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Fetal Monitor as Precordial Doppler in Neuroanesthesia

Cormack, John R. MBBS, FANZCA; Mellios, Anastasia MBBS (Hons); McGlade, Desmond MBBS, FANZCA

doi: 10.1213/XAA.0000000000000483
Case Reports: Letter to the Editor
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Department of Anaesthesia and Pain Management, St. Vincent’s Hospital, Fitzroy, Melbourne, Australia, cormackj@ozemail.com.au

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To the Editor

The precordial Doppler remains a useful noninvasive monitor to aid in the detection of venous air embolism in sitting or head-up patients requiring anesthesia for craniotomy or burr hole.1 The commercially available monitor with an appropriate flat transducer for precordial placement is not always readily available and suffers from diathermy interference.2 We have successfully used a continuous fetal heart Doppler (a component of a CTG cardiotocogram) loaned from the obstetric unit as an excellent alternative (Phillips Avalon FM 30). It utilizes an ultrasound frequency 1 MHz ± 100 Hz compared with the conventional precordial Doppler (Versatone device) that has a frequency of 2.4 MHz. Clearly, the 2 Dopplers are designed for different purposes. However, our experience with the obstetrical Doppler in many neurosurgical cases has led to clear confirmation of tone change with injection of agitated saline and, in one case, picked up a small air embolus following craniotomy upon returning to supine from a semisitting position. In this case, the tone change was associated with a moderate drop in end-tidal co2 and brief bradycardia and hypotension that was self-limiting.

Sonographs clearly demonstrate a graphical representation of the tone change heard with fetal Doppler (CTG) before and after injection of agitated saline through a peripheral cannula (Figure 1). We have compared this with a sonograph generated from the spectral Doppler recording, before and after agitated saline injection, with transthoracic echocardiography and centered on the tricuspid valve (Figure 2).

Figure 1.

Figure 1.

Figure 2.

Figure 2.

The CTG unit is compact, easy to use, and has an 8-hour battery life and no audible interference from bipolar diathermy. It is extremely sensitive, giving a clear, harsh murmur upon injection of 30 mL of agitated saline when placed over the right or left parasternal edge, as demonstrated by Schubert et al.1 In the authors’ opinions, the change of tone is equal to that of current, purpose-built technology.

The precordial Doppler continues to remain relevant. It offers recognized advantages over transesophageal echocardiography because of its noninvasive nature and lack of specialty training required for interpretation of results. Both modes of detection are highly sensitive, with transesophageal echocardiography able to detect as little as 0.02 mL/kg air and precordial Doppler 0.05 mL/kg. The difficulty is in differentiating those patients who have clinically significant air emboli. Further research is being undertaken to correlate the patterns of Doppler signal waveform analysis and gas volumes entrained, to aid in recognition of this.

At our institution, we believe that precordial Doppler monitoring is an underutilized tool in the monitoring of semisitting craniotomy,3 in part because of lack of availability. Surely, other hospitals suffer from the same problems. This simple alternative is a ubiquitous technology available at any health care center that also deals with obstetric population, and it may help to allow neuroanesthesiologists to monitor more often for a rare but potentially fatal complication.

John R. Cormack, MBBS, FANZCA
Anastasia Mellios, MBBS (Hons)
Desmond McGlade, MBBS, FANZCA
Department of Anaesthesia and Pain Management
St. Vincent’s Hospital
Fitzroy, Melbourne, Australia
cormackj@ozemail.com.au

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REFERENCES

1. Schubert A, Deogaonkar A, Drummond JC. Precordial Doppler probe placement for optimal detection of venous air embolism during craniotomy. Anesth Analg. 2006;102:15431547.
2. Warltier DC, Mirski MA, Lele AV, Fitzsimmons L, Toung TJK. Diagnosis and treatment of vascular air embolism. Anesthesiology. 2007; 106:164177.
3. Liutkus D, Gouraud JP, Blanloeil Y. The sitting position in neurosurgical anaesthesia: a survey of French practice [in French]. Ann Fr Anesth Reanim. 2003;22:296300.
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