Looking at the strip and the monitor together, several features point to a physiologic sinus tachycardia rather than a paroxysmal SVT:

1. Rate is in the "sinus" zone, not the classic SVT zone The monitor and rhythm strip show a rate of ~128 bpm. AVNRT/AVRT almost always runs faster and very fixed - typically ≥150 bpm and commonly 160-220 bpm. As Rosen's Emergency Medicine notes, AVNRT is "a regular, narrow-complex rhythm with a ventricular rate of 130 beats/min or greater, commonly more than 160 beats/min." A rate of 128 sits comfortably within the sinus tachycardia range and is on the low end for reentrant SVT.

2. Discrete, normally-directed P waves precede every QRS In the long lead II rhythm strip at the bottom of your image, you can trace an upright P wave before each QRS with a consistent PR interval - this is a true sinus P wave (normal atrial depolarization vector: upright in II, and by extension III/aVF, negative in aVR). In AVNRT the atria and ventricles activate almost simultaneously, so the P wave is typically buried inside the QRS or appears as a subtle terminal deflection (pseudo-R' in V1, pseudo-S in II/III/aVF) - it is not seen as a clean, separate P wave the way it is here. Braunwald's Heart Disease specifically flags this: "the presence of an inferior [upright] P wave axis... excludes AVRT or AVNRT because it suggests an [independent sinus] origin" - Braunwald's Heart Disease, 2 Vol Set.

3. Constant 1:1 P-to-QRS relationship with a physiologic PR Every P is followed by a QRS at a fixed, short-but-normal PR interval - conduction is going through the normal sinus-to-AV-node-to-ventricle pathway, not around a reentrant circuit.

4. Clinical context supports a secondary, physiologic cause The patient is anxious/distressed, tachypneic (RR 26), clutching her chest - a picture consistent with pain, anxiety, hypoxia, fever, or hypovolemia driving the sinus node faster, rather than a sudden electrical short-circuit arrhythmia. Harrison's Principles of Internal Medicine draws exactly this distinction: "Physiologic Sinus Tachycardia - defining feature: normal sinus mechanism precipitated by exertion, stress, exogenous or endogenous stimulants, concurrent illness" versus "Pathologic Supraventricular Tachycardia (SVT)," which is not tied to a physiologic trigger - Harrison's Principles of Internal Medicine, 22nd ed.

5. Rate variability Sinus tachycardia typically shows subtle beat-to-beat rate variability (respiratory/autonomic influence), whereas AVNRT/AVRT is "machine-regular" with essentially no variability once it starts (and starts/stops abruptly, "on-off," rather than ramping up gradually as sinus tach does).

The combination of a rate <150 bpm, clearly visible sinus-morphology P waves with a fixed normal PR interval before every QRS, and a clinical context (anxiety, tachypnea, distress) that plausibly explains an elevated sinus rate - rather than an abrupt-onset, P-wave-obscured, machine-regular narrow-complex tachycardia at ≥150-180 bpm - is what makes this sinus tachycardia rather than SVT (AVNRT/AVRT). If there were any diagnostic doubt at the bedside, a vagal maneuver or adenosine trial would help: sinus tach slows transiently and then returns to its prior rate, while AVNRT/AVRT either terminates abruptly or is unaffected - it does not "slow down and speed back up" the way sinus tachycardia does.

I checked for recent contradicting evidence (systematic reviews/meta-analyses, last 2 years) and found nothing that changes these standard ECG differentiation criteria - the recent literature in this space concerns adenosine efficacy/genetics of SVT, not diagnostic ECG criteria.