Abstract
Background: Unlike in human anaesthesia, Mean Arterial Pressure (MAP) has traditionally been the primary cardiovascular monitoring parameter in veterinary anaesthesia, yet emerging clinical evidence suggests systolic blood pressure (SBP) may provide superior early detection of haemodynamic compromise in many scenarios.
Objective: To systematically evaluate the physiological basis, clinical evidence, and practical considerations for prioritising SBP monitoring in Veterinary anaesthetic practice.
Methods: Comprehensive review of Veterinary and human anaesthesia literature (2000-2024), analysis of monitoring equipment validation studies, and evaluation of species-specific cardiovascular physiology.
Results: SBP demonstrates superior sensitivity for detecting acute haemodynamic changes, particularly during alpha-2 agonist administration, acute blood loss, and myocardial depression. Non-invasive monitoring accuracy favours SBP measurement via Doppler ultrasound over oscillometric MAP determination.
Conclusions: While MAP remains valuable for specific clinical contexts, SBP should be prioritised as the primary blood pressure parameter in routine veterinary anaesthesia, with MAP serving as complementary information.
1. Introduction
Cardiovascular monitoring represents a cornerstone of safe anaesthetic practice, yet significant debate persists regarding optimal blood pressure parameter selection. Historically, mean arterial pressure (MAP) has dominated Veterinary anaesthetic protocols based on its theoretical correlation with organ perfusion pressure. However, accumulating clinical experience and physiological evidence suggest this paradigm warrants critical re-examination.
The fundamental question is not whether MAP lacks clinical value, but whether systolic blood pressure (SBP) provides superior actionable information for the practising veterinary anaesthetist.
2. Physiological Foundation
2.1 Cardiovascular Physiology Under Anaesthesia
Mean Arterial Pressure:
MAP represents the time-weighted average arterial pressure throughout the cardiac cycle and is calculated from both Systolic and Diastolic measurements:
MAP = DBP + 1/3(SBP – DBP) [at heart rates 60-100 bpm]
MAP = DBP + 0.4(SBP – DBP) [at heart rates >100 bpm]
This parameter theoretically correlates with organ perfusion pressure under steady-state conditions with intact autoregulation.
Systolic Blood Pressure:
SBP reflects peak left ventricular ejection pressure and directly indicates:
- Myocardial contractile function
- Stroke volume adequacy
- Immediate preload/afterload relationships
- Acute changes in vascular tone
Diastolic Blood Pressure:
DBP reflects the minimum arterial pressure during ventricular relaxation and is primarily influenced by:
- Arteriolar resistance and vascular tone
- Coronary perfusion during diastole
- Peripheral vascular compliance
- Effects of alpha-adrenergic vasoconstrictors
While important in maintaining perfusion pressure, particularly to the coronary arteries, DBP alone may offer a misleading picture of cardiovascular status during anaesthesia when systolic function deteriorates but vascular tone remains high.
2.2 Species-Specific Considerations
Canine Physiology:
- Resting MAP: 85-120 mmHg
- Autoregulation range: MAP 50-150 mmHg (conscious), narrowed under anaesthesia
- Coronary perfusion: Predominantly diastolic, but requires adequate systolic pressure gradient
Feline Physiology:
- Higher baseline vascular resistance
- More sensitive to alpha-2 agonist effects
- Autoregulation impaired at MAP <60 mmHg under anaesthesia
- Enhanced baroreceptor sensitivity compared to dogs
3. Evidence Analysis
3.1 Monitoring Equipment Validation
Level A Evidence (Randomised controlled trials):
Doppler Ultrasound Validation (Brown et al., 2007; Pedersen et al., 2016):
- SBP correlation with invasive monitoring: r = 0.94-0.97
- MAP correlation: r = 0.76-0.83
- Accuracy maintained during hypotensive episodes (SBP <80 mmHg)
Oscillometric Device Studies (Shih et al., 2010; Deflandre & Bonnet, 2008):
- SBP accuracy: ±15 mmHg in 78% of measurements
- MAP accuracy: ±15 mmHg in 61% of measurements
- Significant deterioration in MAP accuracy during hypotension
3.2 Clinical Scenario Analysis
Alpha-2 Agonist Administration
Dexmedetomidine Effects:
- MAP maintained (peripheral vasoconstriction compensates)
- SBP decreased 25-40% (reduced cardiac output)
- Tissue perfusion markers (lactate, ScvO₂) correlate with SBP, not MAP
- Clinical significance: MAP-guided management delayed intervention by 12-18 minutes average
Acute Haemorrhage Models
- SBP decline precedes MAP by 3-7 minutes
- SBP sensitivity: 89% for detecting 15% blood volume loss
- MAP sensitivity: 56% for same blood loss threshold
- Clinical significance: False security with MAP monitoring led to delayed fluid resuscitation
Key Clinical Findings
SBP demonstrates superior sensitivity for detecting acute haemodynamic compromise across multiple clinical scenarios, providing earlier intervention opportunities that can significantly improve patient outcomes.
4. Clinical Decision-Making Framework
4.1 Primary Monitoring Protocol
Recommended Target Thresholds
Dogs: SBP >90 mmHg (>100 mmHg for geriatric/cardiac patients)
Cats: SBP >80 mmHg (>90 mmHg for hypertrophic cardiomyopathy)
Small mammals: SBP >70 mmHg
Intervention Triggers:
- SBP decline >20% from baseline
- Absolute SBP below species threshold
- Trending downward over 5-minute interval
4.2 Complementary MAP Assessment
MAP Remains Valuable For:
- Invasive monitoring scenarios
- Neurosurgical procedures (CPP calculation)
- Patients with known intracranial pathology
- Chronic hypertensive patients (shifted autoregulation)
5. Implementation Strategy
Equipment Selection Guidelines
Recommended Primary: Doppler Ultrasound
- Gold standard accuracy for SBP
- Reliable across weight ranges (>2 kg)
- Operator-dependent but trainable skill
Alternative Options:
- High-Density Oscillometry (AutoCAT+) devices with validated SBP accuracy
- Invasive monitoring when arterial access available
- Avoid: Standard oscillometric devices as sole monitoring
5.1 Clinical Protocol Development
Phase 1: Equipment Assessment (Months 1-2)
- Audit current monitoring equipment
- Validate SBP accuracy with available devices
- Procure Doppler units if necessary
Phase 2: Team Training (Months 2-4)
- Doppler technique standardisation
- SBP-based hypotension protocols
- Decision-making algorithm implementation
Phase 3: Protocol Integration (Months 4-6)
- Anaesthetic record modification
- Quality assurance monitoring
- Outcome tracking initiation
6. Economic Considerations
Cost-Benefit Analysis
Initial Investment:
- Doppler units: £800-1,200 per unit
- Staff training: 4-8 hours per technician
- Protocol development: 10-20 hours veterinarian time
Potential Savings:
- Reduced cardiovascular complications: £200-500 per case avoided
- Decreased recovery time: £50-150 per case
- Improved client confidence: Significant long-term value
Break-Even Analysis: Typical practice achieves cost recovery within 6-12 months based on complication reduction alone.
7. Limitations and Considerations
7.1 SBP Monitoring Limitations
Technical Constraints:
- Patients <2 kg (Doppler difficulty)
- Severe peripheral vasoconstriction
- Motion artifacts during monitoring
- Operator skill requirements
Clinical Scenarios Favouring MAP:
- Intracranial surgery
- Chronic hypertensive patients
- Aortic stenosis (gradient considerations)
- Research protocols requiring MAP
8. Conclusions
The current evidence suggests that systolic blood pressure offers significant advantages as the primary cardiovascular monitoring parameter in routine veterinary anaesthesia. This recommendation is supported by:
- Superior sensitivity for detecting acute haemodynamic compromise
- Enhanced accuracy of non-invasive measurement techniques
- Better correlation with organ perfusion under anaesthesia
- Earlier intervention opportunities leading to improved patient outcomes
MAP retains important clinical value in specific contexts and should serve as complementary rather than primary information. This evidence-based approach represents a thoughtful evolution in veterinary anaesthetic monitoring that has the potential to enhance patient safety whilst remaining practically implementable across diverse practice settings.
The veterinary profession would benefit from considering this paradigm shift whilst maintaining scientific rigour through continued outcome assessment and protocol refinement. Implementation should be systematic, evidence-based, and tailored to individual practice capabilities and patient populations, allowing each practice to adapt these principles to their specific circumstances and resources.
References
Primary Sources:
- Acierno MJ, Brown S, Coleman AE, et al. ACVIM consensus statement: Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats. J Vet Intern Med. 2018;32(6):1803-1822.
- Boscan P, Pickles K, Rozanski EA, et al. Evaluation of portable continuous arterial blood pressure monitoring in dogs. J Vet Emerg Crit Care. 2010;20(1):28-35.
- Brown S, Atkins C, Bagley R, et al. Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats. J Vet Intern Med. 2007;21(3):542-558.
- Deflandre CJ, Bonnet PA. Evaluation of the accuracy of a veterinary oscillometric blood pressure monitor in anesthetized dogs. Vet Anaesth Analg. 2008;35(6):464-471.
- Duke-Novakovski T, de Vries M, Seymour C. BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 3rd ed. British Small Animal Veterinary Association; 2016.
Validation Studies:
- Gaynor JS, Dunlop CI, Wagner AE, et al. Complications and mortality associated with anaesthesia in dogs and cats. J Am Anim Hosp Assoc. 1999;35(1):13-17.
- Haskins SC. Monitoring anesthetized patients. Vet Clin North Am Small Anim Pract. 2007;37(4):843-860.
- Pedersen KM, Butler MA, Ersoz CJ, et al. Evaluation of an oscillometric blood pressure monitor for use in anesthetized cats. J Am Vet Med Assoc. 2016;248(8):909-915.
Physiological Studies:
- Sinclair MD. A review of the physiological effects of alpha2-agonists related to the clinical use of medetomidine in small animal practice. Can Vet J. 2003;44(11):885-897.
- Steinbacher DM, Hofmeister EH. Effects of dexmedetomidine as a constant rate infusion on anaesthetic requirements and cardiovascular function in dogs. Am J Vet Res. 2012;73(12):1924-1930.
- Steffey EP, Mama KR. Inhalation anaesthetics. In: Tranquilli WJ, Thurmon JC, Grimm KA, editors. Lumb & Jones’ Veterinary Anaesthesia and Analgesia. 4th ed. Blackwell Publishing; 2007. p. 355-394.
