9.3 Shock Classification, Fluid Resuscitation & Hemodynamic Monitoring
Key Takeaways
- Shock is a state of cellular hypoxia caused by inadequate tissue perfusion, categorized into Hypovolemic, Cardiogenic, Distributive (Septic, Anaphylactic, Neurogenic), and Obstructive shock.
- The Singapore MOH Sepsis Management Bundle requires obtaining blood cultures prior to antibiotics, administering empiric IV antibiotics within 1 hour, giving a 30 mL/kg crystalloid bolus for hypotension or lactate ≥4 mmol/L, and starting Noradrenaline as the first-line vasopressor to maintain MAP ≥65 mmHg.
- Continuous invasive arterial blood pressure monitoring requires zeroing and leveling the transducer to the phlebostatic axis (4th intercostal space, mid-axillary line) and conducting the square wave test.
- Central Venous Pressure (CVP) monitoring (normal 2–6 mmHg) evaluates right ventricular preload and guides volume resuscitation, supported by central line bundle care to prevent CLABSI.
- Pulmonary Artery Catheters provide direct assessment of PAWP (normal 6–12 mmHg), Cardiac Index (normal 2.5–4.0 L/min/m²), and Systemic Vascular Resistance (normal 800–1200 dynes·sec/cm⁵) to differentiate complex shock states.
9.3 Shock Classification, Fluid Resuscitation & Hemodynamic Monitoring
Shock is a life-threatening state of acute circulatory failure characterized by inadequate tissue perfusion, resulting in cellular hypoxia, anaerobic metabolism, and systemic lactic acidosis (serum lactate >2 mmol/L). Critical care and emergency nurses in Singapore must master shock classification, hemodynamic interpretation, rapid resuscitation bundles, and vasoactive drug titration to prevent multi-organ dysfunction syndrome (MODS).
Pathophysiology & Shock Classifications
Shock is categorized into four primary categories based on hemodynamic mechanisms:
| Shock Category | Primary Cause | CVP / Preload | Cardiac Output (CO) | SVR (Afterload) | Primary Clinical Interventions |
|---|---|---|---|---|---|
| Hypovolemic | Fluid volume loss (hemorrhage, burns, dehydration) | Decreased (<2 mmHg) | Decreased | Increased | Isotonic crystalloids; blood transfusion (1:1:1 PRBC, FFP, Platelets for bleeding). |
| Cardiogenic | Myocardial pump failure (acute MI, end-stage HF) | Increased (>6–8 mmHg) | Severely Decreased (<2.2 L/min/m²) | Increased | Inotropes (Dobutamine); vasopressors (Noradrenaline); Mechanical support (IABP/ECMO). Avoid fluid loading! |
| Distributive (Septic) | Vasodilation & capillary leak secondary to sepsis | Decreased or Normal | Increased / Normal (Early hyperdynamic) | Severely Decreased (<800 dynes·sec/cm⁵) | 30 mL/kg crystalloids; Noradrenaline (1st choice); empiric IV antibiotics within 1 hour. |
| Distributive (Anaphylactic) | Severe IgE-mediated vasodilation & bronchospasm | Decreased | Decreased | Severely Decreased | IM Adrenaline 0.5 mg (1:1000) thigh; IV fluids; H1/H2 blockers; IV hydrocortisone. |
| Distributive (Neurogenic) | Loss of sympathetic vascular tone from spinal injury | Decreased | Decreased | Severely Decreased | IV fluids; Vasopressors; Atropine for severe bradycardia. |
| Obstructive | Physical obstruction to blood flow (Tamponade, PE, Tension Pneumothorax) | Increased | Decreased | Increased | Pericardiocentesis (Tamponade); Needle decompression/Chest drain (Pneumothorax); Thrombolysis (PE). |
Obstructive Shock Highlights
- Cardiac Tamponade: Displaying Beck's Triad (Hypotension, Jugular Venous Distension, and Muffled Heart Sounds) and Pulsus Paradoxus (>10 mmHg drop in SBP during inspiration). Requires immediate pericardiocentesis.
- Tension Pneumothorax: Characterized by absent breath sounds on the affected side, tracheal deviation, JVD, and severe hypotension. Requires needle decompression at the 2nd ICS MCL followed by chest tube insertion.
Singapore MOH Sepsis Resuscitation Protocols
In alignment with the Surviving Sepsis Campaign and Singapore MOH clinical guidelines, the Hour-1 Sepsis Resuscitation Bundle must be executed rapidly:
- Measure Serum Lactate: Initial lactate >2 mmol/L indicates hypoperfusion. Re-measure within 2 to 4 hours to confirm clearance.
- Obtain Blood Cultures: Draw 2 sets of blood cultures prior to starting antibiotics (one peripheral set and one from vascular access devices).
- Administer Broad-Spectrum IV Antibiotics: Initiate empiric broad-spectrum coverage within 60 minutes of presentation.
- Rapid Fluid Resuscitation: Administer a 30 mL/kg bolus of IV isotonic crystalloid (e.g., Plasmalyte or Ringer's Lactate) within 3 hours for hypotension (MAP <65 mmHg) or lactate ≥4 mmol/L.
- Initiate Vasopressors: Administer Norepinephrine (Noradrenaline) as the first-line vasopressor to maintain a Mean Arterial Pressure (MAP) ≥65 mmHg if fluid resuscitation fails.
Arterial Line & Central Venous Pressure (CVP) Monitoring
Continuous invasive monitoring allows real-time assessment of blood pressure and right-sided cardiac filling pressures.
Arterial Pressure Monitoring (A-Line)
- Allen's Test: Must be performed prior to radial artery cannulation to confirm ulnar collateral circulation patency.
- Transducer Zeroing & Leveling:
- Leveling: Align the transducer to the Phlebostatic Axis (4th intercostal space at the mid-axillary line, level with the right atrium). Transducers positioned too high yield falsely low readings; transducers positioned too low yield falsely high readings.
- Zeroing: Open the transducer stopcock to air and press 'Zero' on the monitor.
- Square Wave Test: Performed by flushing the system. A normal system produces a square wave followed by 1 to 2 oscillations. An overdamped waveform (sluggish peak, no oscillations) understates SBP and overstates DBP. An underdamped waveform (excessive ringing) overstates SBP.
Central Venous Pressure (CVP) Monitoring
- Definition: CVP measures superior vena cava pressure near the right atrium, reflecting right ventricular preload. Normal range is 2 to 6 mmHg (or 3 to 8 cm H₂O).
- Waveform: 'a' wave (atrial contraction), 'c' wave (tricuspid closure), 'v' wave (venous filling during systole).
- CLABSI Prevention Bundle per Singapore MOH standards: Max sterile barriers during insertion, chlorhexidine 2% in 70% alcohol skin prep, transparent dressings changed every 7 days, and daily assessment for prompt line removal.
Pulmonary Artery Catheter (PAC) & Hemodynamic Calculations
The Pulmonary Artery Catheter (Swan-Ganz) provides direct left ventricular hemodynamic assessment.
Key Hemodynamic Parameters & Formulas
- Pulmonary Artery Wedge Pressure (PAWP): Reflects left ventricular end-diastolic pressure. Normal: 6 to 12 mmHg. PAWP >18 mmHg signifies cardiogenic shock or fluid overload.
- Cardiac Index (CI): Normal range is 2.5 to 4.0 L/min/m².
- Systemic Vascular Resistance (SVR): Normal range is 800 to 1200 dynes·sec/cm⁵. SVR <800 indicates distributive/septic shock; SVR >1200 reflects compensatory vasoconstriction.
- Mean Arterial Pressure (MAP): Calculated as $\text{MAP} = \text{DBP} + \frac{1}{3}(\text{SBP} - \text{DBP})$. Target MAP is ≥65 mmHg.
Vasoactive Infusion Administration & Safety
- Central Line Lumen: Concentrate vasopressors (Noradrenaline, Vasopressin) must be infused via a dedicated lumen of a central venous catheter to prevent severe extravasation injury and peripheral necrosis.
- Extravasation Treatment: If extravasation occurs, stop the infusion immediately and infiltrate the site with Phentolamine mesylate (5 to 10 mg diluted in normal saline) to reverse local vasospasm.
SNB Legal & Clinical Governance
Under the Nurses and Midwives Act (Cap. 209) and SNB Code for Nurses and Midwives (2023), nurses managing invasive lines and vasoactive drugs must maintain strict documentation of transducer zeroing, line care, hourly hemodynamic parameters, and fluid balances, escalating clinical changes via ISBAR.
A nurse in the Intensive Care Unit is preparing to zero and level an arterial pressure monitoring system. To what anatomical landmark must the transducer be leveled to ensure accurate blood pressure readings?
According to the Singapore MOH Sepsis Resuscitation Bundle and Surviving Sepsis Campaign guidelines, which initial vasopressor is recommended as the first-choice agent to restore blood pressure in septic shock when fluid resuscitation fails to achieve the target Mean Arterial Pressure (MAP) ≥ 65 mmHg?
A nurse is monitoring a patient following blunt chest trauma. The patient develops distant/muffled heart sounds, severe hypotension, and neck vein distension (Beck's triad). Which condition should the nurse immediately suspect?