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| Rate (beats/min) | R-R interval (s) | |
| Normal – 20 weeks | 120-160 | 0.5-0.375 |
| Normal – 36 weeks | 110-150 | 0.55-0.4 |
| Tachycardia | >180 | <0.33 |
| Bradycardia | <100 | >0.6 |
| Rate (beats/min) | R-R interval (s) | |
| Normal – 20 weeks | 120-160 | 0.5-0.375 |
| Normal – 36 weeks | 110-150 | 0.55-0.4 |
| Tachycardia | >180 | <0.33 |
| Bradycardia | <100 | >0.6 |