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Shock, Dehydration &
Volume Status

Understanding how the body maintains fluid balance, what happens when it fails, and how to recognize the clinical signs of circulatory compromise.

Anatomy & Physiology

What Is Volume Status?

Volume status refers to the total amount of fluid circulating in your blood vessels and tissues. The adult body contains roughly 5 liters of blood, distributed between arterial, venous, and capillary compartments. Maintaining the right volume is essential for delivering oxygen and nutrients to every cell.

Body Fluid Compartments

Total Body Water (~60% body weight) Intracellular Fluid ~2/3 of TBW (~28 L in 70 kg adult) Inside every cell K+, Mg2+, PO4 3- main cation: K+ Extracellular Fluid ~1/3 of TBW (~14 L in 70 kg adult) Plasma ~3.5 L (in vessels) Interstitial ~10.5 L (between cells) main cation: Na+

What Is Dehydration?

Dehydration is a net loss of body water that exceeds intake. It can result from inadequate drinking, excessive sweating, vomiting, diarrhea, or diuretic medications. The severity is often graded by percentage of body weight lost:

Mild (1-3%)
Thirst, dry mouth, slight decrease in urine output. Heart rate may be mildly elevated.
Moderate (3-6%)
Tachycardia, decreased skin turgor, dry mucous membranes, reduced urine (oliguria).
Severe (\( >6\% \))
Hypotension, altered mental status, sunken eyes, anuria. Can progress to shock.

What Is Shock?

Shock is a life-threatening state of inadequate tissue perfusion: cells do not receive enough oxygen and nutrients to maintain normal function. Without treatment, shock leads to organ failure and death.

The Perfusion Equation

MAP Cardiac Output (CO) × SVR Heart Rate (HR) × Stroke Volume (SV) Preload Contractility Afterload Vessel diameter + smooth muscle tone

Four Types of Shock

Hypovolemic
Low volume: hemorrhage, dehydration, burns. Reduced preload drops CO.
Cardiogenic
Pump failure: MI, arrhythmia. Heart cannot generate adequate CO despite normal volume.
Distributive
Vasodilation: sepsis, anaphylaxis, neurogenic. SVR collapses, blood pools peripherally.
Obstructive
Physical blockage: PE, tension pneumothorax, cardiac tamponade. Blood cannot flow forward.

Dehydration vs. Shock

Feature Dehydration Shock
Definition Net water loss exceeding intake Inadequate tissue perfusion
Primary Problem Fluid deficit Oxygen delivery failure
Can Occur Without the Other? Yes: mild dehydration, no shock Yes: cardiogenic shock, no dehydration
Overlap Severe dehydration → hypovolemic shock
Key Sign Thirst, dry mucosa, low urine Hypotension, tachycardia, altered mental status

Compensatory Mechanisms

Volume Loss Detected Baroreceptors (aorta, carotid) Sympathetic Nervous System Activation Heart Rate ↑ Maintains CO Vasoconstriction Raises SVR RAAS + ADH Retains fluid Maintain MAP

Stages of Hypovolemic Shock

Stage I: Compensated
Up to 15% blood loss (~750 mL). HR slightly elevated. BP normal. Body compensates well. Patient may feel anxious.
Stage II: Mild
15-30% loss (~750-1500 mL). Tachycardia, narrowed pulse pressure, pale skin, delayed capillary refill.
Stage III: Moderate
30-40% loss (~1500-2000 mL). Hypotension, tachycardia \( >120 \), confusion, oliguria. Needs immediate fluid.
Stage IV: Severe
\( >40\% \) loss (\( >2000 \text{ mL} \)). Profound hypotension, lethargy/coma, no urine output. Life-threatening emergency.

How to Assess Volume Status

1
Vital signs: Check heart rate, blood pressure, and orthostatic changes (BP drop \( >20 \text{ mmHg} \) or HR rise \( >20 \text{ bpm} \) on standing).
2
Physical exam: Skin turgor, mucous membranes, capillary refill (\( >2 \text{ seconds} \) = concern), jugular venous distension (JVD).
3
Urine output: Normal is \( \geq 0.5 \text{ mL/kg/hr} \). Oliguria (<0.5) suggests low perfusion. Anuria (<50 mL/day) is critical.
4
Lab markers: Elevated BUN:creatinine ratio (\( >20:1 \)), elevated lactate (\( >2 \text{ mmol/L} \)), low central venous pressure (CVP).
5
Mental status: Restlessness → confusion → lethargy → unresponsiveness tracks worsening perfusion to the brain.

Example: Mild Dehydration

A 22-year-old athlete finishes a long run on a hot day. He feels thirsty, has a dry mouth, and his heart rate is 92 bpm (slightly elevated). His blood pressure is 118/76 mmHg (normal). Urine is dark yellow and reduced.

Normal volume Current: ~97% (mild loss) Intravascular Volume HR: 92 bpm BP: 118/76 Status: Compensated

Example: Progressing to Shock

A 68-year-old woman with severe diarrhea for 3 days. She is confused, HR is 128 bpm, BP is 82/54 mmHg, and she has produced no urine in 6 hours. Skin is cool and clammy.

Normal ~50% volume Intravascular Volume HR: 128 bpm BP: 82/54 Status: SHOCK Urine: 0 mL in 6 hrs Mental: Confused

Example: Distributive Shock

A 45-year-old man develops fever of 103.4°F, HR 130 bpm, BP 74/40 mmHg after a urinary tract infection. His skin is warm and flushed (not cool). He has adequate urine output initially but is confused.

What's different?
Warm skin + low BP suggests massive vasodilation, not volume loss. Blood volume may be normal but the container has expanded.
Mechanism
Sepsis causes inflammatory mediators to dilate vessels: SVR collapses. Even though CO may be high initially, MAP still falls.

Common Mistakes

✕ Equating dehydration with shock
Mild dehydration is common and treatable. Shock is a life-threatening emergency. They are related but not the same.
✕ Relying on BP alone
Blood pressure is maintained by compensation until late stages. A normal BP does not rule out significant volume loss.
✕ Ignoring heart rate
Tachycardia is often the earliest sign of volume depletion. A resting HR above 100 bpm should prompt investigation.
✕ One-size-fits-all treatment
Giving large volumes of fluid to cardiogenic shock worsens the problem. Treatment depends on the type of shock.

Shock Types at a Glance

Type Primary Defect Skin Treatment
Hypovolemic Low preload (volume) Cool, pale, clammy IV fluids, stop bleeding
Cardiogenic Pump failure (low CO) Cool, pale, clammy Inotropes, treat cause
Distributive Low SVR (vasodilation) Warm, flushed Fluids + vasopressors
Obstructive Mechanical blockage Variable Relieve obstruction

Question 1 of 5

A patient in shock has warm, flushed skin and a bounding pulse. Which type of shock is most likely?

Question 2 of 5

Which of the following is the earliest compensatory sign of volume loss?

Question 3 of 5

The equation \( \text{MAP} = \text{CO} \times \text{SVR} \) tells us that blood pressure depends on cardiac output and vascular resistance. In hypovolemic shock, which factor is primarily reduced?

Question 4 of 5

A patient with cardiogenic shock from a massive heart attack is given large volumes of IV saline. What is the most likely outcome?

Question 5 of 5

Approximately two-thirds of total body water is found in which compartment?