Mean Arterial Pressure and Total Peripheral Resistance
The pressure that perfuses every organ is not the average of systolic and diastolic.
MAP as the time-weighted average of the pressure curve
Calculate mean arterial pressure from systolic and diastolic values
Explain the relationship between MAP, cardiac output, and total peripheral resistance
Predict the MAP response when cardiac output and resistance change in opposite directions
Distinguish pressure and flow phenotypes in shock, exercise, and hypertension
Interpret how vasoactive drugs alter MAP through resistance and venous return
Quick check
A 68-year-old patient in septic shock has a heart rate of 112 bpm, blood pressure 74/40 mm Hg, and central venous pressure 4 mm Hg. Cardiac index is 4.2 L/min/m2, which is elevated. Surface area is 1.8 m2. The treating team is considering a vasopressor to raise MAP to 65 mm Hg.
What is the primary hemodynamic abnormality that explains the low MAP?
Reason it through
What is the MAP formula?MAP equals diastolic pressure plus one-third of the pulse pressure, or cardiac output times SVR.
If cardiac output is high, what must be low to explain the low MAP?Systemic vascular resistance must be low.
What is the treatment implication?A vasopressor that increases SVR will raise MAP without needing to increase cardiac output further.
A high cardiac output with low MAP defines low SVR as the culprit.
MAP is the organ perfusion pressure
Organs do not see systolic peaks; they see mean driving pressure.
Mean arterial pressure is the time-weighted average of arterial pressure over one cardiac cycle and is the force that drives blood through the systemic circulation.
Because diastole lasts longer than systole at normal heart rates, the MAP is closer to diastolic pressure and is approximated as diastolic plus one-third of pulse pressure.
Organs autoregulate their blood flow across a range of MAP from roughly 50 to 150 mm Hg, but below the lower limit flow becomes pressure-dependent and ischemia ensues.
Calculate the MAP for each blood pressure reading.
MAP = 40 + 1/3(34) = 51 mm Hg, below autoregulatory threshold for most organs
MAP = 60 + 1/3(30) = 70 mm Hg, approaching the lower limit of autoregulation
MAP = 80 + 1/3(40) = 93 mm Hg, normal perfusion pressure
MAP = 110 + 1/3(70) = 133 mm Hg, well above normal autoregulatory range
Cardiac output and SVR are the two determinants
MAP equals cardiac output multiplied by systemic vascular resistance.
Cardiac output and SVR as the two determinants
Cardiac output is the product of heart rate and stroke volume, which itself depends on preload, contractility, and afterload.
Systemic vascular resistance is determined mainly by the diameter of arterioles, the primary resistance vessels, and to a lesser degree by blood viscosity and vessel length.
The pressure-flow relationship is approximately MAP minus central venous pressure equals cardiac output times systemic vascular resistance, so either falling flow or falling resistance can lower perfusion pressure.
Point to the variable that most directly regulates organ perfusion.
1Heart rate
Increases cardiac output but has a secondary effect on MAP through reduced filling time
2Stroke volume
Determined by preload, contractility, and afterload; directly affects pulse pressure
3Arteriolar radius
The dominant controller of SVR; small changes produce large resistance changes
4Blood viscosity
Clinically important in polycythemia and severe anemia but rarely the primary MAP driver
5Venous capacitance
Controls stressed volume and preload but does not directly appear in the MAP equation
The four shock phenotypes
Each shock category has a signature MAP- CO- SVR pattern.
Hypovolemic shock reduces preload and stroke volume, lowering cardiac output; the baroreflex raises SVR to defend MAP.
Cardiogenic shock features low cardiac output from pump failure with a compensatory but insufficient rise in SVR.
Distributive shock has low SVR as the primary defect; cardiac output may be normal, high, or low depending on the phase and compensatory response.
Match each shock type to its hemodynamic fingerprint.
Low CO, high SVR, low preload, low CVP
Low CO, high SVR, high preload, high CVP
Normal to high CO, low SVR, normal to low preload
Low CO, high SVR, high CVP from outflow obstruction
Vasoactive drugs target CO or SVR
Pressors and inotropes are not interchangeable.
Profile classification by CO, SVR, and filling pressures
Norepinephrine acts primarily on alpha-1 receptors to increase SVR and raise MAP, with modest beta-1 effects that support cardiac output.
Epinephrine at low doses stimulates beta-2 vasodilation, but at higher doses alpha effects predominate and increase SVR.
Dobutamine is primarily a beta-1 agonist that increases cardiac output and may lower MAP slightly through reflex withdrawal of sympathetic tone.
Reveal the hemodynamic effect of each drug.
Increases SVR via alpha-1; modest increase in CO via beta-1Increases CO via beta-1; may reduce MAP through beta-2 vasodilationPure alpha-1 agonist; increases SVR with minimal direct CO effectVenous and arterial vasodilator; reduces SVR and preload, lowering MAPInodilator; increases CO and reduces SVR through PDE3 inhibition
The baroreflex defends MAP within seconds
The arterial baroreceptors are the first responders to pressure change.
Carotid sinus and aortic arch baroreceptors sense stretch and increase their firing rate as MAP rises; a fall in MAP reduces firing, releasing the medullary vasomotor center from inhibition.
The efferent response increases sympathetic outflow, raising heart rate, contractility, and arteriolar constriction, while reducing vagal tone.
Resetting occurs in chronic hypertension, so the baroreflex defends a higher baseline MAP and patients may be hypotensive at normal pressures.
Order the baroreflex response from pressure drop to pressure recovery.
Reduced afferent input releases the vasomotor center from tonic inhibition
Sympathetic surge
Increased sympathetic outflow to heart, vessels, and adrenal medulla
Vasoconstriction
Arteriolar constriction raises SVR within seconds
MAP recovery
The combined effect returns MAP toward the baroreceptor set point
Pulse pressure is the CO story
Wide or narrow pulse pressure tells you about stroke volume and arterial compliance.
Pulse pressure is the difference between systolic and diastolic pressure and is determined by stroke volume and the compliance of the large arteries.
A wide pulse pressure occurs in high-output states, aortic regurgitation, and stiff arteries; a narrow pulse pressure suggests low stroke volume from hypovolemia, heart failure, or tamponade.
MAP may remain normal with a wide pulse pressure because the vasodilated resistance network maintains a low runoff resistance.
Select the clinical scenario that best matches each pulse pressure description.
Diagnose the pressure pattern and choose the right drug
In the intensive care unit, the combination of MAP, cardiac output, and pulse pressure guides every vasoactive decision.
A 72-year-old with anterior ST-elevation myocardial infarction has BP 82/50 mm Hg, heart rate 108 bpm, distended neck veins, pulmonary edema on chest radiograph, and cold extremities. Cardiac index is 1.8 L/min/m2 and SVR is calculated as 1400 dynes-s-cm-5.
What is the predominant hemodynamic problem?
Reason it through
What is the target of initial therapy in cardiogenic shock?Improve cardiac output using inotropic support while maintaining adequate MAP.
Would a pure vasoconstrictor be sufficient?No, raising SVR alone increases afterload on a failing heart and may lower output further.
Which drug class addresses both problems?Inotropes such as dobutamine or milrinone increase contractility, while a vasopressor may be needed to maintain coronary perfusion.
Cold extremities and low output make this a pump problem, not a pipe problem.
A 34-year-old with anaphylaxis after penicillin has BP 68/38 mm Hg, heart rate 130 bpm, diffuse urticaria, and wheezing. Cardiac output is high-normal and SVR is low. The patient has stridor, no known cardiac disease, and no signs of hemorrhage. Airway support and crystalloid are being prepared, but the first medication must reverse airway edema, bronchospasm, and distributive vasodilation through one receptor profile.
Which medication and route is the most appropriate first-line treatment?
Reason it through
What is the primary driver of hypotension in anaphylaxis?Low SVR from massive mediator-induced vasodilation and increased capillary permeability.
What other problem does epinephrine treat that pure vasopressors do not?Bronchospasm via beta-2 receptor activation.
What is the preferred route and dose?Intramuscular epinephrine 0.3-0.5 mg in the anterolateral thigh, repeated every 5-15 minutes.
Anaphylaxis needs a drug that fixes pressure, pump, and airways at once; only epinephrine does all three.
A 60-year-old on an esmolol drip for rate control becomes hypotensive with BP 78/52 mm Hg and heart rate 52 bpm. The monitor shows normal sinus rhythm. Extremities are warm. SVR is normal. Bedside ultrasound shows normal ventricular contractility and no pericardial effusion, while hemoglobin and lactate were normal before the infusion. The abrupt temporal relationship between AV-nodal blockade, bradycardia, and hypotension must be translated through cardiac output.
What is the most likely cause of hypotension?
Reason it through
What happens to MAP when heart rate drops?CO = HR x SV, so MAP = (HR x SV) x SVR. A low HR reduces CO unless SV compensates.
Why is the pulse pressure narrow?Stroke volume is reduced by the negative inotropic effect of beta-blockade.
What is the treatment?Reduce or hold the esmolol infusion; if needed, calcium or glucagon can reverse beta-blocker effects.
A slow heart rate with normal SVR is a rate problem, not a resistance problem.
A 45-year-old with cirrhosis and acute variceal bleeding has BP 88/56 mm Hg, heart rate 118 bpm, and cool extremities. He has received 2 units of packed red blood cells. CVP is 3 mm Hg. Hemoglobin remains below the target and active bleeding continues during endoscopy preparation. Bedside ultrasound shows a collapsible inferior vena cava and no pulmonary edema, so the next hemodynamic move must correct underfilling while definitive hemorrhage control proceeds.
What is the most appropriate next intervention to support MAP?
Reason it through
What is the primary driver of low MAP in acute hemorrhage?Low preload and stroke volume from reduced circulating volume.
What is the baroreflex doing?Increasing heart rate and SVR to compensate for the low CO, but even maximal compensation cannot restore MAP without adequate preload.
Why should vasopressors be avoided before volume?Vasoconstriction against an underfilled vascular tree can cause tissue ischemia and worsen outcomes.
Hypovolemic shock needs volume before vasopressors every time.
A 75-year-old with longstanding hypertension undergoes elective surgery. Intraoperative BP drops to 78/46 mm Hg. Heart rate is 68 bpm. CVP is 8 mm Hg. The patient is warm and well-perfused with a cardiac index of 3.8 L/min/m2. There is no surgical bleeding, new wall-motion abnormality, right-heart strain, or pericardial effusion. The preserved flow with warm skin makes a resistance problem more likely than pump failure or obstruction.
What is the most likely cause of the low blood pressure?
Reason it through
How can you distinguish vasodilation from low CO at the bedside?Warm skin, good pulses, and normal or high cardiac output point to vasodilation.
What is the appropriate treatment?A pure vasoconstrictor such as phenylephrine to restore SVR without significantly affecting heart rate.
Why is a beta-agonist less appropriate here?The heart is already performing well; a beta-agonist would increase heart rate and oxygen demand unnecessarily.
A warm hypotensive patient with high output has vasoplegia, not pump failure.
Resident physician and founding medical reviewer at Bone Wizardry, focused on clinical accuracy, clear diagnostic reasoning, and practical board-oriented teaching across the curriculum.
