Sound Waves and Ultrasound Principles

Sound Waves and Ultrasound

• A sound wave consists of one complete positive and negative pressure change (cycle).
• The distance traveled during one cycle is the wavelength.
• Frequency is the number of cycles per second, measured in Hz (Hz).
• Amplitude is the height of a sound wave, which determines its intensity and can be adjusted by adjusting the gain.

Pulse-Echo Principle

• The pulse-echo principle is the basis of ultrasound transducer function.
• Piezoelectric crystals convert electric current into sound waves.
• Sound waves travel through a medium (human tissue), hit tissue, and are reflected back to the crystals.
• The crystals receive the reflected waves and convert them back to electricity as data.
• The data is converted into pixels on the ultrasound screen.

Attenuation

• Attenuation is the loss of intensity or amplitude of sound waves as they travel through a medium.
• It occurs via absorption, reflection, refraction, and scatter.
• Four common factors affecting attenuation:
  • Absorption: conversion of acoustic energy to heat in tissue (e.g., bone).
  • Reflection: sound waves bounce back to the transducer while others continue straight through (e.g., air).
  • Refraction: angle of return of waves becomes different than the angle of transmission due to differences in tissue properties.
  • Scatter: sound waves encounter a medium with heterogeneous surface, causing random directions.

Ultrasound Transducers

• Ultrasound transducers are fixed frequency and cannot adjust frequency.
• Amplitude can be adjusted with gain.
• Different types of transducers have specific frequency ranges:
  • Phased array: 1-5 MHz, high penetration, low resolution.
  • Curvilinear: 5-7 MHz, low frequency.
  • Linear: 6-13 MHz, high frequency, high resolution.
• Lower frequency probes penetrate deeper into tissue but have lower resolution.
• Higher frequency probes penetrate shallow tissue but have higher resolution.

Ultrasound Modes

• Brightness (B) mode: provides structural information using shades of brightness in a 2D image.
• Motion (M) mode: captures returning echoes in one line of B-mode image and displays individual images in sequence over time.
• Doppler modes: examine direction and speed of tissue motion/blood flow, presenting as an audible, color, or spectral display.
• Types of Doppler modes:
  • Color Doppler: shows blood flow/tissue motion in 2D image, coding direction and velocity in color.
  • Power Doppler: detects movement in low-flow states, examining amplitude of returning frequency shifts.

Ultrasound Artifacts

• An ultrasound artifact is a machine-generated image that does not exist in reality.
• Artifacts can cause misinterpretation or be useful if understood.
• Types of artifacts:
  • Acoustic shadowing: caused by strong reflection or absorption in near ultrasound fields.
  • Posterior acoustic enhancement: areas deep to echo weak/echo-free structures appear more echogenic.
  • Reverberation: sound waves bounce between two reflective layers, causing an image to appear in deeper tissue.
  • Mirroring: a duplicate image is depicted opposite a reflective layer.

Point-of-Care Ultrasound (POCUS)

• POCUS is a diagnostic and therapeutic tool that improves safety and efficacy of interventions in anesthesia.
• It addresses specific clinical questions to guide evaluation and management.
• POCUS is complementary to physical examination.
• Conditions that can be identified using focused cardiac US assessment:
  • Severe LV/RV failure
  • Increased PAPs
  • Pericardial effusions
  • Regional wall motion abnormalities
  • Gross valvular pathology
  • Dynamic assessment of IVC

Cardiac Ultrasound Views

• Five common cardiac images:
  • PLAX: probe placed 3/4th ICS L of sternum, indicator towards R shoulder.
  • PSAX: probe placed 3/4th ICS L of sternum, indicator towards L shoulder.
  • Apical 4-chamber: probe placed 4/5th ICS at L nipple/breast, indicator at 2 or 3 o'clock.
  • Subcostal 4-chamber: probe placed subxiphoid with posterior pressure, indicator at 3 or 4 o'clock with overhand grip.
  • Subcostal IVC: probe placed subxiphoid with posterior pressure, indicator at 12 o'clock.
• Expected sonographic and functional anatomical features in each cardiac view:
  • PLAX: "3 tubes and a hole" (RV/LV/LA/desc aorta/(MV/AV)/pericardium).
  • PSAX: RV/LV/PAP muscles.
  • Apical 4-chamber: LA/LV/RA/RV/MV/TV/septum ("5 chamber" = +AV).
  • Subcostal 4-chamber: LA/LV/RA/RV/pericardium.
  • Subcostal IVC: liver/IVC/RA.

Antrum Ultrasound Assessment

• Reasons for using the antrum as a sono-anatomic indicator of gastric contents:
  • It is the area most amenable to US assessment.
  • Liver provides a sonographic window.
  • Limited air.
  • Superficial location.
  • Multiple identifiable tissue layers.
  • Accurate reflection of total gastric contents.
• Patient and probe position for sonographic assessment of gastric antrum:
  • Probe: Indicator towards patient head; probe Subxiphoid, midline or R of midline.
  • Patient: supine, or right lateral decubitus (superior/final definitive).
• Gastric antrum images:
  • No contents: no visible content in supine or RLD.
  • Fluid: clear fluid in RLD but none in supine.
  • Early solid: clear fluid in RLD with some debris.
  • Late solid: significant solid content in RLD.

Regional Anesthesia

• Benefits of regional anesthesia:
  • Decrease surgical stress response.
  • Decrease systemic analgesic requirements.
  • Decrease opioid-related side effects.
  • Decrease general anesthesia requirements.
  • Potential for chronic pain development.
  • Post-op analgesia.
• Patient selection:
  • Risk of general vs. regional anesthesia.
  • Primary vs. post-op analgesia.
  • Profound comorbidity.
  • Intolerance to opioids.
  • OSA.
  • PONV.
• Contraindications:
  • Uncooperative patient.
  • Coagulopathy.
  • Infection at insertion site.
  • Pre-existing neuropathy.
  • Severe pulmonary compromise.

Neural Anatomy

• Endoneurium surrounds individual axon.
• Perineurium surrounds fascicle.
• Epineurium surrounds multiple fascicles, fat, and vessels.
• Mesoneurium surrounds functional unit of nerve (+ artery/vein).
• Axons are dynamic in sheaths.
• Blood supply is microscopic and irregular.

Local Anesthetics

• Mechanism: blocks Na+ channel.
• Ionized form is the active species at the intracellular binding site on Na+ channels.
• Higher affinity for open, inactivated state of Na+ channel.
• Pharmacokinetics/pharmacodynamics:
  • Onset: ↑ concentration/dose = ↑ speed of onset.
  • Duration: ↑ protein binding = ↑ duration.
  • Potency: ↑ lipophilicity = ↑ potency/toxicity.
• Additives:
  • Epi: decreases vascular absorption, prolongs block.
  • Opioids: prolongs block, neuraxial > PNB.
  • A2 agonists: prolongs block, directly inhibits presynaptic conduction.
  • Dexamethasone: prolongs block, MOA unknown.

Ultrasound-Guided Regional Anesthesia (USGRA)

• Advantages:
  • Real-time needle guidance.
  • Decreased LAST risk.
  • Decreased dose/concentration.
• Disadvantages:
  • Cost.
  • Knowledge requirement.
  • Training requirement.
• Scanning axis and plane:
  • Short axis/In plane: nerves viewed in short axis, needle advanced in plane.
• Needle/nerve image acquisition:
  • In plane: look at your hands, tilt to see nerve, slide to see needle.
  • Heel probe to direct beam into needle.
  • To determine how far to insert needle from probe, use pythagorean theorem.

Regional Anesthesia Preparation

• Assessment:
  • Comorbidities.
  • Discussion.
  • Informed consent.
  • Eval anatomy.
• Patient prep:
  • Privacy.
  • Time out.
  • ASA monitors.
  • IV access.
  • O2.
  • Analgesia/anxiolysis.
• Equipment:
  • (Meh)
• Block procedure:
  • Maintain meaningful contact with patient.
  • R(esponse), A(spirate), P(ressure), T(otal volume): verbalize and document.

Sound Waves and Ultrasound

• A sound wave consists of one complete positive and negative pressure change (cycle).
• The distance traveled during one cycle is the wavelength.
• Frequency is the number of cycles per second, measured in Hz (Hz).
• Amplitude is the height of a sound wave, which determines its intensity and can be adjusted by adjusting the gain.

Pulse-Echo Principle

• The pulse-echo principle is the basis of ultrasound transducer function.
• Piezoelectric crystals convert electric current into sound waves.
• Sound waves travel through a medium (human tissue), hit tissue, and are reflected back to the crystals.
• The crystals receive the reflected waves and convert them back to electricity as data.
• The data is converted into pixels on the ultrasound screen.

Attenuation

• Attenuation is the loss of intensity or amplitude of sound waves as they travel through a medium.
• It occurs via absorption, reflection, refraction, and scatter.
• Four common factors affecting attenuation:
  • Absorption: conversion of acoustic energy to heat in tissue (e.g., bone).
  • Reflection: sound waves bounce back to the transducer while others continue straight through (e.g., air).
  • Refraction: angle of return of waves becomes different than the angle of transmission due to differences in tissue properties.
  • Scatter: sound waves encounter a medium with heterogeneous surface, causing random directions.

Ultrasound Transducers

• Ultrasound transducers are fixed frequency and cannot adjust frequency.
• Amplitude can be adjusted with gain.
• Different types of transducers have specific frequency ranges:
  • Phased array: 1-5 MHz, high penetration, low resolution.
  • Curvilinear: 5-7 MHz, low frequency.
  • Linear: 6-13 MHz, high frequency, high resolution.
• Lower frequency probes penetrate deeper into tissue but have lower resolution.
• Higher frequency probes penetrate shallow tissue but have higher resolution.

Ultrasound Modes

• Brightness (B) mode: provides structural information using shades of brightness in a 2D image.
• Motion (M) mode: captures returning echoes in one line of B-mode image and displays individual images in sequence over time.
• Doppler modes: examine direction and speed of tissue motion/blood flow, presenting as an audible, color, or spectral display.
• Types of Doppler modes:
  • Color Doppler: shows blood flow/tissue motion in 2D image, coding direction and velocity in color.
  • Power Doppler: detects movement in low-flow states, examining amplitude of returning frequency shifts.

Ultrasound Artifacts

• An ultrasound artifact is a machine-generated image that does not exist in reality.
• Artifacts can cause misinterpretation or be useful if understood.
• Types of artifacts:
  • Acoustic shadowing: caused by strong reflection or absorption in near ultrasound fields.
  • Posterior acoustic enhancement: areas deep to echo weak/echo-free structures appear more echogenic.
  • Reverberation: sound waves bounce between two reflective layers, causing an image to appear in deeper tissue.
  • Mirroring: a duplicate image is depicted opposite a reflective layer.

Point-of-Care Ultrasound (POCUS)

• POCUS is a diagnostic and therapeutic tool that improves safety and efficacy of interventions in anesthesia.
• It addresses specific clinical questions to guide evaluation and management.
• POCUS is complementary to physical examination.
• Conditions that can be identified using focused cardiac US assessment:
  • Severe LV/RV failure
  • Increased PAPs
  • Pericardial effusions
  • Regional wall motion abnormalities
  • Gross valvular pathology
  • Dynamic assessment of IVC

Cardiac Ultrasound Views

• Five common cardiac images:
  • PLAX: probe placed 3/4th ICS L of sternum, indicator towards R shoulder.
  • PSAX: probe placed 3/4th ICS L of sternum, indicator towards L shoulder.
  • Apical 4-chamber: probe placed 4/5th ICS at L nipple/breast, indicator at 2 or 3 o'clock.
  • Subcostal 4-chamber: probe placed subxiphoid with posterior pressure, indicator at 3 or 4 o'clock with overhand grip.
  • Subcostal IVC: probe placed subxiphoid with posterior pressure, indicator at 12 o'clock.
• Expected sonographic and functional anatomical features in each cardiac view:
  • PLAX: "3 tubes and a hole" (RV/LV/LA/desc aorta/(MV/AV)/pericardium).
  • PSAX: RV/LV/PAP muscles.
  • Apical 4-chamber: LA/LV/RA/RV/MV/TV/septum ("5 chamber" = +AV).
  • Subcostal 4-chamber: LA/LV/RA/RV/pericardium.
  • Subcostal IVC: liver/IVC/RA.

Antrum Ultrasound Assessment

• Reasons for using the antrum as a sono-anatomic indicator of gastric contents:
  • It is the area most amenable to US assessment.
  • Liver provides a sonographic window.
  • Limited air.
  • Superficial location.
  • Multiple identifiable tissue layers.
  • Accurate reflection of total gastric contents.
• Patient and probe position for sonographic assessment of gastric antrum:
  • Probe: Indicator towards patient head; probe Subxiphoid, midline or R of midline.
  • Patient: supine, or right lateral decubitus (superior/final definitive).
• Gastric antrum images:
  • No contents: no visible content in supine or RLD.
  • Fluid: clear fluid in RLD but none in supine.
  • Early solid: clear fluid in RLD with some debris.
  • Late solid: significant solid content in RLD.

Regional Anesthesia

• Benefits of regional anesthesia:
  • Decrease surgical stress response.
  • Decrease systemic analgesic requirements.
  • Decrease opioid-related side effects.
  • Decrease general anesthesia requirements.
  • Potential for chronic pain development.
  • Post-op analgesia.
• Patient selection:
  • Risk of general vs. regional anesthesia.
  • Primary vs. post-op analgesia.
  • Profound comorbidity.
  • Intolerance to opioids.
  • OSA.
  • PONV.
• Contraindications:
  • Uncooperative patient.
  • Coagulopathy.
  • Infection at insertion site.
  • Pre-existing neuropathy.
  • Severe pulmonary compromise.

Neural Anatomy

• Endoneurium surrounds individual axon.
• Perineurium surrounds fascicle.
• Epineurium surrounds multiple fascicles, fat, and vessels.
• Mesoneurium surrounds functional unit of nerve (+ artery/vein).
• Axons are dynamic in sheaths.
• Blood supply is microscopic and irregular.

Local Anesthetics

• Mechanism: blocks Na+ channel.
• Ionized form is the active species at the intracellular binding site on Na+ channels.
• Higher affinity for open, inactivated state of Na+ channel.
• Pharmacokinetics/pharmacodynamics:
  • Onset: ↑ concentration/dose = ↑ speed of onset.
  • Duration: ↑ protein binding = ↑ duration.
  • Potency: ↑ lipophilicity = ↑ potency/toxicity.
• Additives:
  • Epi: decreases vascular absorption, prolongs block.
  • Opioids: prolongs block, neuraxial > PNB.
  • A2 agonists: prolongs block, directly inhibits presynaptic conduction.
  • Dexamethasone: prolongs block, MOA unknown.

Ultrasound-Guided Regional Anesthesia (USGRA)

• Advantages:
  • Real-time needle guidance.
  • Decreased LAST risk.
  • Decreased dose/concentration.
• Disadvantages:
  • Cost.
  • Knowledge requirement.
  • Training requirement.
• Scanning axis and plane:
  • Short axis/In plane: nerves viewed in short axis, needle advanced in plane.
• Needle/nerve image acquisition:
  • In plane: look at your hands, tilt to see nerve, slide to see needle.
  • Heel probe to direct beam into needle.
  • To determine how far to insert needle from probe, use pythagorean theorem.

Regional Anesthesia Preparation

• Assessment:
  • Comorbidities.
  • Discussion.
  • Informed consent.
  • Eval anatomy.
• Patient prep:
  • Privacy.
  • Time out.
  • ASA monitors.
  • IV access.
  • O2.
  • Analgesia/anxiolysis.
• Equipment:
  • (Meh)
• Block procedure:
  • Maintain meaningful contact with patient.
  • R(esponse), A(spirate), P(ressure), T(otal volume): verbalize and document.