MedSimu Surgery

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[MUSIC PLAYING]
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Welcome to MedSimmu Surgery Podcast.
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Today, we're doing a deep dive into something absolutely
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fundamental in surgical practice--
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fluid balance and fluid replacement therapy.
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Absolutely.
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It's a topic that underpins so much of what we do.
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Definitely.
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And getting it right is crucial for patient outcomes.
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And let's be honest, for exams like the MRCS2--
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So we've got a good handle on the basics,
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monitoring what goes wrong and how we fix it,
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resuscitation, maintenance replacement, all the key bits.
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- Right, our mission today is really to unpack all of that,
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pull out the essentials you need
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for managing patients effectively,
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and yes, for easing those exams.
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- Let's get into it.
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- Okay, so starting right at the beginning, fluid balance.
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What exactly are we talking about?
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- Well, at its simplest, it's about equilibrium, isn't it?
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The balance between the fluid coming into the body
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and the fluid going out.
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- Input versus output.
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- Exactly.
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And maintaining this balance is vital for, well,
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pretty much every metabolic process.
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It's core to homeostasis.
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- And reminds us about the composition.
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How much fluid are we actually dealing with?
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- Yeah, it's quite a lot.
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Around 60% of our body weight is water.
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And it's not all in one place, obviously.
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- Right.
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- Roughly two thirds is inside the cells
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that's intracellular fluid.
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- Okay.
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- And the other third is outside the cells extracellular.
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That includes plasma, interstitial fluid.
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And the constant movement between these compartments is key.
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Precisely.
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Water, electrolytes, they're always moving back and forth,
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mainly by diffusion and osmosis.
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All the key things stable.
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It's a very dynamic system.
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Which brings us to monitoring.
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Why is it so critical to keep such a close eye on this?
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Because imbalances can be really dangerous, fast.
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Too little fluid, hypovolemia leads to poor organ perfusion
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shock, essentially.
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That's actually good.
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at all. And too much hypervolemia? Well, that can swamp the system, often pointing towards
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heart or kidney problems.
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And it's not just about the volume, is it? The electrolytes get thrown off too.
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Exactly. Electrolyte imbalances are a major hazard. Things like potassium shifts can cause
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serious cardiac arrhythmias and other imbalances mess up metabolism. Very risky.
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So practically speaking, how do we monitor? What are the go-to methods?
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First up is always the clinical examination. You're looking for signs.
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Like what?
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dehydration or the mucous membranes dry? Check capillary refill time. Is it
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prolonged? Like over two seconds. That suggests poor perfusion. Thunken eyes and
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severe cases. Okay, and for overload? You might see edema, perhaps puffy ankles or
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sacral edema if they're bed bound. A raised JVP jugular venous pressure is
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another classic sign. Listening for crackles in the lungs too. And vitals,
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what story do they tell? Oh, vital signs are crucial. In depletion you'd expect
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low blood pressure, hypotension, and the body often compensates with a faster heart rate,
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tachycardia, and maybe faster breathing.
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An overload.
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Could be high blood pressure, hypertension, maybe a bounding pulse.
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Tachycardia can happen there too actually.
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What about urine output? You hear a lot about that.
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Hugely important. It's often one of the earliest and most sensitive indicators.
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We typically want to see more than half a mil per kilo per hour.
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So 0.5 millialkilia or at?
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Yeah, that's the target. If it drops significantly,
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that's a real warning sign of potential depletion or kidney issues. Are there
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other things? Maybe less immediate checks? Definitely. Daily weights are
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surprisingly useful. Small changes day-to-day can show fluid shifts before
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they become clinically obvious. A sudden gain or loss is a bit of a red flag.
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Makes sense. And lab tests. Labs are essential. We look at urea and creatinine.
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They often rise in dehydration as the kidneys try to conserve water.
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electrolytes obviously are vital for spotting and managing those dangerous imbalances.
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Sodium potassium.
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Exactly. And hematocrit can sometimes be helpful too. It might be elevated in dehydration because
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the blood is more concentrated, hemoconcentration.
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Right. So it's a combination of clinical signs, vitals, urine, weight labs. You need the whole
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picture.
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You really do. No single thing tells the whole story.
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Okay. Let's shift gears. What typically causes these imbalances in surgical patients or, you
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know, general medical patients?
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Oh, lots of things can knock it out of whack.
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Very common causes are simple fluid losses,
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think diarrhea, vomiting, high output from an NG tube,
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or surgical drains.
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Yeah, stuff we see all the time.
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All the time.
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Also, big operations themselves can cause shifts,
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blood loss obviously, but also third spacing,
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where fluid moves out of the circulation into tissues.
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Ah, yes, third spacing.
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Tricky to manage.
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It can be.
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Then you've got underlying conditions.
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Acute kivvy injury is a big one.
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diabetes, especially if blood sugar is high, causing lots of urination
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polyuria. Right, the osmotic diuresis. Exactly. And heart failure or
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chronic kidney disease often lead to fluid retention, so overload. And
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sometimes we cause it. Unfortunately, yes. Iatrogenic causes, using diuretics too
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aggressively, maybe, or even just getting maintenance fluids slightly wrong over
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time. Malnutrition can also play a part affecting colloid osmotic pressure. It's
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a wide net. So let's talk interventions. First, the emergency situation. Fluid
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When do we jump in immediately?
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You need resuscitation when there are clear signs of shock or significant hypovolemia.
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The classic triggers are systolic blood pressure under 100 millimeter Hg.
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Heart rate over 90 beats per minute.
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Cap refill longer than two seconds.
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Breathing fast, maybe over 20 breaths a minute.
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And cold, clammy peripheries.
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So those are the alarm bells.
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What's the first move?
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Get fluids in fast.
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a bolus of 500 milliliters of a crystalloid solution, like Hartman's or saline, given
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quickly over about 15 minutes.
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500 millihertz stat.
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Pretty much.
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Right.
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Crucially, while you're doing that, you have to figure out why they're shocked and treat
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the underlying cause.
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Is it bleeding?
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Sepsis?
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Dehydration?
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Right.
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The bolus buys time, but doesn't fix the root problem.
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Exactly.
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If they're still unstable after that first 500 milliliters, they likely need more, sometimes
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quite a bit more, potentially over two liters, but you reassess constantly how they respond
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tells you a lot. Okay, that's the acute resuscitation. What about less urgent
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situations? Patients who just can't drink, maybe they're NBM before surgery or have
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a bowel obstruction, that's maintenance fluids, right? That's right. Maintenance
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therapy is for patients who aren't actually depleted now but can't take
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fluids orally. So yeah, preoperative NBM, maybe post-operative
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alleles where the gut isn't working. How do we calculate how much they need just
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to tick over? The standard guideline is around 25 to 30 milligrams per kilogram
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of body weight per day. 25-30 milli-seal kilo-gauge. Yeah. But there's a more detailed calculation
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often used, especially for getting the rate right. It's based on weight bands. Ah, the
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105-20 rows. That's the one. 100 millimeters per kilo per day for the first 10 kilos of
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body weight. Okay. Then 50 milliliterlets per kilo per day for the next 10 kilos. Right.
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And then 20 milliliters per kilo per day for every kilogram over 20 kilos. That gives you
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the total daily volume needed for maintenance. Correct. It accounts for the fact that metabolic
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rate and fluid needs aren't perfectly linear with weight.
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Got it. That's helpful. Now the last category, fluid replacement.
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This sounds like it's for patients who are already behind or losing extra fluid.
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Exactly. Replacement is for patients who are stable,
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not shocked, but definitely dehydrated, or they have ongoing losses that need
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correcting. Think someone with persistent diarrhea
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or maybe slow GI bleeding like Melina. So it's correcting a deficit and keeping
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up with ongoing losses? Precisely. You calculate their maintenance needs like we just discussed,
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but then you have to add extra fluid to cover the estimated deficit and whatever they're currently
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losing. And you need to account for anything they are managing to take in? Yes. Definitely factor in
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any oral or enteral intake, even fluids given with IV medications. It all counts towards the
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balance. And electrolytes become really key here too, I imagine. Absolutely critical. If you're
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replacing deficits, especially if there are significant electrolyte abnormalities, you need
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frequent lab monitoring. Correcting things like sodium or potassium too quickly can be just as
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dangerous as the imbalance itself. So it's a careful balancing act adjusting based on clinical
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picture in labs. It really is. Constant reassessment is key for replacement therapy. This has been a
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fantastic run through. We've really hit the core principles of fluid balance, why it matters,
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how we track it with monitoring. Mm-hmm. Covered the common causes that throw things off. And then
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walk through the main strategies,
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that urgent resuscitation, the day-to-day maintenance,
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and the more nuanced replacement therapy.
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- Yeah, the fundamentals are all there.
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- It really underscores how vital careful assessment
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and a solid understanding are.
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You absolutely need this for good patient care.
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- Couldn't agree more.
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Vigilance and knowing these principles inside out
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are essential, not just for practice, but as you said,
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for getting through surgical training and those exams.
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- Well, thank you for breaking that all down
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So clearly, join us next time on MedS