Chapter 3-02: Anatomy, Physiology, and Diagnostics in Critical Care Medicine

INTRODUCTION

Please see Chapters 1 and 2 for cardiac and pulmonary anatomy/physiology, respectively. In this chapter, we combine anatomy, physiology, and diagnostics into a single section, which reflects the use of diagnostics in critical care medicine to characterize altered physiology in critically ill patients.

ASSESSMENT OF FLUID RESPONSIVENESS

• Hypotension is a common problem in the ICU

• Various methods exist to determine whether a patient with hypotension is likely to be “fluid responsive,” which is defined as an increase in cardiac stroke volume with a fluid bolus

• Each of the following methods can help inform this decision, but no single measure is perfect: 1) Assessment of pulse pressure variation (PPV); 2) Central venous pressure (CVP); 3) Dynamic measurement of the inferior vena cava (IVC using POCUS); and 4) Passive leg raise

• Please note that these measures were validated in specific settings and may not be valid in alternative clinical scenarios

Arterial Line and Pulse Pressure Variation (PPV)

• Indications for arterial line placement:

  • - Invasive hemodynamic monitoring

  • - Frequent laboratory draws, often arterial blood gases (ABGs)

• Interpretation of the arterial line tracing:

  • - Waveform shows systolic peak pressure, dicrotic notch, and end-diastolic pressure (Figure 3.1)

  • - Mean arterial pressure (MAP) is calculated and shown on the monitor

  • - Position of the transducer will affect the blood pressure readings

    • Device should be zeroed at the level of the heart

    • When the transducer is ABOVE the patient, then the measured pressure will be LOWER than actual

    • When the transducer is BELOW the patient, then the measured pressure will be HIGHER than actual

• Measuring PPV using the arterial line: See Figure 3.2

  • - PPV is a predictor of fluid responsiveness in mechanically ventilated patients

    • PPV >13% suggests that the patient is fluid responsive

    • PPV 9–13% is indeterminate

    • PPV <9% suggests that the patient is not fluid responsive

  • - Pulse Pressure (PP) = (Systolic Pressure – Diastolic Pressure)

    • PP maximum occurs during inspiration; PP minimum occurs during expiration

    • PPV is calculated with the following formula:

      $Pulse Pressure Variation (PPV)=(PPmax−PPmin)PPmean$