Overview
Blood pressure represents the force exerted by the circulating blood on the walls of blood vessels. It is determined by cardiac output (CO) and systemic vascular resistance (SVR).
(MAP – SVR) = HR X SVR
CO = HR X SVR
Cardiac output depends on an interplay between the sympathetic and parasympathetic nervous system. In infants, stroke volume is relatively fixed and cardiac output primarily depends on heart rate. In adults stroke volume plays a much more important role, especially in conditions where increasing heart rate is not feasible e.g. coronary artery disease, HOCM and aortic stenosis.
Definition of terms
| Term | Definition |
|---|---|
| Cardiac output (CO) | The volume of blood the heart pumps in one minute |
| Mean arterial pressure | The average calculated blood pressure during a single cardiac cycle |
| Central venous pressure | The pressure measured in the vena cava, near the right atrium |
| Stroke Volume | Amount of blood pumped from the heart during contraction. Depends on preload, afterload and myocardial contractility |
| Heart rate | Number of contractions of the heart per minute |
| Cardiac index (CI) | CO/BSA (normal range 2.6 – 4.2 L/min/m2) |
| Preload | Volume of blood in the ventricle at end-diastole (Left ventricular end-diastolic volume – LVEDV) |
| Afterload | Resistance to ejection of the blood from the ventricles. SVR accounts for 95% of the resistance during ejection |
| Systemic Vascular Resistance (SVR) | The resistance that has to be overcome for blood to flow through the circulatory system |
| Contractility | The force and velocity of ventricular contraction when preload and afterload are held constant. Best indicated by the ejection fraction (normal LVEF ~ 60%) |
Pulse pressure (PP)
PP = SBP – DBP
The normal pulse pressure is ~40 mmHg at rest, and upto 100mmHg with strenuous exercise
| Variation | Causes |
|---|---|
| Narrow pulse pressure (<25 mmHg) | Aortic stenosis, coarctation of the aorta, tension pneumothorax, heart failure, shock |
| Wide pulse pressure (> 40 mmHg) | Aortic regurgitation, atherosclerotic vessels, patent ductus arteriosus, high-output states (thyrotoxicosis, arteriovenous malformation, pregnancy, anxiety) |
Intra-operative Hypertension
- Differentials of intra-operative hypertension
- “Light” anaesthesia
- Pain (sympathetic activation from surgical stimuli)
- Chronic hypertension
- Illicit drug use e.g. cocaine, amphetamines
- Hypermetabolic states e.g. Malignant Hyperthermia, Neuroleptic malignant syndrome
- Raised intracranial pressure (Cushing’s triad – Hypertension, bradycardia, irregular respiration)
- Autonomic hyperreflexia i.e. spinal cord lesion higher than T5 = severe; spinal cord lesion lower than T10 = mild)
- Endocrine disorders e.g. Thyrotoxicosis, Phaeochromocytoma, Hyperaldosteronism
- Hypervolemia
- Drug contamination (Local anesthetic + epinephrine): can be intentional or unintentional
- Hypercarbia
- Treatment of intra-operative hypertension
- Deepen anesthesia: propofol, volatile agents, opioids (increases analgesia, histamine release causes hypotension)
- Short-acting vasodilators: Clevidipine, Nitroglycerine (venous > arterial), Nitroprusside (arterial > venous)
- Beta-blockers: Labetalol (greater effect on beta receptors when given IV – a:B ration 1:4 → 1:7), Esmolol (effect on HR >> BP)
- Long-acting vasodilators: Hydralazine
Intra-operative Hypotension
- Differentials of intra-operative hypotension
- Measurement error (confirm cuff size, cuff position, transducer level e.t.c.)
- Hypovolemia (blood loss, dehydration, diuresis, sepsis)
- Drugs (IV induction agents, volatile agents, opioids, anticholinesterases, LAST, vancomycin, protamine, vasopressor/vasodilator infusion problem, syringe swap, drugs given by surgeon)
- Regional or neuraxial anesthesia: presents with vasodilation, bradycardia, respiratory failure, LAST, high spinal
- Surgical events: vagal reflexes, obstructed venous return, pneumoperitoneum, retractors, positioning
- Cardiopulmonary problems: tension pneumothorax, hemothorax, tamponade, embolism, sepsis, myocardial depression
- Treatment of intraoperative hypotension
- Turn down or turn off the anesthetic (can give midazolam if indicated)
- Drugs
- Vasoconstrictors (phenylephrine, vasopressin, norepinephrine)
- Positive inotropes (ephedrine, epinephrine)
- HR control (glycopyrrolate, atropine, pacing)
- Volume
- Reevaluate estimated blood loss and replace
- Consider placing an arterial line
- Can consider: central venous pressure, PAC, TEE
- Ventilation
- Reduce PEEP (decreases intrathoracic pressure → improves venous return)
- Decrease I:E ratio (shortens inspiratory time → improves venous return
- Rule out pneumothorax
- Metabolic
- Treat acidosis and/or hypocalcemia
- Most vasoactive drugs will not act effectively if the patient is acidotic or hypocalcemic
- Consider using bicarbonate if pH < 7.15 (source: surviving sepsis)
Pressor/ionotropes
| Drug | Initial bolus | Onset | Time to peak | Duration of action | Infusion rate range |
|---|---|---|---|---|---|
| Phenylephrine | 50-100 mcg | < 1 min | 1 min | 10 – 15 min | 0.2 – 2 mcg/kg/min |
| Vasopressin | 0.5 – 1 unit | < 1 min | 1 min | 30 – 60 min | 0.01 – 0.05 units/min |
| Norepinephrine | 5 – 10 mcg | < 1 min | 1 min | 1 – 2 min | 0.02 – 0.3 mcg/kg/min |
| Ephedrine | 5 – 10 mg | 1 – 2 min | 2 – 5 min | 60 min | N/A |
| Epinephrine | 5 – 10 mcg | < 1 min | 2 min | < 5 min | 0.02 – 0.3 mcg/kg/min |
