3.5 Vascular Resistance, Microcirculation, and Systemic Hemodynamics

3.5 Vascular Resistance, Microcirculation, and Systemic Hemodynamics

Systemic hemodynamics and microcirculatory fluid exchange govern organ perfusion and tissue oxygenation. This section examines vascular resistance equations, arterial pulse contour analysis, venous return mechanics, the revised endothelial glycocalyx model, autoregulation, and cardiovascular autonomic reflexes.

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1. Systemic and Pulmonary Vascular Resistance and Blood Pressure Determinants

Applying the hydraulic analogue of Ohm's Law ($\Delta P = Q \times R$), vascular resistance represents the ratio of the pressure gradient across a vascular bed to the volumetric flow rate (cardiac output):

Systemic Vascular Resistance (SVR)

$SVR = \frac{MAP - CVP}{CO} \times 80$

• $MAP$: Mean Arterial Pressure ($\text{mmHg}$).
• $CVP$: Central Venous Pressure / Right Atrial Pressure ($\text{mmHg}$).
• $CO$: Cardiac Output ($\text{L/min}$).
• Conversion Factor ($80$): Converts $\text{mmHg} \cdot \text{min} / \text{L}$ to absolute metric units of $\text{dynes} \cdot \text{s} \cdot \text{cm}^{-5}$.
• Normal Value: $800-1200\text{ dynes}\cdot\text{s}\cdot\text{cm}^{-5}$ (or $10-15\text{ Wood units}$; $1\text{ Wood unit} = 80\text{ dynes}\cdot\text{s}\cdot\text{cm}^{-5}$).
• Arterioles (diameter $10-100\,\mu\text{m}$) are the primary site of systemic vascular resistance, responsible for $\approx 60\%$ of total SVR, modulated by vascular smooth muscle tone.

Pulmonary Vascular Resistance (PVR)

$PVR = \frac{MPAP - PAWP}{CO} \times 80$

• $MPAP$: Mean Pulmonary Arterial Pressure ($\text{mmHg}$).
• $PAWP$: Pulmonary Artery Wedge Pressure / Left Atrial Pressure ($\text{mmHg}$).
• Normal Value: $100-200\text{ dynes}\cdot\text{s}\cdot\text{cm}^{-5}$ (or $1.2-2.5\text{ Wood units}$). PVR is roughly one-sixth to one-tenth of SVR.

  Pulmonary Vascular
  Resistance (PVR)
     ^
     |  \                                   / High Volume: Alveolar vessels
     |   \                                 /  stretched and compressed
     |    \       PVR Nadir at FRC        /
     |     \              |              /
     |      \_____________v_____________/
     |       Low Volume: Extra-alveolar
     |       vessels compressed by lack
     |       of radial tethering
     +-----------------------------------------> Lung Volume (RV to TLC)
             RV          FRC          TLC

The U-Shaped PVR Curve: PVR varies non-linearly with lung volumes:

• At High Lung Volumes ($>\text{FRC}$ toward TLC): Alveoli expand and stretch, mechanically compressing small intra-alveolar capillaries, increasing capillary resistance.
• At Low Lung Volumes ($<\text{FRC}$ toward RV): Loss of radial elastic parenchymal traction causes larger extra-alveolar conduit vessels to narrow and collapse, increasing resistance.
• PVR is minimal at FRC: Mechanical ventilation titrated to maintain lung volumes near normal FRC minimizes RV afterload.

Determinants of Arterial Blood Pressure

1. Mean Arterial Pressure (MAP): The time-weighted average pressure driving systemic tissue perfusion. At normal resting heart rates ($60-80\text{ bpm}$), diastole comprises two-thirds of the cardiac cycle: $MAP = DBP + \frac{1}{3}(SBP - DBP)$ At significant tachycardia ($>100\text{ bpm}$), systole and diastole approach equal duration: $MAP \approx DBP + \frac{1}{2}(SBP - DBP)$
2. Hemodynamic Determinants: Rearranging Ohm's Law demonstrates that arterial pressure depends on cardiac output, vascular tone, and venous pressure: $MAP = (CO \times SVR) + CVP$
3. Pulse Pressure ($PP$): The difference between systolic and diastolic pressure ($PP = SBP - DBP$). Governed directly by ventricular stroke volume and inversely by arterial compliance: $PP \approx \frac{SV}{C_{\text{art}}}$

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2. Pulse Pressure and Peripheral Arterial Wave Amplification

As the arterial pressure pulse propagates away from the ascending aorta toward peripheral conduit arteries (such as the radial, femoral, and dorsalis pedis arteries), the contour of the arterial waveform changes dramatically:

  Pressure
  (mmHg)   AORTIC ROOT WAVEFORM               RADIAL ARTERY WAVEFORM
  140 ^                                            /\ Elevated SBP
      |                                           /  \ (Amplification)
  120 |         /\ SBP                           /    \
      |        /  \                             /      \
  100 |       /    \___                        /        \_ Dicrotic Notch
      |      /     |   \ (Dicrotic Notch)     /         |  (Delayed/Smoothed)
   80 | ----/------+----\--------------------/----------+-------------------
      |    /             \ DBP              /            \ Depressed DBP
      +-------------------------> Time     +-------------------------> Time
      [ MAP ~ 93 mmHg; Narrow PP ]         [ MAP ~ 92 mmHg; Wide PP ]

Peripheral Changes and Mechanisms

1. Systolic Blood Pressure (SBP): Increases progressively (radial artery SBP is often $10-20\text{ mmHg}$ higher, and dorsalis pedis SBP up to $20-40\text{ mmHg}$ higher than aortic SBP).
2. Diastolic Blood Pressure (DBP): Decreases progressively.
3. Pulse Pressure ($PP$): Widens substantially in peripheral arteries.
4. Mean Arterial Pressure (MAP): Remains nearly constant, decreasing only slightly ($1-2\text{ mmHg}$) due to viscous energy dissipation in large conduit arteries.
5. Dicrotic Notch: Becomes delayed, smoothed, and less distinct peripherally.

Physical Mechanisms of Peripheral Amplification

• Reduced Arterial Caliber: Peripheral arteries are smaller in cross-sectional diameter.
• Decreased Vascular Compliance (Increased Stiffness): The central aorta contains abundant distensible elastin fibers, dampening pressure peaks (Windkessel effect). Peripheral muscular arteries contain higher collagen-to-elastin ratios, making them substantially stiffer.
• Wave Reflection: When the forward (incident) pressure wave hits downstream high-resistance arteriolar branching points, reflection waves travel backward toward the heart. In peripheral vessels, reflected waves summate constructively with the forward incident wave during systole, augmenting the systolic peak.

Clinical Significance: Peripheral non-invasive or intra-arterial radial cuffs overestimate central aortic systolic pressure and underestimate central diastolic pressure, but accurately track central MAP.

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3. Venous Return Mechanics and Cardiopulmonary Interactions

Arthur Guyton established that cardiac output is ultimately governed by the ability of the systemic vasculature to deliver blood back to the right atrium (Venous Return, $VR$). Under steady-state conditions, cardiac output must equal venous return ($CO = VR$).

  Flow (L/min)
   10 ^                           Cardiac Function Curve (Starling)
      |                                   /---
      |                                 /--
    5 |                     OPERATING /--
      |                       POINT *   \ Venous Return Curve
      |                            /      \
      |                          /          \
    0 +-------------------------+------------+---------> RAP / CVP (mmHg)
                                0     2      8 (MSFP)

Mean Systemic Filling Pressure ($MSFP$)

• Definition: The equilibrium pressure that would exist throughout the entire systemic circulation if cardiac output were instantaneously arrested and blood redistributed uniformly.
• Normal Value: $\approx 7-10\text{ mmHg}$.
• Determinants:
  1. Stressed Blood Volume: The fraction of circulating blood volume ($\approx 30\%$) that actively stretches vascular walls beyond their unstressed capacity. (The unstressed volume, $\approx 70\%$, merely fills the vascular bed without exerting transmural pressure).
  2. Venomotor Tone: Sympathetic venoconstriction shifts blood from the unstressed into the stressed volume compartment, acutely increasing $MSFP$.

The Venous Return Equation

Venous return is driven by the pressure gradient between the peripheral microcirculation ($MSFP$) and the right atrium ($RAP$, clinically indexed as $CVP$):

$VR = \frac{MSFP - CVP}{R_{vr}}$

Where $R_{vr}$ is the resistance to venous return (determined primarily by venous and venular caliber).

• Driving Gradient for Venous Return: Under normal conditions, $MSFP - CVP \approx 8 - 2 = 6\text{ mmHg}$. Because resistance to venous flow is exceptionally low, this tiny pressure head is sufficient to drive the entire resting cardiac output ($5\text{ L/min}$).

Impact of Positive Pressure Ventilation on Venous Return

Mechanical ventilation dramatically influences right and left ventricular hemodynamics through cyclic changes in intrathoracic pressure (ITP):

1. Inspiratory Phase: Positive pressure gas delivery elevates intrathoracic pressure.
2. Reduction in RV Preload: Elevated ITP is transmitted directly to the right atrium, elevating $CVP$. This narrows the upstream pressure gradient driving venous return ($MSFP - CVP$), acutely reducing venous return and right ventricular end-diastolic volume ($EDV$).
3. Transient Augmentation of LV Preload: Concurrently, positive alveolar pressure compresses pulmonary capillaries, squeezing blood into the left atrium and transiently augmenting LV preload.
4. Expiratory Phase: Two to three heartbeats later, the reduced RV stroke volume traverses the pulmonary vasculature, resulting in diminished LV filling, decreased LV stroke volume, and a decline in systolic blood pressure.

Dynamic Predictors of Fluid Responsiveness: In mechanically ventilated patients without spontaneous breathing efforts or cardiac arrhythmias, cyclic swings in stroke volume (Stroke Volume Variation, $SVV > 12-13\%$) and arterial pulse pressure (Pulse Pressure Variation, $PPV > 13\%$) accurately identify patients whose ventricles reside on the ascending limb of the Frank-Starling curve and will augment cardiac output following fluid boluses.

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4. Microcirculation and the Revised Endothelial Glycocalyx Model

The Classical Starling Hypothesis

Ernest Starling (1896) proposed that transvascular fluid filtration ($J_v$) was governed by the net balance between hydrostatic and colloid oncotic pressure gradients across the semipermeable capillary endothelium:

$J_v = K_f \cdot [(P_c - P_{if}) - \sigma (\pi_c - \pi_{if})]$

Where $K_f$ is capillary filtration coefficient, $P_c$ is capillary hydrostatic pressure, $P_{if}$ is interstitial hydrostatic pressure, $\sigma$ is the reflection coefficient for albumin ($0 \le \sigma \le 1$), $\pi_c$ is plasma colloid oncotic pressure, and $\pi_{if}$ is interstitial oncotic pressure.

• Classical Model Prediction: Net fluid filtration occurred at the arterial end of the capillary ($P_c > \pi_c$), and net fluid reabsorption occurred at the venular end ($P_c < \pi_c$).

  REVISED ENDOTHELIAL GLYCOCALYX MODEL:
   =================== ENDOTHELIAL CELL ===================
   [ Luminal Blood ] ---> Endothelial Glycocalyx Layer (EGL)
                            | (Semipermeable molecular sieve)
   [ Subglycocalyx Space ] -+ (Very low protein: pi_sg ~ 0)
   =================== INTERCELLULAR CLEFT ================
   [ Interstitial Space ]  (pi_if does NOT drive reabsorption!)
   --------------------------------------------------------
   * Hydrostatic gradient (Pc - Pif) favors outward filtration throughout.
   * Oncotic gradient opposes filtration via (pi_c - pi_sg).
   * NO STEADY-STATE REABSORPTION AT VENULAR END!
   * All filtered fluid returned exclusively via LYMPHATICS.

The Revised Endothelial Glycocalyx Model (Levick and Michel)

Modern ultrastructural research demonstrated that the primary semipermeable oncotic barrier is not the endothelial cell junction, but the Endothelial Glycocalyx Layer (EGL)—a web of membrane-bound proteoglycans (syndecans, glypicans), glycosaminoglycans (heparan sulfate, chondroitin sulfate), and adsorbed plasma albumin lining the luminal surface.

1. The Subglycocalyx Space: Beneath the glycocalyx and inside the intercellular cleft lies the subglycocalyx space. Fluid filtration constantly flushes proteins outward through the wide inter-endothelial clefts; consequently, the oncotic pressure in the subglycocalyx space ($\pi_{sg}$) is extremely low (near zero).
2. The Revised Equation: $J_v = K_f \cdot [(P_c - P_{if}) - \sigma (\pi_c - \pi_{sg})]$
3. The "No Reabsorption" Rule: Because the subglycocalyx space is virtually devoid of protein, the effective opposing oncotic pressure gradient is $\pi_c - \pi_{sg}$, which is much higher than $\pi_c - \pi_{if}$. Even at the venular end of the capillary (where $P_c$ drops to $15\text{ mmHg}$), $P_c$ still exceeds the true opposing transvascular oncotic force. Consequently, there is NO steady-state venous fluid reabsorption in continuous capillaries (skin, muscle, lung, connective tissue). Filtration continues along the entire length of the capillary (decreasing in magnitude, but never reversing).
4. Role of Lymphatics: $100\%$ of all filtered interstitial fluid and extravasated plasma proteins are returned to the vascular system exclusively via the lymphatic system.
5. Glycocalyx Pathology in Anaesthesia: Sepsis, ischemia-reperfusion injury, hyperglycemia, and hypervolemia (via Atrial Natriuretic Peptide release) degrade the glycocalyx. Destruction of the glycocalyx abolishes $\sigma$, precipitating massive transudation of fluid and albumin into the interstitial space (interstitial edema and third-spacing).

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5. Autoregulation Mechanisms and Vasomotor Reflexes

Vascular Autoregulation

Autoregulation is the intrinsic ability of vital vascular beds to maintain relatively constant blood flow despite wide variations in perfusion pressure:

• Cerebral Circulation: Constant between $MAP = 50-150\text{ mmHg}$.
• Renal Circulation: Constant between $MAP = 80-180\text{ mmHg}$.
• Coronary Circulation: Constant between perfusion pressures of $60-140\text{ mmHg}$.

Three Core Mechanisms of Autoregulation

1. Myogenic Mechanism (Bayliss Effect): Intrinsic smooth muscle response. Acute elevations in intravascular transmural pressure stretch vascular smooth muscle cells in small arterioles, activating stretch-sensitive non-selective cation channels. The resulting depolarization opens voltage-gated L-type calcium channels, triggering rapid vasoconstriction. When pressure falls, reduced stretch produces relaxation.
2. Metabolic Mechanism: Increased tissue metabolic activity or reduced blood flow causes local accumulation of vasodilatory metabolites: adenosine, $\text{H}^+$, lactate, $\text{K}^+$, interstitial hyperosmolarity, and $\text{CO}_2$, combined with local hypoxia. These metabolites relax arteriolar sphincters, restoring flow.
3. Endothelial Mediators: Endothelial cells release vasoactive factors in response to flow shear stress and receptor activation:
   • Vasodilators: Nitric Oxide (NO; activates soluble guanylyl cyclase $\rightarrow \uparrow$ cGMP) and Prostacyclin ($PGI_2$; activates adenylyl cyclase $\rightarrow \uparrow$ cAMP).
   • Vasoconstrictors: Endothelin-1 (ET-1; acts on $ET_A$ receptors on vascular smooth muscle).

Cardiovascular Autonomic Reflexes