VITALS Newsletter — by Niki DeValk, AAS, LVT, SRS
Interventional & Surgical Specialist | Owner, NiKara Preclinical
Let's Dive In
This month's theme came straight from my community. After I posted about training, a colleague suggested ECG interpretation, and I couldn't think of a better way to spend November. All month, VITALS is about all things cardio: the trace, the physiology behind it, and the decisions it drives in the surgical suite.
The ECG is one of the most continuous streams of information we have under anesthesia. It updates with every beat, it responds to every drug we give, and it often tells us a story before the blood pressure or the end-tidal CO2 does. Reading it well isn't about memorizing waveforms. It's about knowing what normal looks like for the animal on the table, and noticing quickly when the story changes.
Why This Topic Matters
In preclinical work, the ECG does double duty. It is a real-time safety monitor for the animal, and in many studies it is also study data. Cardiac device trials, interventional procedures, and safety pharmacology programs all depend on clean, well-interpreted traces. A rhythm change that goes unrecognized can affect the animal and the dataset at the same time.
The challenge is that "normal" is species-specific. A trace that would be alarming in a dog can be completely routine in a horse. A pig's heart conducts differently from a sheep's. Rodent hearts beat so fast that some familiar landmarks barely exist. Add the predictable effects of anesthetic drugs and electrolyte shifts, and ECG interpretation becomes a true multispecies skill. That's exactly the kind of skill I love to build in surgical and anesthesia teams.
Key Insights
A clean trace starts with placement
Good contact matters more than equipment. Clip, clean, and use fresh gel or adhesive electrodes, and keep leads away from cautery cables, warming devices, and fluid pumps.
Large animals in dorsal recumbency often read best with a base-apex lead, which produces large, easy-to-follow complexes in horses, cattle, sheep, and goats.
Small patients like rabbits, rodents, and guinea pigs often do better with fine subcutaneous needle electrodes or padded clips, since their low-amplitude signals are easy to lose in artifact.
Consistent placement across animals and timepoints is what makes ECG data comparable within a study.
"Normal" looks different in every species
Horses: High resting vagal tone means second-degree AV block and sinus arrhythmia can be completely physiologic.
Swine: Purkinje fibers run through nearly the full thickness of the ventricular wall. This changes how the ventricles activate and helps explain why pigs are known for ventricular arrhythmias during cardiac manipulation.
Dogs: Respiratory sinus arrhythmia is a normal, healthy finding.
Birds: The QRS complex is typically negative in lead II, the reverse of what most mammal-trained eyes expect.
Mice and rats: Heart rates of several hundred beats per minute compress the trace, and mice lack a clearly separated ST segment.
Anesthetic drugs leave fingerprints
Alpha-2 agonists (dexmedetomidine, xylazine) reliably cause bradycardia, and first- or second-degree AV block is an expected pattern. Ruminants are especially sensitive to xylazine.
Opioids often slow the heart through vagal effects.
Anticholinergics raise heart rate, and can briefly produce AV block just after administration before the rate climbs.
Ketamine tends to support heart rate through sympathetic stimulation.
Inhalant anesthetics can prolong the QT interval, which matters in studies where QT is an endpoint.
Electrolytes write on the ECG
Rising potassium shows up as tall, peaked T waves, then flattened P waves, widened QRS complexes, and slowing heart rate.
Low potassium can bring ST depression, prominent U waves, and ectopic beats.
Calcium shifts the QT interval: longer with low calcium, shorter with high calcium.
Recognizing these patterns early means you can check a blood gas and correct the cause rather than just treating the rhythm.
What's Changing
Nonclinical ECG data carries more regulatory weight. The 2022 ICH E14/S7B Q&As describe how well-designed in vivo QT studies, combined with in vitro hERG data, can support an integrated risk assessment and in some cases reduce the need for a clinical thorough QT study. That raises the bar for how ECG data are collected and analyzed in animal studies.
Best-practice in vivo QT methods are now spelled out. The same Q&As include recommendations on study quality, which puts a spotlight on consistent lead placement, stable anesthesia or conscious recording, and validated heart rate correction.
Telemetry is getting smarter. Automated analysis tools now flag artifacts, abnormal RR intervals, and arrhythmias across very long recordings, with skilled human review still confirming what the software finds.
Deep learning is entering the trace. Researchers are applying deep learning models to animal ECGs, including transfer learning from human ECGs to detect early conduction changes in mouse models. It's early, but promising.
Less invasive recording options keep growing. Jacketed external telemetry and group-housed recording let many studies collect high-quality ECG data with fewer surgical implants and less stress on the animals.
Clinical Connections
Here's a quick reference I use when moving between species. Heart rates are approximate awake ranges for adult research animals and vary with age, breed, strain, and body size. Young, growing animals such as weanling and grower pigs often run higher. Expect lower values under most anesthetic protocols, especially with alpha-2 agonists and opioids.
Species Typical awake HR (bpm) Lead that reads well under anesthesia Normal finding worth knowing
Horse 28–44 Base-apex Physiologic second-degree AV block at rest
Cattle 48–84 Base-apex Large, clear complexes in base-apex
Sheep / goat 60–120 Base-apex or lead II Very sensitive to alpha-2 bradycardia
Swine 70–120 Lead II Transmural Purkinje network; arrhythmia-prone with cardiac manipulation
Dog 60–160 Lead II Respiratory sinus arrhythmia
Ferret 180–250 Lead II Small complexes; good contact is key
Rabbit 180–300 Lead II Low amplitude; needle electrodes help
Guinea pig 230–380 Lead II Widely used in QT-focused studies
Chicken 250–350 Lead II QRS typically negative in lead II
Rat 300–450 Lead II Short, compressed intervals
Mouse 500–700 Lead II No clearly separated ST segment
Case Study: The Broiler With "Normal" Vitals and a Compromised Airway
The scenario: An ovine model is premedicated with dexmedetomidine before induction for a peripheral interventional procedure. Shortly after premedication, the monitor shows a slow heart rate with intermittent P waves that aren't followed by a QRS complex. A newer team member flags it right away and asks whether to give atropine.
The pivot: Instead of reacting to the rhythm alone, I looked at the whole picture. The pattern was second-degree AV block, a well-known fingerprint of alpha-2 agonists. Blood pressure was well maintained, mucous membranes were pink, and the conducted beats were regular. Reaching for an anticholinergic by reflex can drive the heart rate up while the alpha-2 vasoconstriction is still in place, which increases cardiac workload without solving a real problem. So I kept monitoring closely, with a clear threshold for when I would step in.
The outcome: The AV block resolved on its own as the drug's peak effect faded and the induction and maintenance phases went smoothly. The procedure ran on schedule, and the ECG record documented an expected drug effect rather than an unplanned intervention.
The takeaway: Knowing what each drug is supposed to do to the trace turns a startling rhythm into an expected one. Read the rhythm in context with pressure, perfusion, and timing, then decide.
Practical Takeaways
Know the species-normal trace before the first incision, not after the first surprise.
Invest in electrode contact and consistent placement. Clean data starts there.
Learn each drug's expected fingerprint so you can tell a drug effect from a developing problem.
Read the ECG alongside blood pressure, perfusion, and end-tidal CO2. No single number tells the whole story.
When the T waves or QT interval change, think electrolytes and check a blood gas.
In studies where the ECG is an endpoint, standardize everything you can: lead, position, timing, and anesthetic depth.
Wrapping Up
I'm grateful to the colleague who suggested this theme, and I'm excited to spend the whole month on it. On Friday, I'll publish a deep dive on comparative electrocardiography across preclinical species, with more detail on conduction physiology, anesthetic influences, and how ECG data supports safety pharmacology. I'll also be sharing ECG tips on the company page every Tuesday and Thursday.
If your team would like ECG interpretation training, perioperative monitoring support, or help building cardiac monitoring into your SOPs, I'd love to talk. VITALS Academy training can be tailored to your species, procedures, and study needs.
Stay sharp. Stay supported. Stay vital.
— Niki
Niki DeValk, AAS, LVT, SRS
Independent Surgical & Anesthetic Specialist | Owner, NiKara Preclinical
📧 niki@nikarapreclinical.com 🌐 www.nikarapreclinical.com
