Neuromuscular Monitoring

Why It Matters

• Impaired pharyngeal function and airway obstruction
• Increased risk of aspiration
• Impaired hypoxic ventilatory control
• Unpleasant symptoms of muscle weakness (diplopia, facial weakness, dysphagia)
• Prolonged PACU length of stay

"Quantitative neuromuscular monitoring is the only method of determining whether full recovery of muscular function has occurred and whether pharmacologic reversal can be safely avoided." — Miller's Anesthesia, 10e

Methods of Monitoring

1. Clinical / Time-Based Assessment (Unreliable)

• Time since last NMBD dose
• Physical tests: head lift ×5 seconds, handgrip, tidal volume, vital capacity, tongue protrusion, eye opening

2. Peripheral Nerve Stimulators (Qualitative Monitoring)

Figure 21-5: Ulnar nerve location and electrode placement — Barash Clinical Anesthesia, 9e

3. Quantitative Monitoring (Objective) — Gold Standard

Technologies:

TOF Ratio Interpretation

At TOF ratios < 0.90, awake volunteers exhibit impaired pharyngeal function, airway obstruction, and increased aspiration risk even though they may appear clinically recovered. — Miller's Anesthesia, 10e

Depth of Block Classification

Clinical Practice Recommendations

1. Quantitative monitoring should be used whenever NMBDs are administered — subjective assessment alone is insufficient.
2. Monitoring guides dosing of NMBDs intraoperatively and reversal drug selection and dosing at end of surgery.
3. Neostigmine is most effective for reversal of minimal block (TOF count 4, minimal fade); it is unreliable for deep block.
4. Sugammadex can reverse moderate-to-deep block but quantitative monitoring is still needed to confirm recovery.
5. Time elapsed since last NMBD dose is not a reliable substitute for monitoring — up to 37% of patients have TOF ratio < 0.90 two or more hours after a single intubating dose of intermediate-duration NMBD.
6. International guidelines from Canada, France, Spain, Australia, New Zealand, Czech Republic, Portugal, Japan, and the USA all recommend quantitative monitoring.

Summary

Neuromuscular Monitoring — Definition & Physiology

Definition

• Guide intraoperative dosing of NMBDs
• Determine the depth of block for optimal surgical conditions
• Confirm adequate recovery of neuromuscular function before tracheal extubation
• Guide timing and dosing of reversal agents

"Good evidence-based practice dictates that clinicians always quantitate the extent of neuromuscular block by objective monitoring." — Miller's Anesthesia, 10e

Physiology of the Neuromuscular Junction

1. Anatomy of the NMJ

Figure 6-11B: Structure of the neuromuscular junction — Ganong's Review of Medical Physiology, 26e

2. Sequence of Events: Nerve Impulse → Muscle Contraction

1. Action potential propagates down the myelinated lower motor neuron; becomes unmyelinated at the terminal
2. Depolarization of the nerve terminal opens voltage-gated Ca²⁺ channels
3. Ca²⁺ influx triggers exocytosis of ACh-containing synaptic vesicles into the synaptic cleft
4. ACh diffuses across the cleft and binds to nicotinic cholinergic receptors (N_M type) concentrated at the tops of the junctional folds
5. Receptor activation → increased Na⁺ and K⁺ conductance → Na⁺ influx → endplate potential (EPP)
6. The EPP depolarizes the adjacent muscle membrane to its firing threshold
7. Action potentials propagate in both directions along the muscle fiber → excitation-contraction coupling → muscle contraction
8. ACh hydrolysis: AChE rapidly breaks down ACh into acetate and choline, terminating the signal; choline is re-taken up for ACh re-synthesis

Figure 6-12: Events at the NMJ — Ganong's Review of Medical Physiology, 26e

3. The Nicotinic Acetylcholine Receptor (nAChR)

• Mature (junctional) receptor: α₁α₁βδε subunits — located at the motor endplate; requires binding of 2 ACh molecules (one at each α subunit) for activation
• Immature (extrajunctional) receptor: α₁α₁βδγ subunits — expressed throughout the entire muscle membrane in fetal tissue or pathological states (denervation, burns, immobilization)

• Extrajunctional receptors have prolonged channel opening times and increased sensitivity to depolarizing NMBDs
• If succinylcholine is administered ≥24 hours after denervation injury (spinal cord injury, severe burns, stroke, Guillain-Barré, prolonged ICU immobilization), massive K⁺ efflux through these receptors → life-threatening hyperkalemia

• Located on the nerve terminal
• Nondepolarizing NMBDs bind here → reduce mobilization of ACh vesicles → contribute to the fade phenomenon (progressive decrease in twitch height during sustained or repetitive stimulation)

4. Receptor Occupancy and Stimulation Response

5. Mechanism of Neuromuscular Blockade

Depolarizing Block (Phase I) — Succinylcholine

• Structurally resembles ACh (two ACh molecules linked by methyl groups)
• Binds and activates both postsynaptic and extrajunctional nAChRs → sustained depolarization (fasciculations followed by flaccid paralysis)
• NOT hydrolyzed by AChE; metabolized by pseudocholinesterase (plasma cholinesterase) — slower degradation than ACh → prolonged depolarization
• The continuously depolarized membrane cannot respond to subsequent ACh release → paralysis
• At TOF stimulation: no fade (all 4 twitches equally reduced)

Nondepolarizing Block — Rocuronium, Vecuronium, Cisatracurium, etc.

• Bind to one or both α subunits of the nAChR without activating the ion channel — competitive antagonism
• Also block presynaptic α₃β₂ receptors → impair ACh mobilization → fade on repetitive stimulation
• At TOF stimulation: fade is present (T4 < T3 < T2 < T1)
• Reversed by: anticholinesterases (neostigmine) or selective binding agents (sugammadex)

6. Why Interpatient Variability Demands Monitoring

Summary: The Physiological Basis of NMJ Monitoring

Motor neuron action potential
       ↓
Voltage-gated Ca²⁺ channel opens at nerve terminal
       ↓
ACh vesicle exocytosis → ACh into synaptic cleft
       ↓
ACh binds 2 α subunits of postsynaptic nAChR
       ↓
Na⁺/K⁺ channel opens → Na⁺ influx → Endplate Potential
       ↓
Muscle membrane depolarizes → Action potential → Contraction
       ↓
AChE hydrolyzes ACh → channel closes → repolarization

Physiological Changes During Positive Pressure Ventilation (PPV) & Effects on Fluid Balance

Comparison: Spontaneous vs. Positive Pressure Breathing

"With the initiation of positive-pressure ventilation (PPV), the opposite occurs: venous return is diminished, cardiac output falls, and there is a decreased pressure gradient between the left ventricle and aorta." — Rosen's Emergency Medicine, 9e

I. Cardiovascular Effects

A. Reduced Venous Return and Preload

• PPV increases mean intrathoracic pressure, which diminishes the venous pressure gradient from peripheral capillaries to the right atrium
• Normally this gradient is relatively small (~5–8 mmHg), so even modest increases in intrathoracic pressure significantly impair right heart filling
• PEEP further amplifies mean intrathoracic pressure → greater ventricular preload reduction
• Conditions with low lung compliance (asthma, obesity, ascites, ARDS, light anesthesia) exaggerate this effect

B. Reduced Cardiac Output

• Decreased RV preload → decreased RV stroke volume → decreased LV filling → decreased cardiac output
• Hypotension is common after initiation of PPV, especially in:
  • Hypovolemic patients
  • Vasodilatory states (sepsis, anaphylaxis)
  • Patients taking diuretics or ACE inhibitors
  • Cyanotic heart disease (preload-dependent)

C. Effect on Left Ventricular Afterload

• Increased intrathoracic pressure reduces LV transmural pressure (the effective afterload on the LV)
• This is actually beneficial in LV failure — PPV can improve LV ejection in cardiogenic pulmonary oedema by reducing afterload

II. Pulmonary Effects

A. Lung Volumes and V/Q Matching

• Under general anaesthesia with spontaneous breathing: reduced tidal volume, reduced FRC, increased closing volumes → V/Q mismatch and atelectasis
• PPV with PEEP counters atelectasis, improves minute ventilation, and reduces intrapulmonary shunt
• However, PPV distributes ventilation to non-dependent lung zones, while perfusion remains gravity-dependent → potential V/Q mismatch

B. Alveolar Pressures

• Peak inspiratory pressure (PIP) and plateau pressure are monitored as surrogates of lung stretch
• Excessive pressures → barotrauma (pneumothorax, pneumomediastinum) and volutrauma
• Exhalation is passive via chest wall recoil when ventilator pressure is removed

III. Effects on Fluid Balance

A. Haemodynamic Mechanism

• ↑ Intrathoracic pressure → ↑ Inferior vena cava (IVC) pressure → ↑ Renal venous pressure
• Elevated renal venous pressure → ↑ peritubular capillary pressure → increased tubular sodium reabsorption → antinatriuresis
• ↓ Cardiac output → ↓ Systemic arterial pressure → baroreceptor-mediated ↑ sympathetic tone to the kidney → renal vasoconstriction, ↓ renal blood flow (RBF), ↓ GFR, ↓ urine output

B. Neurohumoral Mechanism (the dominant pathway)

1. Atrial Natriuretic Peptide (ANP) Suppression

• Decreased cardiac filling → decreased atrial stretch → decreased ANP secretion
• ANP normally promotes natriuresis and diuresis; its suppression → salt and water retention
• Decreased ANP also leads to ↑ sympathetic tone and ↑ renin activation

2. Renin–Angiotensin–Aldosterone System (RAAS) Activation

• ↓ Renal perfusion pressure + ↑ sympathetic tone → ↑ Renin release → ↑ Angiotensin II → ↑ Aldosterone
• Aldosterone → ↑ sodium reabsorption and ↑ potassium excretion in distal tubule
• With PEEP, renin and aldosterone levels are further elevated proportional to the degree of haemodynamic compromise

3. ADH (Vasopressin / AVP) Release

• Decreased atrial stretch → impulses project from atrial stretch receptors → hypothalamus → ↑ ADH synthesis and release
• PPV is a recognised nonosmotic stimulus for ADH secretion
• ADH → ↑ water reabsorption in collecting tubules → oliguria and water retention
• This results in a syndrome resembling SIADH:

  "Positive-pressure ventilation of the lungs... can promote the release of ADH." — Barash Clinical Anesthesia, 9e "Several chronic, nonmalignant pulmonary conditions, including positive-pressure ventilation, impede venous return... decreased atrial stretch increases AVP release... resulting in a syndrome of inappropriate AVP release — SIADH." — Medical Physiology (Boron & Boulpaep)

C. Summary: Net Renal Effects of PPV

"The renin–angiotensin–aldosterone system undoubtedly augments the renal responses to positive pressure ventilation. An increase in PEEP can depress cardiac output, RBF, GFR, and urine volume, and increase renin and aldosterone." — Miller's Anesthesia, 10e

D. Effect of PEEP on Fluid Balance

• PEEP further increases mean airway and intrathoracic pressure beyond PPV alone
• Each additional cmH₂O of PEEP worsens haemodynamic compromise → greater RAAS activation and ADH release
• The magnitude of renal depression correlates with mean airway pressure, not the specific ventilator mode (no difference between volume-controlled and pressure support in creatinine clearance and FENa)

E. Clinical Consequences

1. Intraoperative oliguria — may occur regardless of IV fluid administered, due to ADH secretion and PPV effects combined with surgical stress; not automatically a sign of hypovolaemia
2. Positive fluid balance — water and sodium retention during PPV contributes to perioperative fluid accumulation
3. Fluid overload risk — aggressive fluid resuscitation during PPV can cause dangerous positive fluid balance; in AKI, positive fluid balance is independently associated with increased mortality
4. Post-extubation diuresis — return to negative intrathoracic pressure with spontaneous breathing restores venous return, increases ANP, suppresses ADH → brisk diuresis often follows extubation
5. Fluid responsiveness assessment — PPV enables the use of dynamic indices: respiratory variation in pulse pressure (PPV), stroke volume variation (SVV), and IVC collapsibility to predict preload responsiveness

"The suppression of renal urine production is related to perioperative ADH secretion and may also be influenced by the effects of positive-pressure ventilation. Increased intrathoracic pressure reduces venous return and cardiac output, which combine with a variety of neurohumoral responses such as sympathetic activation and suppression of ANP release to decrease GFR and urine output. As a result, intraoperative urine output may be low regardless of the volume of IV fluid administered." — Miller's Anesthesia, 10e

Summary Diagram

PPV / PEEP
    ↓
↑ Mean Intrathoracic Pressure
    ├──→ ↓ Venous Return → ↓ RV preload → ↓ CO → ↓ BP
    │         └──→ Baroreceptors → ↑ Sympathetic → Renal vasoconstriction
    │                                              ↓ RBF, ↓ GFR, ↓ UO
    │
    ├──→ ↑ IVC pressure → ↑ Renal venous pressure → ↑ Na⁺ reabsorption
    │
    └──→ ↓ Atrial stretch
              ├──→ ↓ ANP → Na⁺ / water retention
              ├──→ ↑ Renin → ↑ Angiotensin II → ↑ Aldosterone → Na⁺ retention
              └──→ ↑ ADH (AVP) → Water retention → Oliguria / SIADH-like state

Thermoregulation Under Anaesthesia & Impact of Perioperative Hypothermia

Part 1: Normal Thermoregulation

Part 2: Thermoregulation Under Anaesthesia

A. How Anaesthetics Impair Thermoregulation

• Impair hypothalamic thermoregulatory reflex integration
• Volatile agents: Produce a concentration-dependent reduction in the vasoconstriction threshold — e.g., isoflurane decreases the vasoconstriction trigger temperature by ~3°C per 1% inhaled concentration
• Shift both the sweating threshold upward and the shivering/vasoconstriction threshold downward → the body tolerates a much wider temperature range without responding
• Neuromuscular blocking agents (paralytics) abolish shivering entirely, removing the only significant heat-generating response

• Blocks efferent sympathetic fibres → prevents compensatory vasoconstriction in the anaesthetized dermatomes
• Blocks afferent thermal sensory input from the lower body → the hypothalamus receives falsely normal temperature information (altered perception of temperature from anaesthetised dermatomes)
• This miscommunication allows continued heat loss without triggering corrective responses
• Lowers the shivering threshold and vasoconstrictive response to hypothermia

• Effects are roughly additive — the greatest degree of thermoregulatory impairment
• Particularly relevant given increasing use of regional anaesthesia in modern ERAS pathways

"General anesthesia lowers the cold-response threshold of the body. Volatile and intravenous anesthetics impair thermoregulation, paralytics prevent the body's shivering response to hypothermia, and compensatory vasoconstriction is downregulated." — Sabiston Textbook of Surgery, 7e

B. The Three Phases of Heat Loss Under Anaesthesia

Figure 52-1: Unintentional hypothermia during general anaesthesia — Morgan & Mikhail's Clinical Anaesthesiology, 7e

C. Environmental and Surgical Factors Contributing to Heat Loss

D. Temperature Monitoring Sites

Part 3: Impact of Perioperative Hypothermia

A. Cardiovascular Effects

• Cardiac arrhythmias — increased risk at temperatures below 35°C; ventricular fibrillation below 28°C
• Myocardial ischaemia and infarction — postoperative shivering dramatically increases myocardial oxygen demand; vasoconstriction raises afterload; catecholamine surge increases heart rate and BP
• Increased peripheral vascular resistance — tissue hypoperfusion and ↑ cardiac workload
• Hypertension on emergence — sympathetic activation from cold stress
• In the PACU, hypothermic patients have a significantly increased incidence of postoperative myocardial ischaemia

B. Haematological Effects — Coagulopathy

• Hypothermia impairs platelet aggregation and activation — platelets sequester in the spleen and peripheral vasculature
• Reduces activity of coagulation factor enzymes (the coagulation cascade is highly temperature-dependent; standard laboratory coagulation tests are performed at 37°C and will appear falsely normal)
• Associated with nearly a 20% increase in operative blood loss
• Increased transfusion requirements
• This coagulopathy is reversible with rewarming

C. Surgical Site Infection (SSI)

• Hypothermia causes peripheral vasoconstriction → decreased oxygen delivery to the wound
• Tissue hypoxia impairs:
  • Neutrophil oxidative killing of bacteria
  • Collagen synthesis and wound healing
• Associated with significantly increased rate of SSI — this is one of the strongest evidence-based drivers for maintaining normothermia intraoperatively

D. Drug Metabolism

• Most anaesthetic agents are hepatically metabolised by cytochrome P450 enzymes — all enzyme activity is temperature-dependent
• Hypothermia prolongs the duration of action of:
  • Inhalational agents
  • Opioids
  • Neuromuscular blocking drugs (NMBDs) — prolonged neuromuscular blockade
  • Benzodiazepines, propofol
• May cause delayed awakening from anaesthesia
• Impairs correction of metabolic acidosis and electrolyte derangements (e.g., hyperkalaemia)

E. Postoperative Shivering

• The body's attempt to generate heat once anaesthetic-induced thermoregulatory inhibition wanes
• Shivering can increase oxygen consumption by up to 5-fold
• Increases CO₂ production, cardiac output, heart rate, and systemic BP
• Increases intraocular pressure
• Poorly tolerated in patients with cardiac or pulmonary disease
• May cause metabolic acidosis if intense enough
• Treatment: meperidine (pethidine) 12.5–25 mg IV in adults; active rewarming is the better strategy

F. Renal Effects

• Impaired renal function
• Hypothermia reduces GFR and may contribute to intraoperative oliguria

G. Neurological / CNS Effects

• Altered mental status and confusion
• Cognitive impairment in PACU
• Delayed emergence from anaesthesia

H. Summary of Consequences

"Patients who are hypothermic on arrival in the PACU should be actively warmed to avoid these immediate complications as well as delayed consequences of hypothermia... long-term deleterious effects include an increased incidence of myocardial ischemia and myocardial infarction, delayed wound healing, and increased perioperative mortality." — Miller's Anesthesia, 10e

Part 4: Prevention and Management

Preoperative

• Prewarming for 30 minutes with convective forced-air blankets before induction → reduces Phase 1 redistribution significantly by narrowing the central–peripheral temperature gradient

Intraoperative

Postoperative

• Forced-air warming device is first choice in PACU
• Alternatively: warming lights, heating blankets
• Continue until core temperature ≥ 36°C
• IV meperidine 12.5–25 mg for symptomatic shivering while rewarming

"Hypothermia is most drastic within the first hour after induction because of the rapid effect of anesthesia-induced vasodilation. Therefore, it is during this period that rewarming techniques are most critical." — Sabiston Textbook of Surgery, 7e