loss of circulation
- You can also just simply tell is someone is hypo- or hyperventilating
- Dorsch JA, Dorsch SE. 2007. Gas monitoring. In: Understanding anesthesia equipment (Dorsch and Dorsch, Eds.) Lippincott Williams & Wilkins, Philadelphia PA.
- Kodali BS. 2013. Capnography outside the operating rooms. Anesthesiology; 118:192.
Volume control (VC) and pressure control (PC) are two common modes of positive pressure mechanical ventilation. In VC, the clinician sets the tidal volume that is given for every breath; pressure is allowed to vary over the course of the breath. In PC, the ventilator is programmed to deliver the same pressure throughout inspiration, so tidal volume is allowed to vary based on the pressure and timing settings, as well as the patient’s own lung compliance.
The timing of ventilation can be set according to a trigger. Continuous mandatory ventilation (CMV) involves setting the respiratory rate and having the ventilator deliver breaths at exactly that rate. This is generally used in paralyzed patients (e.g., general anesthesia), where the patient is not expected to trigger any breaths. In Synchronized Intermittent Mandatory Ventilation (SIMV), mandatory breaths are still given but they are synchronized to the patients’ own respiratory efforts (if present). Also, the patient is allowed to take additional breaths on their own. SIMV is often used to wean patients from the ventilator, by decreasing the rate of mandatory breaths and having patients take more of their breaths spontaneously.
Pressure support (PS) is another mode that is used for weaning. No mandatory breaths are programmed. The patient actively takes their own breaths, and the ventilator simply gives an additional boast of inspiratory pressure to help them out.
Positive End Expiratory Pressure (PEEP) is a setting that is used to prevent alveolar collapse, increase functional residual capacity, and generally improve gas exchange. PEEP involves programming a small amount of additional airway pressure (often ~5-10 cmH2O) to be present at the end of expiration.
- Nugent K, Nourbaksh E (Eds.). 2011. A bedside guide to mechanical ventilation. Createspace.
- Owens W. 2012. The ventilator book. First Draught.
- Kacmarek RM, Hess DR. 2008. Mechanical ventilation for the surgical patient. In: Anesthesiology (Longnecker DE, Brown DL, Newman MF, Zapol WM, Eds.). McGraw Hill, New York.