Table 1.

Modified Spencer and Eftedal–Brubakk scales for audio Doppler and 2D echo bubble grading as adapted for our study. Both are categorized, non-linear scales, and a direct comparison of single grades is difficult

Modified Spencer scale for audio Doppler bubble detection	Bubble grades	Eftedal–Brubakk scale for echocardiographic bubble detection
No adequate signal	X	No adequate signal
No bubbles detectable	BG0	No bubbles visible
Occasional bubbles	BG1	Occasional bubbles
Bubble signals in less than half of heartbeats	BG2	At least 1 bubble/4 heartbeats
Bubble signals in most of heartbeats	BG3	At least 1 bubble/heartbeat
Bubble signals continuously and predominantly	BG4	At least 1 bubble at every cm2 in every view
	BG5	Whiteout—no single bubble discrimination

However, a rough relation of lower, medium and higher bubble grades between such scales and their different underlying measurements is considered adequate in this study