Hemodynamics – Non-Invasive Monitoring

• Hemodynamics refers to the flow of blood through the body.

• Compromised hemodynamics → decreased perfusion to end tissues.

• Non-invasive monitoring is crucial because technology is not always available, and physical assessment often provides earlier insights than monitors.

1. Core Principle: Look, Listen, Feel

• Observation is the first and most essential form of monitoring.

• Even experienced physicians rely heavily on direct patient assessment:

  • Look – patient’s appearance, skin color, posture, distress.

  • Listen – breath sounds, heart sounds, bowel sounds, patient complaints.

  • Feel – pulses, skin temperature, capillary refill, tactile cues.

• Non-invasive monitoring gives context to numerical values and often explains discrepancies in invasive measurements.

2. Body’s Response to Decreased Perfusion

• The body shuts down less vital organs first to preserve perfusion to the brain and heart.

• Perfusion compromise occurs in stages: Primary → Secondary → Final.

Primary Signs – Skin & Gut (Least Vital)

• Skin: cool, clammy, pale, ashen, cyanotic.

• Gut: decreased bowel sounds, diarrhea or constipation.

• NG tube: increased output.

• Rationale: skin and GI tract are low-priority organs, so blood flow is diverted from them first.

Secondary Signs – Kidneys, Liver, Lungs

• Lungs: ↑ respiratory rate, ↑ effort, shortness of breath, ↓ PaO₂ or SpO₂.

• Kidneys: ↓ urine output, concentrated urine, ↑ BUN, creatinine, potassium.

• Liver: ↑ ALT, AST, prolonged clotting times.

• Indicates the body is shutting down mid-priority organs to maintain vital organ perfusion.

Final Signs – Brain & Heart (Most Vital)

• Brain: altered or decreased level of consciousness, disorientation, slow-reacting pupils.

• Heart: chest pain or pressure, tachycardia/bradycardia, ectopy, ST elevation.

• At this stage, perfusion is critically compromised, and these signs indicate severe hemodynamic instability.

3. ICU Non-Invasive Tools

• Cardiac monitor – continuous heart rate and rhythm.

• Pulse oximetry – oxygen saturation monitoring.

• Blood pressure cuff – basic BP measurement.

4. Blood Pressure & Perfusion

• BP = Cardiac Output × Systemic Vascular Resistance (CO × SVR)

• Key metric: Mean Arterial Pressure (MAP) – best indicator of organ perfusion.

  • Goal: MAP > 60 mmHg to ensure end-organ perfusion.

MAP Approximation Formula

$$MAP \approx \frac{(2 \times DBP) + SBP}{3}$$

• Weighted toward diastole (spend ~2/3 of cardiac cycle in diastole).

5. Pulse Pressure

• Equation: SBP − DBP.

• Purpose: evaluates the width between systolic and diastolic pressures.

• Clinical Use: trends in widening or narrowing pulse pressure can indicate changes in cardiac output or vascular resistance.

Key Takeaways

• Physical assessment is fundamental; monitor numbers supplement, not replace, bedside assessment.

• Early recognition of perfusion compromise through look, listen, feel can precede invasive monitoring.

• Understand hierarchy of organ perfusion: skin & gut → kidneys/liver/lungs → brain & heart.