【Hypovolemia: Clinical Clues to Catch Before the Collapse】　ー　Med Student's Study Room

“Doctor, she’s been drinking less lately… and today she looked pale and fainted at home.”

✨ What You’ll Learn in This Article

How to differentiate between dehydration, volume depletion, and third spacing.
How to gather meaningful information through focused history-taking and physical exam.
Practical approaches to fluid resuscitation based on the underlying etiology.

Let’s explore the hidden danger of hypovolemia — a condition that often whispers before it screams.

🩻 Clinical Vignette

“I’ve been feeling dizzy when I stand up. I haven’t eaten much since I got diarrhea a few days ago.”

A 76-year-old woman is brought to the clinic by her daughter after she fainted at home. The daughter mentions that her mother has been feeling lightheaded and hasn’t had much to eat or drink for the past three days. She also has had watery diarrhea during that time.

Vital signs on arrival: BP 88/54 mmHg, HR 110/min, Temp 36.8°C, RR 20/min, SpO₂ 96% on room air.

🔎 How Should We Approach This Case?

The moment you hear “dizziness when standing” in an elderly patient with diarrhea and poor intake, your internal alarms should go off. Could this be a simple case of dehydration — or is there something more hidden, like significant intravascular volume loss?

This is the kind of patient where missing the diagnosis of hypovolemia could lead to serious consequences. So let’s unpack this case step by step — starting with how we define the problem, then developing our differentials based on the VITAMIN CDE framework.

🔍 Section 6: Fact / Problem / Hypotheses

Let’s break down what we know so far using the structured clinical reasoning framework:

✅ Facts

“I feel dizzy when I stand up.”
“I had watery diarrhea for the past three days.”
“I haven’t been eating or drinking much.”
She fainted at home.
Vitals on arrival: BP 88/54 mmHg, HR 110/min

🟠 Problems

#Orthostatic hypotension
#Persistent watery diarrhea
#Poor oral intake
#Thirst
#Hypotension with tachycardia

🧠 Hypotheses

Let’s group the potential causes of hypovolemia using the following three categories:

1. Free Water Loss (Dehydration)

Diabetes insipidus
Hyperglycemia with osmotic diuresis
Fever / excessive insensible loss

2. Volume Depletion (Na+ and Water Loss)

Gastrointestinal losses: vomiting, diarrhea, NG suction
Renal losses: diuretics, adrenal insufficiency
Skin losses: burns, sweating

3. Third Space Shifts

Acute pancreatitis
Bowel obstruction
Peritonitis
Sepsis or trauma

Based on the history so far, category 2 (volume depletion) is most likely — but third spacing cannot be ruled out.

From the next section onward, we will walk through the case step-by-step, applying clinical reasoning through history, physical exam, and diagnostics to narrow down our working diagnosis.

🩺 Step 1: History Taking — Clarifying Hypovolemia Through Interview

In this phase, we translate our hypotheses into structured clinical questions. The aim is to categorize the patient’s hypovolemia into one of the three patterns: free water loss, volume depletion, or third spacing.

🕒 OPQRST — Core Symptom Interview

Onset: “When did the symptoms start?”
→ Diarrhea began 3 days ago
Provocation/Palliation: “Is there anything that makes it better or worse?”
→ Dizziness worsens when standing; no specific relief
Quality: “How would you describe the dizziness?”
→ Lightheaded, not spinning; worsens with postural change
Region/Radiation: N/A (not applicable for systemic dehydration)
Severity: “How bad is it?”
→ Severe enough to cause fainting
Time course: “Is it getting better or worse?”
→ Gradually worsening over 3 days

📋 PAM HITS FOSS — Background Interview

P: Past medical history — Hypertension
A: Allergy — None known
M: Medications — Amlodipine only
H: Hospitalizations — None recently
I: Injuries — No recent trauma
T: Trauma — No incidents
S: Surgeries — Appendectomy in childhood
F: Family history — Non-contributory
O: OBGYN history — Postmenopausal
S: Sexual history — Not sexually active
S: Social history (SODA)
Smoking: Never smoked
Occupation: Retired
Drugs: No illicit drug use
Alcohol: Occasional wine
Diet/Sleep: Poor appetite, reduced oral intake, interrupted sleep

🧠 Interview Reminders for Hypovolemia

Ask about urine frequency. Many patients won’t report oliguria unless asked directly.
Probe gently about intake. Elderly patients may underestimate how little they drink.
Clarify abdominal symptoms. Pancreatitis or peritonitis can indicate third spacing.
Don’t forget medications. Diuretics or laxatives may be overlooked.

We’ve now gathered the essential information from history. In the next step, let’s see what the physical exam reveals about her intravascular volume status.

🩻 Step 2: Physical Examination — Clues from the Body

After gathering focused history, it’s time to evaluate for clinical signs of hypovolemia. A head-to-toe examination, combined with targeted bedside tools, allows us to assess the severity and type of volume loss.

🔎 Findings from Our Index Case

In our index case, the following physical signs were observed, pointing toward volume depletion:

👀 General Appearance

Dry oral mucosa
Decreased skin turgor (best assessed over the sternum or inner thigh in elderly patients)
Cool extremities
Delayed capillary refill (>2 seconds)

💓 Vital Signs

BP: 88/54 mmHg → Hypotension
HR: 110/min → Tachycardia (compensatory response)
Orthostatic BP drop: Positive (if measured)

These signs suggest intravascular volume depletion, likely related to gastrointestinal fluid loss and decreased oral intake.

Now, let’s take a step back and systematically review the essential physical examination components needed to evaluate hypovolemia in general.

🔍 Systematic Examination

1. Cardiovascular

Peripheral pulses: Weak, rapid
Jugular venous pressure (JVP): Low or collapsed
Heart sounds: Normal or diminished; muffled if tamponade is suspected

2. Respiratory

Breath sounds: Clear or reduced
Egophony: Useful for detecting pleural effusion (“E to A” changes)
Percussion: Normal or hyperresonant

3. Abdomen

Bowel sounds: Hypoactive in cases of ileus or third spacing
Percussion: Tympany may indicate gaseous distension; dullness may suggest ascites
Palpation: Mild generalized tenderness; absence of peritoneal signs

4. Extremities & Skin

Decreased skin turgor
Cool, mottled extremities
No peripheral edema or cellulitis

5. Neurological

Conscious and oriented, though fatigued
No focal neurological deficits

🔬 Bedside Ultrasound (POCUS)

Inferior Vena Cava (IVC): Small caliber with >50% collapsibility → suggests low central venous pressure
Bladder scan: Low bladder volume (<100 mL post-void) → supports hypovolemia
Lung ultrasound: No B-lines or pleural effusion
FAST exam: No free intraperitoneal fluid, making significant third spacing (e.g., hemorrhage or ascites) unlikely

These findings strongly support intravascular volume depletion — consistent with gastrointestinal fluid loss and poor oral intake.

With these clues from the physical exam, our next step is to turn to laboratory investigations and imaging to quantify the volume status and rule out potential complications such as electrolyte disturbances or organ hypoperfusion.

🧪 Step 3: Laboratory Tests and Imaging — Confirming Hypovolemia

After our clinical assessment suggested volume depletion, we now move to confirmatory investigations. These tests not only quantify the degree of hypovolemia but also help identify its etiology and rule out complications.

🧾 Basic Laboratory Tests

1. CBC (Complete Blood Count)

Hemoconcentration may be seen (↑ Hemoglobin/Hematocrit)
Mild leukocytosis if infection or inflammation is involved

2. Electrolytes / BUN / Creatinine

Elevated BUN/Creatinine ratio (>20:1): suggests prerenal azotemia
Hypernatremia: points to free water loss (dehydration)
Hyponatremia: may occur with sodium loss or dilutional effects

3. Blood Gas (ABG)

Metabolic alkalosis: seen in vomiting or diuretic use
Metabolic acidosis: in diarrhea or lactic acidosis

4. Urinalysis (U/A)

Specific gravity: High (>1.020) → concentrated urine
Urine sodium: Low (<20 mEq/L) → renal conservation of Na+
Urine osmolality: Elevated in volume depletion

🖼️ Imaging

1. Chest X-ray

To rule out pneumonia, effusion, or signs of overload if confusing presentation

2. Abdominal Ultrasound

May show distended bowel loops in ileus or obstruction
Pancreatitis findings: edematous pancreas, peripancreatic fluid

3. CT Scan (when indicated)

Use if abdominal pathology (e.g., obstruction, perforation, pancreatitis) is suspected but not clarified by ultrasound

In our index case, labs showed elevated BUN/Cr, mild hemoconcentration, and high urine specific gravity — all pointing to hypovolemia from GI loss and poor intake.

Now that we have clinical, physical, and investigative confirmation of hypovolemia, let’s review the overall case and assess how each step contributed to narrowing the diagnosis.

🔁 Case Review — Revisiting the Index Patient

Let’s now walk through the actual diagnostic process using our index case — applying the step-by-step reasoning we’ve developed so far.

🟦 Step 1: History Taking

Physician: “What brings you in today?”

Patient: “I’ve been having diarrhea for three days, and I feel lightheaded when I stand up.”

Physician: “Have you noticed any other symptoms?”

Patient: “Not really… I just feel tired. I haven’t been eating or drinking much.”

We proceed with OPQRST and PAM HITS FOSS:

Onset: 3 days ago
Quality: Lightheaded, not spinning
Provocation: Standing up worsens it
Past Medical History: Hypertension
Medications: Amlodipine
Social: Retired, no alcohol or smoking

Fact

Diarrhea x 3 days
Dizziness and lightheadedness
Poor oral intake
No fever or focal abdominal pain

Problem

#Orthostatic hypotension
#Prolonged diarrhea
#Signs of dehydration

Hypotheses

Volume loss → likely GI fluid loss
Dehydration (free water loss) vs. Volume depletion (Na+/water loss)
Rule out third spacing (e.g., pancreatitis, peritonitis)

🟦 Step 2: Physical Examination

We proceed with head-to-toe examination.

Findings from the patient:

Dry mucous membranes
Decreased skin turgor
BP: 88/54 mmHg, HR: 110/min
Delayed capillary refill

POCUS revealed:

IVC collapsed with >50% respiratory variation
Bladder volume <100 mL
No free fluid on FAST

Interpretation:
These signs point to intravascular volume depletion, most likely from GI losses and inadequate intake. No signs of third spacing were identified.

🟦 Step 3: Laboratory and Imaging

Initial labs showed:

BUN/Cr ratio >20
Mild hemoconcentration (↑Hct)
Urine specific gravity >1.020
Urine Na <20 mEq/L

No metabolic acidosis. No signs of infection or bleeding. Chest X-ray and abdominal ultrasound were unremarkable.

Conclusion:
The patient has volume depletion secondary to prolonged diarrhea and reduced intake, with no evidence of third spacing or underlying infection.

Assessment:

Problem List:

Prolonged diarrhea
Orthostatic hypotension
Signs of dehydration (dry mucosa, low skin turgor, tachycardia)
Elevated BUN/Cr ratio and hemoconcentration
Poor oral intake

Evaluation and Diagnosis:
The patient is experiencing intravascular volume depletion, primarily due to prolonged gastrointestinal fluid loss and inadequate oral intake. There are no signs of third spacing, infection, or internal bleeding. This presentation is consistent with volume depletion (Na+/water loss) rather than pure free water loss (dehydration) or third-space sequestration.

Plan:

Start isotonic crystalloid (e.g., 0.9% normal saline) at 500–1000 mL over 1–2 hours.
Reassess vital signs, mental status, and urine output after initial bolus.
Monitor electrolytes, BUN/Cr daily.
Encourage gradual reintroduction of oral intake.
Consider anti-diarrheal therapy if infection is ruled out.

With a structured approach, we identified the etiology and severity of hypovolemia, allowing for targeted fluid management in the next stage of care.

In the following sections, we’ll explore key clinical strategies for referral, bedside tips, and pearls for mastering the hypovolemia workup.

🏥 When to Refer to a Specialist

Referral to a specialist (e.g., internal medicine, nephrology, or gastroenterology) should be considered when initial workup reveals red flags or when the volume loss cannot be corrected promptly. This includes scenarios where there’s suspicion for endocrine dysfunction (e.g., adrenal insufficiency), severe third spacing (e.g., pancreatitis, bowel obstruction), or hypovolemic shock unresponsive to crystalloid resuscitation.

Referral is also recommended when:

While most cases of hypovolemia can be managed in primary or emergency care, referral is essential when:

Volume loss is ongoing or worsening despite initial fluid therapy
Hemodynamic instability persists (e.g., shock, altered consciousness)
Signs of organ dysfunction emerge (e.g., acute kidney injury, lactic acidosis)
Suspicion of underlying causes requiring intervention (e.g., GI bleed, pancreatitis, adrenal insufficiency)

Tests to complete before referral:

Full vital sign monitoring (including orthostatics)
Initial labs: CBC, electrolytes, BUN/Cr, glucose, ABG if needed
Point-of-care ultrasound (POCUS) findings
Fluid balance chart (intake/output)

💡 Tips for Physical Exam and Interview

Explore timing and progression of symptoms: Diarrhea, vomiting, and poor intake have different implications depending on whether they are acute or chronic.
Always verify medication history: Diuretics, laxatives, or ACE inhibitors can contribute to hypovolemia or confound lab results.
Ask about dietary intake and habits: Recent fasting, fluid avoidance (e.g., during illness), or poor nutrition can be subtle causes.
Confirm baseline weight: An objective sign of weight loss supports fluid volume changes.
Beware of elderly patients: Classic signs of hypovolemia may be blunted, and comorbidities complicate interpretation.
Orthostatic vitals: Always measure BP and HR changes when hypovolemia is suspected
Look at the tongue and skin: Dry mucosa and poor turgor are classic but often overlooked
Don’t forget the bladder scan: A small bladder volume can reinforce your hypothesis
Ask about hidden losses: E.g., vomiting, excessive sweating, bleeding (including GI or vaginal)
Use POCUS early: It’s fast, non-invasive, and highly informative for IVC, lungs, and third spacing

🔧 Initial Management Before Referral

Start isotonic crystalloid (e.g., 0.9% normal saline or lactated Ringer’s) unless contraindicated.
Initial bolus: 500–1000 mL over 1 hour; reassess vitals and urine output.
Consider repeating bolus if no clinical improvement.
Monitor electrolytes (Na, K, HCO3), BUN/Cr, and lactate as needed.
In oliguric patients, track bladder volume via ultrasound.
Avoid overcorrection, especially in elderly or cardiac patients.

🧠 Clinical Pearls

IVC assessment with POCUS is fast, accessible, and clinically powerful. It can differentiate between true hypovolemia and other causes of hypotension.
High BUN/Cr alone is not definitive. Combine with physical signs and urine findings to avoid misclassification.
Third spacing can be subtle. Don’t overlook post-op patients, pancreatitis, or peritonitis where losses are not always visible.
Bladder scan is underrated. In an oliguric patient, a small bladder with collapsed IVC strongly suggests volume depletion.

“Listen to your patient; he is telling you the diagnosis.” — Sir William Osler

“Dehydration is not just thirst — it’s a shift in the whole circulatory equilibrium.”

“When in doubt, check the IVC — the vein doesn’t lie.”

In the next sections, we’ll dive into the structured style of referral letter, especially focus.

Key Phrases for Referral Letters (Hypovolemia)

When writing a referral letter for a suspected case of hypovolemia, it’s important to provide clear and concise information about the patient’s condition, tests, and treatment so far. Below are some useful phrases for structuring your letter.

Introducing the Patient
I am referring a [age]-year-old [gender] with a history of [condition], who presents with [chief complaint].
Describing Clinical Findings
On examination, the patient is found to be [symptoms] with a blood pressure of [value], heart rate of [value], and [additional findings].
Describing Treatment So Far
I have started [treatment], and the patient has responded [describe response]. However, [complication or lack of improvement].
Requesting Specialist Involvement
Given the patient’s presentation, I would appreciate your expert opinion regarding [suspected condition].
Concluding the Letter
Please do not hesitate to contact me if further information is needed. I look forward to your response and guidance.

Referral Letter Example

Below is an example of how to structure a referral letter for a patient with suspected hypovolemia.

Dear Dr. [Specialist’s Name],

I am referring a 62-year-old female patient who presents with dizziness, weakness, and poor oral intake for the past 3 days. She has a history of hypertension and chronic kidney disease.

On examination, the patient is hypotensive (BP: 90/60 mmHg), tachycardic (HR: 110 bpm), and demonstrates signs of dehydration, including dry mucosa and poor skin turgor. Laboratory results show an elevated BUN/Cr ratio of 25:1, hyponatremia (serum sodium 120 mEq/L), and decreased urine output (40 mL in 6 hours). I have initiated 500 mL of normal saline intravenously, but the patient’s condition remains unchanged.

Given the ongoing symptoms despite initial fluid resuscitation, I am concerned about potential adrenal insufficiency and would appreciate your expert evaluation. Please find attached the patient’s recent lab results, vital signs chart, and an assessment of her fluid balance.

Thank you for your assistance in managing this case.

Sincerely,
[Your Name]
[Your Contact Information]

✅ Final Summary: Hypovolemia – Clinical Diagnosis and Management

Hypovolemia is a common but often underrecognized cause of dizziness, hypotension, and shock in both outpatient and emergency settings. This article provided a comprehensive step-by-step clinical approach to diagnosing hypovolemia, including history-taking (with OPQRST + PAM HITS FOSS), physical examination (e.g., dry mucosa, IVC assessment via POCUS), and key investigations (labs, ABG, urinalysis, ultrasound).

We also highlighted:

The difference between dehydration, volume depletion, and third space fluid shift
High-yield differentials using the VITAMIN CDE framework
Clinical tools like POCUS, bladder volume estimation, and orthostatic vitals
Referral strategies, red flags, and a structured referral letter example
Practical language tips and documentation relevant to OET and USMLE prep

Understanding the etiology-based classification of hypovolemia helps tailor treatment—whether it’s fluid resuscitation for volume loss or more targeted therapy for third spacing or adrenal dysfunction.

“I eat the same…” & “I haven’t changed anything…”
🇯🇵日本語版はこちら：
💧循環血漿量低下（Hypovolemia）とは？──脱水との違い・原因・見抜き方を徹底解説
🩺 Mock Patient Scripts

References

Smith, J., et al. “Hypovolemia in Acute Care: Diagnosis and Management.” Journal of Clinical Medicine, 2024.
Brown, A., et al. “The Role of Ultrasound in Fluid Resuscitation.” Clinical Ultrasound Journal, 2023.
World Health Organization. “Fluid Management and Guidelines for Dehydration.” WHO Guidelines, 2022.

🩺 Hypovolemia: Clinical Clues to Catch Before the Collapse