Heart- and Cardiovascular Surgery
Emboli detection using transcranial Doppler ultrasound is based on HITS detection. HITS means "high intensity transient signals". HITS are short, overshooting Doppler patterns. In contrast to artefacts, HITS generate solely unidirectional signals.
|Emboli Differentiation||Emboli Detection|
Emboli detection using transcranial Doppler ultrasound is based on HITS detection. HITS means “high intensity transient signals”. HITS are short, overshooting Doppler patterns. In contrast to artefacts, HITS generate solely unidirectional signals.
Identification of HITS should be based on the guidelines of “Consensus Committee of the Ninth International Cerebral Hemodynamic Symposium“, 1995:
- A Doppler microembolic signal is transient, usually lasting less than 300 milliseconds. Its duration depends on its time of passage through the Doppler sample volume.
- The amplitude of a Doppler microembolic signal is usually at least 3 dB higher than that of the background blood flow signal and depends on the characteristics of the individual microembolus.
- Within the appropriate dynamic range of bidirectional Doppler equipment, a signal is unidirectional within the Doppler velocity spectrum.
- The maximum of the signal belongs to the envelope courve.
- Depending on the equipment used and its own velocity, a microembolic signal is accompanied by a “snap,” “chirp,” or “moan” on the audible output.
Modified from: Dissertationsschrift von Alexandra Schröder, Rhein-Ruhr-Universität Bochum, 2004
Generally, in terms of pathophysiology, two different types of emboli should be distinguished: solid (e.g. plaque material) and gasous (e.g. air embolism).
According to size, origin and count, emboli do have different clinical relevance. Whereas small amounts of air embolism can be reabsorbed and metabolised within brain capillaries before hypoperfusion or even stroke occurs, solid emboli (e.g. plaque material scattered during carotid stenting) are resorbed or broken down significantly slower.
Because of missing collateralisation beyond the Circle of Willis, occlusions of cerebral arteries result in hypoperfusion or according to size and localisation of the embolus in stroke.
Different studies concordantly show that emboli detection is an adequate technical method to narrow the probability of embolic aetiology of stroke [Lund C. et al., 2000; Grosset D.G. et al., 1994].
Whereas measurement of blood flow velocity by change of reflected Doppler frequency belongs to clinical routine in most hospitals, change of received Doppler intensity is important to detect embolic signals.
Such intensity modulations result from US reflections at interfacial tissues with different acoustic impedances (e.g. between air and blood), which can be detected by ultrasound sensitively. Typically, short, high-frequent signals (HITS) can be derived, which are not easy to distinguish from artefacts. Doppler justification should consider a low gain amplitude and a small sample volume to reduce mismeasurement.
Literature references to reduce gain as far as possible that HITS impress more obviously within the pattern, which therefore reduces rate of nonobservence.
Aetiology and clinical relevance
Cerebral embolism is a major reason of ischemic strokes and account for about 20-30% of strokes.
Since intraoperative strokes are predominantly caused by embolism instead of inefficient hemodynamic circulation (Spencer et al., 1997), the occurrence of embolic signals may lead to a change of surgical treatment and thereby help to reduce the risk of intraoperative stroke. Emboli detection becomes particularly relevant in vascular surgery, when the arterial system needs to be opened (e.g. carotid surgery). Among emboli detection, TCD even allows a continuous monitoring of cerebral hemodynamics.
Embolic signals during carotid surgery should be classified into three categories, which occur at different stages and do have different clinical relevance.
- Preparation of the vascular segment
- a. Normally: fragile plaques: high risk of neurological symptoms.
- Re-opening of vascular flow
- a. Normally: gasous emboli, which are rather non-hazardous.
- Postoperatively (within 1-2 hours)
- a. Postoperatively, in about 20% of patients microembolic signals are detectable, which correlates with an increased risk for new strokes. Regarding this purpose, scientific Literature outlines a threshold of about 50 counts per minute.
Up to now, referring to carotid angioplasty (e.g. stenting), there was no significant correlation between count of MES and postoperative ischemic events.
q.v. separate chapter.
Surgery on cardiopulmonary bypass circulation
Supposedly, three different sources of microembolic signals (MES):
- Gasous MES, which are generated due to oxygenation with the extracorporal circulation. These emboli may be reduced by particular filters.
- Inadequate air elimination of the cardiac cavern following cardiac surgery. Clinical relevance is depending on the amount of air.
- MES during clamping and re-opening of the aortic arch (plaque material) – often mixed with gasous signals. Clinical relevance of these signals would only be significantly assessable by emboli differentiation (e.g. EmboDop©)
Regarding symptomatic patients with heart valve implants, particularly mechanical valves, differentiation between gasous and solid emboli becomes clinically relevant. Microembolic signals ware supposed to be a marker for efficiency of anticoagulant drugs. A long time, the exact benefit was unclear.
Since a couple of years it is well known that mechanical heart valves generate gasous bubbles, which are caused by cavitation (differences in pressure release/dissolve nitrogen bubbles) and are considered as relatively harmless. However, in case of neurological symptomatic patients, the heart valve should otherwise be considered as a source of thrombotic material.
Differentiation between “harmless” gaseous and “dangerous” solid emboli could be importantly helpful in administration of anticoagulant drugs.
Herold Innere Medizin, 2011.
Doppler- und Duplexsonographie der hirnversorgenden Arterien. Widder et Görtler, Springer Verlag, 6. Auflage, 2004