1 Introduction

Capnography measures the levels of excreted carbon dioxide (CO2) into the blood stream. End tidal CO2 (PETCo2) is the partial pressure or maximal concentration of CO2 at the end of expiration [1]. In patients with normal lung and reasonable cardiac function such as head injury patients, the PETCo2 can be used as a surrogate for partial pressure of CO2 (PaCO2) in mechanically ventilated patients [2]. Intensive care society capnography guidelines updated in 2015 recommend continuous capnography monitoring in all ventilated ICU patients [3]. Thus early interpretation and accurate assessment of capnogram is crucial in neurocritical care patients. We present and discuss a capnogram artifact in an adult head injury, mechanically ventilated patient.

2 Case Detail

A 46 year old male patient with no co-morbidities was admitted to neuro-critical care ward after left front temporal decompressive craniotomy for acute sub dural haemorrhage. With an admission Glasgow Coma Score (GCS) of E1V1M5 he was immediately intubated and transported to the operating room. Post surgery his GCS was E1Vt M5 and he was shifted to ICU for further management. In the ICU the brain protection strategy followed included maintaining mean arterial pressure >70 mm Hg, head end elevation to 30°, hyperosmolar therapy with mannitol and maintaining the PETCo2 in the range of 30–35 mm Hg. On day 2 of his ICU stay the nurse informed the physician of the low PETCo2 alarm on the patient monitor. On observation the PETCo2 was 21 mm Hg, haemodynamically stable and there was no recent change in ventilator settings (MAQUET, Servo i, Sweden). (Fig. 1a) But on careful enquiry we noted that (a) the event occurred after the nurse connected the jet nebulizer (Cirrus TM 2 nebulizer with 22 mm T piece kit and tube, Intersurgical, UK) in the patients breathing circuit between the main stream capnometer and the tracheostomy tube (Fig. 1b) (b) the PETCo2 value returned to 34 mm Hg (Fig. 1c) after disconnecting the nebulizer from the patient circuit.

Fig. 1
figure 1

Snapshot of the patient monitor with stable haemodynamics. Decreased PETCo2 to 21 mm Hg from a base line of 34 mm Hg. The capnograph waveform shows normal phase I and II, but there is gradual decrease of the phase III to baseline leading to decreased PETCo2 value. Snapshot of the patient monitor after disconnecting the jet nebulizer from the ventilator circuit. We can see the normal capnogram waveform with stable haemodynamics. Image showing the location of jet nebulizer between the main stream capnometer and the tracheostomy tube

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3 Discussion

Capnography is not used widely in ICU compared to its use in operation theatre. Only 25% of intensive care units in UK use capnography for the continuous monitoring of mechanically ventilated patients as of 2009 [4]. According to the national patient safety agency (NPSA) database approximately 500 patient safety incidents associated with the use of airway devices occurred in ICUs every year of which 10% are associated with more than temporary harm [5]. It is important to observe that though the disconnection or accidental removal of a tracheostomy or endotracheal tube can be easily recognized by the ventilator alarm, the partial dislodgement of the ventilator circuit especially when changing patient’s position or during airway procedure can be easily identified by capnogram morphology [6].

With the main stream capnometer (Philips M2501A Mainstream Co 2 sensor, Einthoven, Netherlands) used in our case, a sample cell is connected directly between the breathing circuit and the tracheostomy tube. Given our concern with the possible obstruction by condensation of water and accumulation of secretions on the windows in the sample cell we placed the sensor distal to the heat moist exchanger (HME). It is known that with a sudden abrupt fall in PETCo2 due to venous air embolism/ pulmonary thrombo-embolism/decreased cardiac output [7] capnogram morphology is preserved i.e., all the four phases are identifiable. But in this case along with the abrupt fall in PETCo2 to 22 mm Hg from a baseline of 34 mm Hg there was a gradual change of phase III of capnogram to zero. Also in our case there were no associated clinical signs such as a fall in blood pressure, oxygen saturation or no recent changes in ventilatory settings.

The reason for this capnogram artifact can be explained by connection of the jet nebulizer between the capnometer and tracheostomy tube, leading to the dilution of expiratory gases by the forward flow of fresh gas flow (oxygen) at 10 L/min. This occurs during the latter part of expiration when there was a decline in expiratory flow rate below forward fresh gas flow leading to a gradual decrease of the phase III of the capnograph to base line and a falsely low PETCo2 value. Additionally, it is important to understand that this dilutional effect varies throughout the expiratory cycle and can be quantified by observing the expiratory flow waveform. From this it can be observed that the peak expiratory flow values relative to the jet nebulizer flow occur at the start of expiration versus the minimal flow at the end which results in nearly no CO2 being “seen” in the sample cell of the mainstream sensor.

It is important to note that jet nebulizers are the commonly used device for nebulization during mechanical ventilation as they are simple to use and cost effective [8, 9]. Since the operation of jet nebulizer requires a gas flow of 6–8 L/min they can interfere with the tidal volume delivered by the ventilator [10]. To overcome these problem jet nebulization systems may be integrated into modern ventilators which by synchronizing delivery of nebulization with inspiration reduce the loss of aerosolized medication during expiration and provide precise delivery of tidal volumes thus avoiding use of external gas source for providing nebulization [11].

Prolonged prophylactic hyperventilation with PaCo2 ≤ 25 mm Hg is not recommended as it can lead to cerebral ischemia. Since PETCo2 can be used as a surrogate for PaCO2 the continuous monitoring of capnogram can help maintain eucapnia in head injury patients and help decrease the utilization of arterial blood gas analysis to determine PaCO2. Continuously monitoring PETCo2 in head injury patient on mechanical ventilation will help avoid hyperventilation.

4 Conclusion

This report highlights the importance of the proper placement of breathing circuit components to ensure accurate CO2 readings in particular the use of a jet nebulizer. The addition of nebulizer between the mainstream capnometer and the endotracheal tube can lead to falsely low PETCo2.values. It is critical for the clinician to be able to see the capnogram in order to better troubleshoot situations such as this.

Thus the clear understanding of the waveform morphology can help avoid confusion by differentiating this scenario from other life threatening conditions causing abrupt low PETCo2 such as pulmonary thrombo-embolism/ decreased cardiac output and help maintain eucapnia.