5.4 Pulmonary Artery Catheters, Intracranial Pressure & Depth of Anesthesia Monitoring

5.4 Pulmonary Artery Catheters & Depth of Anesthesia Monitoring

Advanced intraoperative monitoring combines invasive cardiopulmonary hemodynamics with electroencephalographic depth-of-anesthesia processing. The Certified Anesthesia Technologist plays an indispensable role in preparing pulmonary artery catheters, executing thermodilution cardiac output calibrations, troubleshooting mixed venous oximetry fiberoptics, and applying processed EEG sensors.

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Pulmonary Artery Catheter (PAC / Swan-Ganz) Architecture

The standard adult pulmonary artery catheter is a flexible, flow-directed balloon catheter fabricated from radiopaque polyvinyl chloride. It measures 7.0 to 7.5 French in external diameter and 110 cm in length, featuring black circumferential depth rings stamped every 10 cm (single narrow band = 10 cm, single wide band = 50 cm):

PULMONARY ARTERY CATHETER (PAC) LUMEN LAYOUT:

                                                   [Balloon: 1.5 mL Air]
                                                              |
[Proximal CVP Port: Blue (at 30 cm)]                          v
  |                                                 (---[Tip]---)
  +---------------------------------------------------/     |   \
  |                                                         |    [Distal PA Port:
  |  [Thermistor Connector Cable]                           |     Yellow (at tip)]
  |    |                                                    v
  |    +--------------------------------------------[Thermistor Bead: 4 cm from tip]
  |
  +===[Red Gate Valve: 1.5 mL Dedicated Luer-Lock Syringe]===> [Balloon Inflation Lumen]

The Standard Catheter Lumens & Clinical Functions

1. Distal Pulmonary Artery Lumen (Yellow): Terminates at the extreme catheter tip. Connected to a continuous pressurized transducer system to record pulmonary artery (PA) pressure and, when the balloon is inflated, Pulmonary Artery Occlusion Pressure (PAOP/wedge). Also serves as the aspiration port for sampling true mixed venous blood (SvO₂).
2. Proximal Injectate / CVP Lumen (Blue): Terminates 30 cm proximal to the catheter tip. In an average adult, this orifice rests in the right atrium when the catheter tip is in the pulmonary artery. Used for continuous CVP monitoring, fluid/medication infusions, and rapid administration of cold or room-temperature injectate during thermodilution cardiac output determinations.
3. Balloon Inflation Lumen (Red Gate Valve): Encloses a small internal conduit leading to a latex or non-latex balloon at the catheter tip. Equipped with a dedicated, volume-limited 1.5 mL slip-lock or Luer-lock syringe. Carbon dioxide is used for inflation in patients with suspected intracardiac right-to-left shunts (due to rapid solubility in blood should the balloon rupture); ambient room air is standard for all other patients. Liquid must never be injected into the balloon.
4. Thermistor Lumen: Contains fine, insulated electrical wires connected to a solid-state temperature bead positioned 4 cm proximal to the catheter tip. Measures core body temperature and senses rapid downstream temperature changes during thermodilution cardiac output measurement.
5. Continuous Mixed Venous Oximetry (SvO₂) Optical Bundle (Optional): Incorporates fiberoptic bundles that transmit light into the blood and return the reflected light, allowing continuous SvO₂ calculation by reflectance spectrophotometry.
6. Additional Lumens (Optional): Some models add venous infusion ports or a right ventricular pacing lumen for a transvenous pacing wire.

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Catheter Insertion Waveform Transitions & Intracardiac Pressures

The catheter is introduced through an 8.5 French percutaneous introducer sheath (Cordis) into the right internal jugular vein. The technologist connects the distal lumen to the bedside pressure monitor and primes all lines. As the catheter floats forward with venous blood return, distinct pressure waveforms demarcate each cardiac chamber:

PAC INSERTION WAVEFORM TRANSITIONS:

Pressure (mmHg)
  30 |                                                 /\    /\    /\
  25 |                      /\    /\    /\            /  \  /  \  /  \  <- PA (Dicrotic Notch)
  20 |                     /  \  /  \  /  \          /    \/    \/    \
  15 |                    /    \/    \/    \        /                
  10 |    a   c   v      /                  \      /                   Phase IV: PAOP
   5 |   / \ / \ / \    /                    \    /                    (Wedge: 6-12 mmHg)
   0 +--+---+---+---+--+----------------------+--+---------------------+-----> Time
      RIGHT ATRIUM      RIGHT VENTRICLE           PULMONARY ARTERY     PULMONARY CAPILLARY
      (0-8 mmHg)        (20-30 / 0-8 mmHg)        (20-30 / 8-15 mmHg)  (PAOP: 6-12 mmHg)
      Depth: ~20 cm     Depth: ~30-35 cm          Depth: ~40-45 cm     Depth: ~45-50 cm

Detailed Insertion Waveform Sequence

1. Right Atrium (RA):
   • Insertion Depth: Approximately 20 to 25 cm from RIJ.
   • Normal Pressures: Mean pressure 0 to 8 mmHg (identical to CVP).
   • Waveform Morphology: Low-amplitude venous waveform displaying distinct a, c, and v waves.
   • Action: Once the RA waveform appears, the balloon is inflated with 1.5 mL of air and locked. The buoyant balloon acts as a sail, carrying the catheter tip forward with the flow of blood.
2. Right Ventricle (RV):
   • Insertion Depth: Approximately 30 to 35 cm from RIJ.
   • Normal Pressures: Systolic pressure 20 to 30 mmHg; diastolic pressure 0 to 8 mmHg.
   • Waveform Morphology: Sudden dramatic upward jump in systolic pressure with a rapid drop to near-zero diastolic baseline.
   • Clinical Warning (Dysrhythmia Risk): The catheter tip mechanically irritates the sensitive right ventricular endocardium and interventricular septum, frequently triggering premature ventricular contractions (PVCs), ventricular bigeminy, or sustained ventricular tachycardia (VT). The technologist must observe the ECG trace continuously; if sustained ventricular ectopy occurs, the physician must quickly advance the catheter into the PA or withdraw it back into the right atrium.
3. Pulmonary Artery (PA):
   • Insertion Depth: Approximately 40 to 45 cm from RIJ.
   • Normal Pressures: Systolic pressure 20 to 30 mmHg; diastolic pressure 8 to 15 mmHg (Mean PA pressure: 10 to 20 mmHg).
   • Waveform Morphology: Characterized by two distinct features:
     1. A sudden, marked elevation in diastolic pressure (stepping up from the RV diastolic level of 0–8 mmHg to the PA diastolic level of 8–15 mmHg due to the resistance of the pulmonary vascular bed).
     2. The appearance of a sharp, prominent dicrotic notch on the downstroke, representing mechanical closure of the pulmonic valve.
4. Pulmonary Artery Occlusion Pressure (PAOP / PCWP / Wedge):
   • Insertion Depth: Approximately 45 to 50 cm from RIJ.
   • Normal Pressures: Mean pressure 6 to 12 mmHg.
   • Waveform Morphology: The inflated balloon wedges into a small terminal branch of the pulmonary artery, isolating the tip from upstream right ventricular pulsations. Flow ceases, creating a static fluid column extending through the pulmonary capillary bed and pulmonary veins directly to the left atrium. The pressure trace drops to a damped, low-pressure venous waveform displaying left atrial a and v waves.
   • Physiological Reflection: In the absence of mitral stenosis, left atrial myxoma, or pulmonary veno-occlusive disease, PAOP directly reflects Left Atrial Pressure (LAP) and Left Ventricular End-Diastolic Pressure (LVEDP), providing an index of left ventricular preload.

Balloon Inflation Safety Protocols

• Passive Balloon Deflation: Immediately upon recording the wedge pressure, the balloon must be deflated passively by removing the syringe from the red gate valve and allowing the air to vent. Never leave a balloon wedged continuously!
• Complications of Persistent Wedging: Sustained occlusion causes ischemic pulmonary infarction or catastrophic pulmonary artery rupture, which carries a high reported mortality (around 50%) from massive endobronchial hemorrhage.

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Mixed Venous Oxygen Saturation (SvO₂)

Mixed venous blood represents the integrated, pooled venous return from the superior vena cava, inferior vena cava, and coronary sinus, thoroughly mixed by the churning action of the right ventricle:

• Sampling Site: True mixed venous blood can only be drawn from the distal pulmonary artery lumen of a PAC. Blood drawn from a central venous line (CVP) represents central venous saturation (ScvO₂), which in healthy patients is normally a few percent lower than true SvO₂ but can exceed it in shock states.
• Normal Range: 65% to 75% (corresponding to a mixed venous oxygen tension, PvO₂, of approximately 40 mmHg).

The Fick Principle of Oxygen Balance

Mixed venous saturation provides an immediate global index of the balance between systemic oxygen delivery (DO₂) and systemic oxygen consumption (VO₂):

$SvO_2 = SaO_2 - \left[ \frac{VO_2}{CO \times 1.34 \times [Hb] \times 10} \right]$

Where CO is cardiac output, SaO₂ is arterial oxygen saturation, and [Hb] is hemoglobin concentration.

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Thermodilution Cardiac Output: The Stewart-Hamilton Equation

Thermodilution is the clinical gold standard for intermittent cardiac output (CO) measurement:

• Procedure: A known volume (typically 10 mL) of cold (0°C to 4°C) or room-temperature (18°C to 22°C) 5% dextrose or 0.9% normal saline is injected rapidly and smoothly (within about 4 seconds) into the proximal CVP port.
• Thermodynamic Detection: The cold injectate mixes thoroughly with blood in the right ventricle. The resulting drop in blood temperature is sensed downstream by the thermistor bead in the pulmonary artery, generating a time-versus-temperature cooling curve.

The Stewart-Hamilton Equation

$CO = \frac{V_i \times (T_b - T_i) \times K_1 \times K_2}{\int_0^\infty \Delta T_b(t) \, dt}$

Where Vi is injectate volume, Tb is blood temperature, Ti is injectate temperature, K₁ and K₂ are specific gravity and computation constants, and the denominator represents the Area Under the Curve (AUC) of temperature change over time.

• Inverse Proportionality: Cardiac output is inversely proportional to the area under the curve.
  • High Cardiac Output: Blood flows rapidly, carrying the cold bolus past the thermistor quickly. The temperature drop is brief and clears rapidly, generating a small, narrow area under the curve.
  • Low Cardiac Output: Blood moves sluggishly through the heart. The cold injectate lingers, producing a prolonged, deep temperature deflection with a large, wide area under the curve.

THERMODILUTION TEMPERATURE CURVES:

Temperature Drop (Delta Tb)
  ^
  |      /---\                <- HIGH CARDIAC OUTPUT (Small Area Under Curve)
  |     /     \
  |    /       \_______
  |   /                 \     <- LOW CARDIAC OUTPUT (Large, Prolonged AUC)
  |  /                   \________
  +--+-------------------+--------> Time (seconds)

Sources of Thermodilution Measurement Error

1. Tricuspid Regurgitation: Severe regurgitation allows injectate to slosh back and forth across the tricuspid valve, delaying transit past the thermistor and causing falsely enlarged AUC (falsely low calculated CO).
2. Slow or Erratic Injection: Taking longer than 3 to 4 seconds to inject allows thermal warming to occur in the syringe, underestimating cardiac output.
3. Intracardiac Shunts: Left-to-right or right-to-left shunts invalidate thermodilution math due to abnormal mixing and recirculation.

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Central Nervous System Monitoring: ICP and Cerebral Oximetry

The ASATT content outline lists intracranial pressure (ICP) under central nervous system monitoring devices, and the ASATT Scope of Practice lists ICP and near-infrared spectroscopy (NIRS) among the monitors technologists support.

Intracranial Pressure Monitoring

• Normal values: Adult ICP is normally about 5 to 15 mmHg; sustained values above about 20 mmHg are commonly treated.
• Cerebral perfusion pressure (CPP) = mean arterial pressure − ICP. For example, a MAP of 80 mmHg with an ICP of 25 mmHg leaves a CPP of only 55 mmHg.
• Device types:
  • External ventricular drain (EVD, ventriculostomy): A catheter in a lateral ventricle that measures ICP and drains cerebrospinal fluid. It is the reference standard and can be re-zeroed.
  • Intraparenchymal monitors: Fiberoptic or strain-gauge tip transducers placed in brain tissue. They are zeroed before insertion and cannot be re-zeroed afterward, so readings can drift over days.
  • Subarachnoid bolts and epidural or subdural sensors: Less invasive options that are generally less accurate.
• Leveling an EVD: The transducer and drip chamber are referenced to the external auditory meatus (tragus), which approximates the foramen of Monro. Because bed height or head position changes the reference, the drain is typically clamped during repositioning or transport and re-leveled afterward according to orders.
• Waveform: The ICP pulse has three peaks: P1 (percussion wave from arterial pulsation), P2 (tidal wave reflecting brain compliance), and P3 (dicrotic wave). A P2 taller than P1 suggests poor intracranial compliance, and sustained plateau elevations (Lundberg A waves) signal dangerous intracranial hypertension.

Cerebral Oximetry (NIRS)

Near-infrared spectroscopy sensors on the forehead estimate regional cerebral oxygen saturation (rSO2) in the frontal cortex, a value weighted toward venous blood. Clinicians follow the trend from the patient's own pre-induction baseline; a sustained relative drop (commonly 20% or more below baseline) prompts a check of blood pressure, oxygenation, carbon dioxide, hemoglobin, and head position. NIRS is common in cardiac surgery and beach-chair shoulder surgery.

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Depth of Anesthesia Monitoring: The Bispectral Index (BIS)

The Bispectral Index (BIS) monitor evaluates the hypnotic component of anesthesia by applying sophisticated mathematical processing to raw frontal electroencephalogram (EEG) signals.

Mathematical Processing & The BIS Algorithm

The BIS monitor acquires single-channel or dual-channel frontal cortical EEG data, digitizes the signal, and extracts three primary sub-parameters:

1. Power Spectrum Analysis (Fast Fourier Transform): Evaluates the distribution of EEG frequencies (beta, alpha, theta, delta).
2. Bispectral Analysis (Phase Coupling): Quantifies non-linear phase relationships and harmonization between different frequency components of the EEG, measuring the degree of cortical synchronization.
3. Burst Suppression Ratio (BSR): Quantifies the percentage of time over the preceding 63 seconds that the EEG voltage is quiescent / isoelectric (<5 μV).

The algorithm integrates these features into a single, dimensionless, empirical score ranging from 0 to 100:

Sensor Application & Impedance Checking

• Skin Preparation: The patient's forehead is wiped firmly with an alcohol prep pad to eliminate sebum, cosmetic oils, and dead skin cells, then allowed to dry completely.
• Electrode Array Placement: The peel-and-stick sensor strip is positioned diagonally across the forehead following the numbered diagram on its package:
  • One electrode sits at the center of the forehead, about 2 inches (5 cm) above the bridge of the nose.
  • One electrode sits directly above the eyebrow.
  • One electrode sits on the temple between the lateral corner of the eye and the hairline.
• Impedance Verification: The technologist presses the sensor edges firmly for 5 seconds to activate the conductive gel. The monitor runs an automated sensor impedance check. If an electrode fails, press it again or replace the sensor until the monitor reports acceptable impedance for every electrode.

Confounding Factors & Clinical Anomalies

Several pharmacological agents and environmental artifacts profoundly distort processed EEG numbers:

1. Ketamine (The Paradoxical Spike): Ketamine is an NMDA receptor antagonist that produces a dissociative anesthetic state characterized by high-frequency frontal cortical beta and gamma oscillations (30 to 50 Hz) along with disorganized theta activity. When ketamine is administered to a deeply anesthetized patient, the BIS monitor algorithm misinterprets this high-frequency beta surge as clinical wakefulness, causing the BIS reading to paradoxically spike from the 40s up to 70 to 85, despite the patient remaining completely unconscious and unarousable.
2. Nitrous Oxide (N₂O): Nitrous oxide adds analgesia and anesthetic depth but changes the processed EEG very little. Adding N₂O to an anesthetic often produces little or no change in the displayed BIS number, so BIS can underestimate the depth that nitrous oxide contributes.
3. Electrosurgical Interference (ESU): Radiofrequency energy generated during monopolar electrocautery radiates into the frontal electrodes, flooding the amplifier with high-frequency noise. The monitor either displays a falsely elevated BIS or drops the Signal Quality Index (SQI) to zero, suppressing the display.
4. Neuromuscular Blocking Agents & Electromyography (EMG): The frontal electrodes detect high-frequency bioelectrical potentials generated by the frontalis muscle (frequency band: 30 to 300 Hz). In an inadequately anesthetized patient, frontalis muscle shivering or tension artificially inflates the BIS score. Administering a muscle relaxant completely abolishes this EMG activity, causing a sudden artifactual decline in the BIS value (e.g., dropping from 65 to 45) without altering true cortical depth.
5. Hypothermia: Hypothermia reduces cerebral metabolic rate (CMRO₂) by approximately 7% per degree Celsius drop, progressively slowing EEG frequencies and lowering the BIS score independent of anesthetic depth.