The Heart, Blood Vessels, and Perfusion

Clinical Orientation

A 76-year-old man is admitted with "not feeling right." His blood pressure is 108/68 — not alarming. But his feet are mottled and cool, capillary refill is 4 seconds, he is confused to place, and his urine output has been 15 mL/hour for the past 6 hours. His SpO2 is 94% on room air. Later that shift, a 55-year-old woman with heart failure has a BP of 142/86. She is alert, warm, and making urine — but her JVD is elevated, her liver is tender, and her ankles are swollen to her knees. Two patients, very different perfusion pictures, and BP alone tells you almost nothing. This chapter answers: What mechanism links the heart, blood vessels, and perfusion to bedside findings, tests, red flags, and nursing priorities?

Cross-reference: Book 1 covers the fluid and electrolyte foundations that determine preload and vascular volume.

What Is Normal?

Four chambers and valves: The heart has two atria (receiving chambers) and two ventricles (pumping chambers). Four valves ensure one-way flow: tricuspid (RA→RV), pulmonary (RV→pulmonary artery), mitral (LA→LV), aortic (LV→aorta). Valve failure causes either backward leak (regurgitation) or forward obstruction (stenosis) — both reduce effective cardiac output.

Coronary circulation: The heart muscle receives its own blood supply through the coronary arteries, which branch off the aorta just above the aortic valve. The left coronary artery supplies most of the left ventricle via the LAD and circumflex branches. The right coronary artery supplies the right ventricle and often the SA and AV nodes. Coronary flow occurs primarily during diastole — when the heart is relaxed and the myocardium is not compressing its own vessels. This is why tachycardia is dangerous for the heart: faster rate = less diastolic time = less coronary perfusion.

Systemic and pulmonary circuits: The right heart pumps blood through the low-pressure pulmonary circulation (normal pulmonary artery pressure ~25/10 mmHg). The left heart pumps through the high-pressure systemic circulation. These circuits are in series — output from the right heart must equal output from the left heart over time. When one side fails, blood backs up into the circuit behind it.

Oxygen content and delivery: Oxygen delivery (DO2) = Cardiac Output × Arterial Oxygen Content. Arterial oxygen content depends on hemoglobin concentration, hemoglobin saturation, and dissolved oxygen. A patient can have normal PaO2 and SaO2 but still have inadequate oxygen delivery if cardiac output is low or hemoglobin is low. This is why "normal vitals" can be deceptive.

Capillary exchange: At the capillary level, oxygen diffuses from blood to cells, and CO2 diffuses from cells to blood. This requires: adequate blood flow to the capillary (perfusion), adequate oxygen in that blood, and a short enough diffusion distance. If any of these fail, cells become hypoxic.

What Goes Wrong?

Pump, vessel, volume, or blood-content failure: Perfusion can fail at any point:

• Pump failure (cardiogenic): Heart cannot generate adequate output despite adequate volume.
• Vessel failure (distributive/obstructive): Vasodilation reduces effective volume, or obstruction blocks flow.
• Volume failure (hypovolemic): Not enough blood to fill the pump.
• Blood-content failure (anemic/hypoxemic): Enough flow but not enough oxygen in the blood.

The BP trap: A normal or even elevated blood pressure does NOT guarantee adequate perfusion. The body will vasoconstrict to maintain BP even when cardiac output is falling. The patient may be in shock with a "normal" blood pressure. Perfusion is assessed at the organ level: brain (mentation), skin (temperature, color, capillary refill), kidneys (urine output).

Venous congestion: When forward flow fails, blood backs up. Left heart failure backs blood into the lungs → pulmonary edema. Right heart failure backs blood into the systemic veins → JVD, hepatomegaly, ascites, peripheral edema. Congestion causes symptoms even when arterial pressure appears adequate.

Causes and Risk Factors

• Cardiac disease: Coronary artery disease, myocardial infarction, heart failure, valvular disease, cardiomyopathy, arrhythmias.
• Vascular obstruction: Atherosclerosis, embolism, thrombosis, vasospasm.
• Low volume: Hemorrhage, dehydration, third spacing.
• Anemia or hypoxemia: Reduced oxygen-carrying capacity or low oxygen content.
• Distributive states: Sepsis, anaphylaxis, neurogenic shock — vasodilation reduces effective circulating volume.

What Happens Inside the Body?

Causal Chain 1: Oxygen delivery failure

Cardiac output × arterial oxygen content → oxygen delivery
→ if delivery inadequate or demand excessive → cells shift to anaerobic metabolism
→ lactic acid production → metabolic acidosis
→ cellular dysfunction → organ-specific manifestations:
→ brain: confusion → coma
→ kidneys: oliguria → acute kidney injury
→ skin: cool, mottled, delayed capillary refill
→ heart: ischemia → decreased contractility → further reduced output

Key assessment finding: Altered mental status — the brain is the most oxygen-sensitive organ. Confusion is often the earliest sign of inadequate perfusion, preceding hypotension.

Causal Chain 2: Forward failure vs. backward congestion

Forward-flow failure → decreased cardiac output → decreased organ perfusion → oliguria, confusion, cool skin, lactic acidosis

Backward pressure → increased venous pressure upstream of failing ventricle:
→ Left heart failure: pulmonary venous congestion → pulmonary edema → crackles, dyspnea, orthopnea
→ Right heart failure: systemic venous congestion → JVD, hepatomegaly, ascites, peripheral edema

What the Nurse May See

Assessing perfusion at the bedside:

• Brain: Level of consciousness, orientation. Confusion = brain not being perfused.
• Skin: Temperature (cool = vasoconstriction from low output), color (pale, mottled, cyanotic), capillary refill (>3 seconds = poor peripheral perfusion).
• Kidneys: Urine output <0.5 mL/kg/hour = renal hypoperfusion.
• Pulses: Weak, thready = low stroke volume. Absent = occlusion. Compare sides.
• Blood pressure: May be normal despite poor perfusion. Look at the TREND — falling BP is late. Narrowing pulse pressure is earlier.

Congestion signs:

• Lungs: Crackles, dyspnea, orthopnea, paroxysmal nocturnal dyspnea, pink frothy sputum.
• Neck veins: JVD (elevated when head of bed >30 degrees).
• Liver: Hepatomegaly, tenderness, hepatojugular reflux.
• Extremities: Pitting edema, worse in dependent areas.

Tests, Labs, and Monitoring

• Vital signs with trend: A single BP reading is almost useless. The trend — especially with HR, MAP, and pulse pressure — tells the story. Narrowing pulse pressure = falling stroke volume.
• ECG: Reveals rate, rhythm, ischemia, infarction, electrolyte effects.
• SpO2 and oxygen delivery: Normal SpO2 does NOT mean adequate oxygen delivery. Hemoglobin and cardiac output determine delivery.
• Lactate: Elevated → anaerobic metabolism → inadequate oxygen delivery. Rising lactate despite treatment = intervention isn't working.
• Hemoglobin: Low Hgb reduces oxygen content even with normal saturation.
• Organ function markers: BUN/Creatinine (renal perfusion), LFTs (hepatic congestion), troponin (cardiac injury).

Nursing Priorities

1. Assess global AND regional perfusion: Don't stop at BP. Check brain (mentation), skin (temperature, cap refill), kidneys (urine output), and compare sides.
2. Compare sides: Unequal pulses, temperature, or capillary refill = vascular occlusion.
3. Correlate BP with mentation, skin, and urine: A patient with BP 90/60 who is alert, warm, and making urine may be adequately perfused. A patient with BP 110/70 who is confused, cool, and oliguric is in shock.
4. Escalate new organ dysfunction: New confusion, falling urine output, rising lactate, or new chest pain — notify the provider immediately.

Complications and Red Flags

Patient and Family Teaching

• "Your heart is a pump that sends oxygen-rich blood to every organ. When the pump weakens or a blood vessel gets blocked, organs don't get the oxygen they need. That's why we check your mental status, your skin temperature, and your urine output — they tell us if your organs are getting enough blood flow."
• Teach signs of poor perfusion: new confusion, chest pain, cool/blue extremities, decreased urination.

Common Student Mistakes

1. Trusting blood pressure alone: BP can be maintained by vasoconstriction even as cardiac output falls. Always check mentation, skin, and urine output.
2. Equating hypoxemia with tissue hypoxia: A patient can have normal SpO2 but still have tissue hypoxia if cardiac output or hemoglobin is low.
3. Missing right-sided heart failure: Focus only on lungs and miss JVD, liver tenderness, and peripheral edema.