The Collapsing Cat with a Rate-Dependent Murmur
A 9-year-old castrated male domestic shorthair, 6.8 kg, is presented collapsed at 23:00. The owner reports a 3-day history of hiding and reduced appetite. On triage: obtunded, pale pink mucous membranes, CRT 3 seconds, temperature 37.1 C, heart rate 210 bpm, weak femoral pulses, respiratory rate 48 with mild effort. Doppler systolic blood pressure is 78 mmHg. You place a 22 g cephalic catheter and draw a PCV/TS (PCV 41%, TS 7.8 g/dL), glucose 6.1 mmol/L, and a venous lactate of 6.4 mmol/L. The attending clinician is 20 minutes away. A colleague suggests a 10 mL/kg bolus of a synthetic colloid while you wait. The practice has LRS, 0.9% NaCl, and a small supply of hypertonic saline (7.2%).
1. Classify this patient's perfusion status and justify an immediate fluid strategy using the available fluids. Explicitly address the colloid suggestion.
Read the worked answer
This is decompensated (cold, hypotensive) shock: obtunded, HR 210, weak pulses, MAP-equivalent 78 mmHg, prolonged CRT, hyperlactatemia. The murmur the clinician will hear is likely dynamic outflow (HCM-associated or rate-related) - do not anchor on it.
Choice: LRS is the default crystalloid. Start a small-volume bolus (10–15 mL/kg = ~70–100 mL over 10–15 min) and reassess pulse quality, mentation, HR, and BP after each aliquot. Cats with suspected cardiac disease tolerate volume poorly; small, repeated boluses with reassessment are safer than a large single load.
Hypertonic saline: 7.2% HTS at 3–4 mL/kg (≈20–27 mL) over 10–15 min, diluted or followed by crystalloid, is reasonable for refractory hypotension while awaiting the clinician - but it is a bridge, not a resuscitation endpoint, and it worsens hypokalemia and hypernatremia risk.
Colloid: decline. Synthetic colloids are associated with coagulopathy and renal injury and are not recommended for routine resuscitation in veterinary patients; with no evidence of albumin loss (TS 7.8 g/dL is high, consistent with hemoconcentration), there is no oncotic rationale.
Reassess lactate and perfusion in 30–60 min; a falling lactate and improved pulse quality are the endpoints, not a target BP number alone.
2. The clinician arrives and asks you to prepare for a fluid challenge while ruling out cardiac vs. distributive causes. List the monitoring parameters you will record before and after each bolus and explain what change in each would push you toward stopping fluids.
Read the worked answer
Record a paired set before and 10–15 min after each bolus:
| Parameter | Stop/withhold trigger |
|---|---|
| Respiratory rate/effort | Rising RR, new crackles, or increased effort → stop |
| Lung auscultation | New or worsening crackles |
| Heart rate | Paradoxical rise or failure to fall with improved perfusion |
| Pulse quality/CRT | Improving → good sign; worsening despite volume → reassess cause |
| Doppler BP | Sustained rise without respiratory change is reassuring; a rise accompanied by tachypnea is not |
| Lactate | Falling trend supports continued cautious resuscitation; rising despite volume suggests ongoing hypoperfusion or sepsis |
| Mentation | Improvement is a key endpoint |
| Body weight | Serial weights track cumulative volume |
The judgement point: in a cat with a murmur and possible HCM, "more fluid" is not automatically better. If perfusion improves but respiratory signs appear, stop and consider inotropes/vasopressors and cardiology input rather than more volume.