10.1 Vital Signs Measurement & Physiological Interpretation

Introduction to Vital Signs & NEWS2 Framework

Measurement and physiological interpretation of vital signs are core nursing responsibilities essential for evaluating patient baseline status, monitoring clinical trajectories, and identifying early signs of physiological decline. In the United Kingdom, the Royal College of Physicians National Early Warning Score (NEWS2) standardises the assessment and response to acute illness in adult patients. NEWS2 measures six physiological parameters: respiratory rate, oxygen saturations, systolic blood pressure, pulse rate, level of consciousness (via the ACVPU scale), and body temperature.

Accurate measurement requires meticulous technique, calibrated equipment, and a clear understanding of normal adult physiological parameters alongside individual patient baselines. Deviations from normal ranges trigger weighted scores (0 to 3) that aggregate to guide clinical urgency, escalation pathways, and monitoring frequency.

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Respiratory Rate (RR) & Oxygen Saturation (SpO2)

Respiratory Rate Measurement & Clinical Significance

The normal resting respiratory rate for an adult is 12 to 20 breaths per minute (bpm). Respiratory rate is widely recognised as the single most sensitive indicator of acute physiological deterioration and clinical risk, often altering hours before changes manifest in heart rate or blood pressure.

• Measurement Technique: Respiratory rate must be assessed manually by observing or feeling thoracic chest excursions for one full minute (60 seconds). It should be conducted unobtrusively while the patient is unaware (for example, while continuing to hold the wrist after assessing the radial pulse) to prevent voluntary control or alteration of breathing patterns.
• Abnormalities:
  • Tachypnoea (>20 bpm): Associated with hypoxia, metabolic acidosis (Kussmaul breathing), pulmonary embolism, sepsis, anxiety, or pain. A respiratory rate ≥25 bpm triggers a high individual parameter score of 3 on NEWS2.
  • Bradypnoea (<12 bpm): Indicates central nervous system depression, opioid overdose, severe hypothermia, or raised intracranial pressure.

Oxygen Saturation (SpO2) & Target Ranges

Pulse oximetry provides a continuous non-invasive estimation of arterial oxygen saturation (SpO2) by measuring differential light absorption of oxygenated and deoxygenated haemoglobin.

In patients with chronic obstructive pulmonary disease (COPD) who exhibit chronic hypercapnia, the respiratory centre becomes desensitised to elevated arterial carbon dioxide (PaCO2). Consequently, ventilation is driven primarily by arterial hypoxaemia (hypoxic drive). Administering high-concentration oxygen can abolish this hypoxic drive, leading to acute severe hypercapnia, respiratory acidosis, coma, and death. Target saturations for these patients must be strictly maintained at 88–92% using Scale 2 of NEWS2.

• Limitations of Pulse Oximetry: Readings can be rendered inaccurate by poor peripheral perfusion (shock, hypothermia, vasoconstriction), peripheral vascular disease, severe anaemia, carbon monoxide poisoning (carboxyhaemoglobin registers as oxygenated haemoglobin), bright ambient light, skin pigmentation, or dark nail varnish.

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Blood Pressure (BP) & Arterial Hemodynamics

Normal Adult Ranges & Measurement Rules

Arterial blood pressure reflects the force exerted by circulating blood against the walls of major arteries. Healthy resting adults commonly sit around 90–120 mmHg systolic and 60–80 mmHg diastolic, but escalation scoring must follow NEWS2, not a narrow textbook “normal” band:

• Systolic Blood Pressure (SBP) under NEWS2: score 0 = 111–219 mmHg; 101–110 scores 1; 91–100 scores 2; ≤90 scores 3 (reflects peak ventricular contraction force).
• Diastolic Blood Pressure (DBP): commonly 60–90 mmHg at rest (reflects resting arterial tone during ventricular relaxation); NEWS2 scores systolic pressure only.

Manual blood pressure determination utilises auscultation over the brachial artery using a sphygmomanometer and stethoscope to identify Korotkoff sounds:

• Phase I Korotkoff Sound: The first clear tapping sound heard as cuff pressure falls below systolic pressure, marking the Systolic BP.
• Phase V Korotkoff Sound: The complete disappearance of sound as cuff pressure falls below diastolic pressure, marking the Diastolic BP.

Cuff Sizing & Equipment Rules

Selecting the correct cuff size is vital to prevent significant measurement errors:

• Bladder Length: Must encircle at least 80% of the patient's upper arm circumference.
• Bladder Width: Must equal at least 40% of the arm circumference.
• Clinical Consequences of Sizing Errors:
  • Undersized Cuff (too small): Overestimates arterial pressure, yielding a falsely high blood pressure reading.
  • Oversized Cuff (too large): Underestimates arterial pressure, yielding a falsely low blood pressure reading.
• Positioning: The patient's arm must be fully supported at the level of the heart. Positioned below heart level causes falsely elevated readings; positioned above heart level causes falsely reduced readings.

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Heart Rate (HR) & Peripheral Pulse Assessment

Normal Adult Range & Palpation Technique

The normal resting adult heart rate is 60 to 100 beats per minute (bpm).

• Radial Pulse Palpation: Palpated over the radial artery at the anterior lateral wrist. If the rhythm is completely regular, count for 30 seconds and multiply by 2. If the rhythm is irregular, or if the rate is outside normal parameters, count for a full 60 seconds.
• Apical Pulse Assessment: Evaluated via auscultation over the 5th intercostal space at the midclavicular line (the cardiac apex) using a stethoscope for a full 60 seconds. Indicated prior to administering cardiac glycosides (e.g., digoxin) or when peripheral pulses are weak or irregular.
• Pulse Deficit: The numerical difference between the apical heart rate and the peripheral radial pulse rate. A pulse deficit indicates that some ventricular contractions are too weak to produce a palpable peripheral pulse, commonly observed in atrial fibrillation.
• Clinical Distinctions:
  • Tachycardia (>100 bpm): Triggered by fever, hypovolaemia, sepsis, pain, anxiety, heart failure, hyperthyroidism, or sympathomimetic agents.
  • Bradycardia (<60 bpm): Seen in physiological conditioning (athletes), beta-blocker therapy, vagal stimulation, heart block, severe hypothermia, or hypothyroidism.

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Body Temperature Measurement

Normal Parameters & Core Measurement Routes

Normal core body temperature in adults is 36.5°C to 37.5°C.

• Tympanic Route: Measures infrared thermal radiation from the tympanic membrane, sharing blood supply with the hypothalamus (the central thermoregulatory centre). Rapid, accurate, and preferred in acute adult settings.
  • Adult Technique: Pull the pinna upward and backward to straighten the external auditory canal for accurate sensor alignment.
  • Paediatric Technique (<3 years): Pull the pinna downward and backward.
• Alternative Routes: Oral (sublingual, requiring patient cooperation and no recent hot/cold ingestion), axillary (adds 0.3–0.5°C adjustment, prone to ambient temperature interference), and core/rectal (used in severe hypothermia or ICU environments).
• Definitions:
  • Pyrexia (Fever): Core temperature >37.5°C (or >38.0°C depending on trust policy).
  • Hypothermia: Core temperature <35.0°C, leading to shivering, confusion, cardiac arrhythmias, and eventual cardiac arrest.

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Level of Consciousness: ACVPU & Glasgow Coma Scale (GCS)

The ACVPU Scale in NEWS2

NEWS2 incorporates the rapid ACVPU tool to screen for neurological deterioration:

• A – Alert: Fully awake, spontaneously conscious, aware of surroundings.
• C – New Confusion: Fresh onset of confusion or delirium not baseline for the patient. Automatically scores 3 points on NEWS2.
• V – Responds to Voice: Opens eyes or responds only when spoken to.
• P – Responds to Pain: Responds only to painful stimuli (trapezius squeeze or supraorbital pressure).
• U – Unresponsive: Completely unresponsive to verbal or physical stimuli.

Glasgow Coma Scale (GCS)

The GCS provides a structured 15-point assessment of neurological function across three clinical domains:

• Total GCS Score: Ranges from 3 (lowest, deep coma) to 15 (fully conscious).
• Critical Airway Threshold: A patient with a GCS score ≤ 8 is unable to maintain or protect their own airway due to loss of pharyngeal and laryngeal reflexes, requiring immediate airway support and anaesthetic intervention.