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What We Already Know about This TopicSeveral ultrasound parameters for assessing fluid responsiveness have been describedTransfontanelle ultrasound can be easily used in small children undergoing surgery, and the anterior fontanelle is an optimal site for Doppler examination of the internal carotid arteryPrevious studies have identified a relationship between fluid responsiveness and respiratory variation in the arterial blood flow peak velocity in the ascending aorta and/or the proximal branches of the aorta What This Article Tells Us That Is NewIn infants having cardiac surgery, the respiratory variation of the internal carotid artery blood flow peak velocity as measured using transfontanelle ultrasound predicts an increase in stroke volume in response to an intravenous fluid bolus Background: Cranial sonography is a widely used point-of-care modality in infants. The authors evaluated that the respiratory variation of the internal carotid artery blood flow peak velocity as measured using transfontanelle ultrasound can predict fluid responsiveness in infants. Methods: This prospective observational study included 30 infants undergoing cardiac surgery. Following closure of the sternum, before and after the administration of 10ml · kg–1 crystalloid, the respiratory variation of the aorta blood flow peak velocity, pulse pressure variation, and central venous pressure were obtained. The respiratory variation of the internal carotid artery blood flow peak velocity was measured using transfontanelle ultrasound. Response to fluid administration was defined as an increase in stroke volume index, as measured with transesophageal echocardiography, greater than 15% of baseline. Results: Seventeen subjects (57%) were responders to volume expansion. Before fluid loading, the respiratory variation of the internal carotid artery and the aorta blood flow peak velocity (means ± SD) of the responders were 12.6 ± 3.3% and 16.0 ± 3.8%, and those of the nonresponders were 8.2 ± 3.2% and 10.9 ± 3.5%, respectively. Receiver operating characteristic curve analysis showed that the respiratory variation of the internal carotid artery and the aorta blood flow peak velocity could predict fluid responsiveness; the area under the curve was 0.828 (P < 0.0001; 95% CI, 0.647 to 0.940) and 0.86 (P = 0.0001; 95% CI, 0.684 to 0.959), respectively. The cutoff values of the respiratory variation of the internal carotid artery and the aorta blood flow peak velocity were 7.8% (sensitivity, 94%; specificity, 69%) and 13% (sensitivity, 77%; specificity, 92%), respectively. Conclusions: The respiratory variation of the internal carotid artery blood flow peak velocity as measured using transfontanelle ultrasound predicted an increase in stroke volume in response to fluid. Further research is required to establish any wider generalizability of the results.