An arrhythmia is any rhythm outside the 140 to 220 beats per minute a calm adult cat should hold at rest, and three families cover almost every case seen in practice: slow rhythms, fast rhythms, and isolated premature beats. A 30-second ECG samples about 1/2880 of the day, so a cat that faints needs a 24-hour recording rather than a longer stare at the monitor. The grade of the arrhythmia, not its name, sets the recheck date.
Three families carry different risks
Slow rhythms mean a rate under 100 beats per minute in an awake cat, or a pause longer than 2 seconds between beats. Third-degree atrioventricular block and sinus node disease live here, and a 3-second pause is enough to drop a cat mid-stride. Fast rhythms start above 240 beats per minute at rest and stay there; atrial tachycardia, atrial fibrillation and ventricular tachycardia are the three labels a cardiologist picks between, and the width of the QRS complex is what separates the last from the first two. Premature beats are single atrial or ventricular complexes pushed into an otherwise normal rhythm. They are common in cats over 10 years old and harmless until they arrive in pairs or runs of 3 or more.
What a 30-second strip proves, and what it does not
A clinic ECG runs 6 or 12 leads for 30 to 60 seconds. It is excellent for confirming a rate, measuring PR and QT intervals, and documenting a rhythm that never stops. It is close to useless for a cat that collapses twice a week: the odds that a 3-second run lands inside a 30-second window are small, and most cats with syncope leave the clinic with a normal strip. Extend the recording to 5 or 10 minutes while the cat is restrained for a blood draw, then repeat it after 2 minutes of quiet, because a rate that falls from 260 to 190 rules out a persistent tachycardia. Before any rhythm diagnosis, run 4 basics: packed cell volume, potassium (3.5 to 5.8 mmol/L), total thyroxine, and blood pressure. Anemia, low potassium, hyperthyroidism and hypertension all imitate a primary arrhythmia, and all 4 are cheap to exclude.
A 24-hour Holter answers the question the strip cannot
Three adhesive electrodes, one recorder in a vest, 24 hours at home. Shave 3 small patches, stay off the sternum, and tape the lead wires so a 4 kg cat cannot pull them loose in 10 minutes. The owner keeps a diary to the minute: sleep, play, meals, litter box, any collapse. The report returns 4 numbers worth reading — minimum rate (often 60 to 90 during sleep), average rate, maximum rate, and ventricular premature complex count per 24 hours — plus couplet and run counts with a strip for each event. A cat with 0 ventricular beats and a maximum of 210 beats per minute is a different patient from the same cat with 900 beats and 7 runs, even when both fainted once.
Grading decides the recheck interval
Ventricular ectopy is graded by frequency and complexity, and the grade maps onto a calendar. Lone beats under 50 per 24 hours rarely change management. A run of 4 beats, or more than 10 couplets, changes it the same day.
| Grade | Ventricular beats per 24 h | Couplets | Runs of 3 or more | Next recheck |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 12 months |
| 1 | 1 to 50 | 0 | 0 | 6 months |
| 2 | 51 to 500 | 1 to 10 | 0 | 3 months |
| 3 | over 500 | over 10 | 0 to 1 | 4 to 6 weeks |
| 4 | any | any | 2 or more | 7 days, cardiology |
Three numbers the owner can collect at home
Resting respiratory rate is the best home measurement: count for 30 seconds while the cat sleeps, multiply by 2, and log it twice a day for 2 weeks to fix a baseline. Under 30 breaths per minute is the target, and a rise of 10 or more that holds for 2 days is a call to the clinic. Weigh weekly on the same scale, because a 200 g loss in 2 weeks matters in a 4 kg cat. Feel the pulse at the inner thigh for 15 seconds and note whether it is even; an irregularity that comes and goes with breathing is common and benign, one that persists for 60 seconds is not.
Recheck intervals by grade
Grade 0 to 1: auscultation, blood pressure and a 6-lead ECG every 6 to 12 months. Grade 2: every 3 months, with a repeat Holter if the owner reports a new collapse. Grade 3 to 4: 4 to 6 weeks until 2 consecutive recordings are stable, then every 3 months. Echocardiography every 6 to 12 months, since a rhythm problem often sits on top of structural disease. A cat on cardiac medication needs a renal panel and potassium at 7 to 14 days after any change, then at 3 months.
What counts as an emergency
Call ahead and drive now for any of these: a faint longer than 10 seconds, or 2 faints inside 24 hours; a resting respiratory rate above 40 that does not fall after 10 minutes of sleep; open-mouth breathing, grey or blue gums, or a cat that will not lie down; a heart rate above 260 or below 60 with weakness; a pause longer than 3 seconds on any strip; cold, painful hind legs with no femoral pulse, which means a clot and leaves a 6-hour window before muscle damage becomes permanent. A quiet cat with a rate of 200 can wait until morning. A cat that has fainted once cannot.
The workable sequence
- Book the Holter within 7 days of the first faint; do not wait for a second episode to justify it.
- Run the 4 basic tests the same week, so a thyroid or potassium cause is excluded before the recorder goes on.
- Start the respiratory rate log the day the vest comes off, and bring 14 days of numbers to the next visit.
- Repeat the Holter at 3 months if the first grade was 2 or higher, and after any change of medication at 7 to 14 days.
- Photograph the ECG report and keep it in one folder, because comparing 2 strips beats remembering one.
Price the workup before the clinic does: a 24-hour Holter, an echocardiogram and a full panel are itemized in the cost tool. When the same cat also has weight loss, the panel in fecal test types covers the parasites that mimic it, and a curve in insulin types is the next branch if bloodwork shows high glucose.
Frequently asked
Can a normal ECG in the clinic rule out arrhythmia?
No. A 30 to 60 second strip covers roughly 1/2880 of a day, so a run that causes fainting every few days will usually be missed. A normal strip only means nothing abnormal happened during that minute; a cat with faints still needs a 24-hour Holter before the heart rhythm is called normal.
How is a 24-hour Holter fitted on a cat?
Three adhesive patches are shaved and placed on the chest, wires are taped down, and the recorder rides in a light vest for 24 hours. Most cats sleep and eat normally in it. The owner keeps a minute-by-minute diary so each collapse or play session can be matched to the rhythm strip.
When does a fainting cat need to be seen immediately?
Any faint longer than 10 seconds, 2 faints in 24 hours, a resting respiratory rate above 40, open-mouth breathing, or cold hind legs with no pulse all mean go now. A pause over 3 seconds on any recording is also an emergency, because the next one may not restart.