Mv basics lecture

Mechanical Ventilation

IM 2013 (AVM)

Indications for Mechanical Ventilation

• Airway Compromise/ Respiratory Failure Hypoxemic Respiratory Failure (Type I)

• PaO2 < 60 mmHg in an otherwise healthy individual• SaO2 < 90 %

Hypercapnic Respiratory Failure (Type 2)• PaCO2 > 50 mmHg in an otherwise healthy individual• AKA “Ventilatory Failure”• Caused by increased WOB, ↓ventilatory drive, or muscle fatigue

• Airway Protection/ Prophylactic Ventilatory Support Clinical conditions in which there is a high risk of future respiratory failure

• Examples: Brain injury, heart muscle injury, major surgery, prolonged shock, smoke injury

• Ventilatory support is instituted to:– Decrease the Work of Breathing (WOB)– Minimize O2 consumption and hypoxemia– Reduce cardiopulmonary stress– Control airway with sedation

Criteria

• Clinical Assessment (most important) Clinical deterioration Tachypnea: RR >35; Bradypnea, Apnea Co-morbid conditions

• ABG’s Hypoxia: pO2<60mm Hg Hypercarbia: pCO2 > 50mm Hg

* ABGs and Physiological parameters cannot distinguish between acute and chronic respiratory Failure* What may be “normal” in a COPD patient (RR > 30/min, PCO2 > 60 mm Hg) may be indications for intubation in a young, otherwise healthy adult

V/Q Matching. Zone 1 demonstrates dead-space ventilation (ventilation without perfusion). Zone 2 demonstrates normal perfusion. Zone 3 demonstrates shunting (perfusion without ventilation).

Principles (1): Ventilation• The goal of ventilation is to facilitate CO2 release and maintain

normal PaCO2

• Minute ventilation (VE)• Total amount of gas exhaled/min.• VE = (RR) x (TV)• VE comprised of 2 factors

• VA = alveolar ventilation• VD = dead space ventilation

• VD/VT = 0.33• VE regulated by brain stem, responding to pH and PaCO2

• Ventilation in context of ICU• Increased CO2 production

• fever, sepsis, injury, overfeeding• Increased VD

• atelectasis, lung injury, ARDS, pulmonary embolism• Adjustments: RR and TV

Principles (2): Oxygenation

• The primary goal of oxygenation is to maximize O2 delivery to blood (PaO2)

• Alveolar-arterial O2 gradient (PAO2 – PaO2)• Equilibrium between oxygen in blood and oxygen in alveoli• A-a gradient measures efficiency of oxygenation• PaO2 partially depends on ventilation but more on V/Q

matching

• Oxygenation in context of ICU• V/Q mismatching

• Patient position (supine)• Airway pressure, pulmonary parenchymal disease, small-

airway disease• Adjustments: FiO2 and PEEP

Terminologies

• A/C: Assist-Control• SIMV: Synchronized Intermittent Mandatory

Ventilation• Bi-level/Biphasic: Non-inversed Pressure

Ventilation with Pressure Support (consists of 2 levels of pressure)

• PRVC: Pressure Regulated Volume Control • PEEP: Positive End Expiratory Pressure• CPAP: Continuous Positive Airway Pressure• PSV: Pressure Support Ventilation• NIPPV: Non-Invasive Positive Pressure Ventilation

Pressure Ventilation vs. Volume Ventilation

• Pressure-cycled modes deliver a fixed pressure at variable volume (neonates)

• Volume-cycled modes deliver a fixed volume at variable pressure (adults)

• Pressure-cycled modes• Pressure Support Ventilation (PSV)• Pressure Control Ventilation (PCV)• CPAP• BiPAP

• Volume-cycled modes• Control• Assist• Assist/Control• Intermittent Mandatory Ventilation (IMV)• Synchronous Intermittent Mandatory Ventilation (SIMV)

Pressures in Positive Pressure Ventilation

• Baseline Pressure• Peak Pressure• Plateau Pressure• Pressure at End of Exhalation


• Baseline Pressure– Pressures are read from a zero baseline value– At baseline pressure (end of exhalation), the

volume of air remaining in the lungs is the FRC.


• Baseline Pressure– Continuous Positive Airway Pressure (CPAP)


• Baseline Pressure– Positive End-Expiratory Pressure (PEEP)

• Prevents patients from exhaling to zero (atmospheric pressure)

• Increases volume of gas left in the lungs at end of normal exhalation – increases FRC


• Peak Pressure (PIP)– The highest pressure recorded at the end of inspiration (PPeak,

PIP)– It is the sum of two pressures

• Pressure required to force the gas through the resistance of the airways and to fill alveoli


• Plateau Pressure– At the end of inspiration, before exhalation starts, the

volume of air in the lungs is the VT plus the FRC. The pressure measured at this point with no flow of air is plateau pressure

– Measured after a breath has been delivered and before exhalation

• Ventilator operator has to perform an “inflation hold” – Like breath holding at the end of inspiration

– Reflects the effect of elastic recoil on the gas volume inside the alveoli and any pressure exerted by the volume in the ventilator circuit that is acted upon by the recoil of the plastic circuit



• Pressure at End of Expiration– Pressure falls back to baseline during expiration– Auto-PEEP

• Air trapped in the lungs during mechanical ventilation when not enough time is allowed for exhalation

• When PEEPi occurs, the lung volume at end expiration is greater then the FRC

• Need to monitor pressure at end of exhalation


• Pressure at End of Expiration– Auto-PEEP


• Auto-PEEP– Why does hyperinflation occur?

• Airflow limitation because of dynamic collapse• No time to expire all the lung volume (high RR or Vt)• Expiratory muscle activity• Lesions that increase expiratory resistance

• Measured in a relaxed pt with an end-expiratory hold maneuver on a mechanical ventilator immediately before the onset of the next breath

• Adverse effects:– Predisposes to barotrauma– Predisposes hemodynamic compromises– Diminishes the efficiency of the force generated by respiratory

muscles– Augments the work of breathing– Augments the effort to trigger the ventilator

Modes of Ventilation


• Mode– Description of a breath type and the timing of

breath delivery• Basically there are three breath delivery techniques

used with invasive positive pressure ventilation– CMV – Controlled ventilation– IMV – Intermittent ventilation– Spontaneous modes


• CMV– All breaths are mandatory and can be volume

or pressure targeted• Controlled Ventilation – when mandatory breaths are

time triggered• Assist/Control Ventilation – when mandatory breaths

are either time triggered or patient triggered


• CMV– when mandatory breaths are time triggered

• ventilator determines the start time (time triggered) and/or the volume or pressure target

• Adequate alarms must be set to safeguard the patient– E.g., disconnection

• Sensitivity should be set so that when the patient begins to respond, they can receive gas flow from the patient


• CMV– Appropriate when a patient can make no

effort to breathe or when ventilation must be completely controlled• Drugs• Cerebral malfunctions• Spinal cord injury• Phrenic nerve injury• Motor nerve paralysis


• CMV– In other types of patients, controlled

ventilation is difficult to use unless the patient is sedated or paralyzed with medications• Seizure activity• Tetanic contractions• Inverses I:E ratio ventilation• Patient is fighting (bucking) the ventilator• Crushed chest injury – stabilizes the chest• Complete rest for the patient


• Assist/Control Ventilation– A time or patient triggered CMV mode in which the

operator sets a minimum rate, sensitivity level, type of breath (volume or pressure)

– The patient can trigger breaths at a faster rate than the set minimum, but only the set volume or pressure is delivered with each breath


• Assist/Control Ventilation– Indications

• Patients requiring full ventilatory support• Patients with stable respiratory drive• The INITIAL mode usually set upon advent of mechanical

ventilation


• Assist/Control Ventilation– Advantages

• Decreases the work of breathing (WOB)• Allows patients to regulate respiratory rate• Helps maintain a normal PaCO2• Useful in patients with neuromuscular weakness or CNS disturbances

– Complications• Tachypnea may result in significant hypocapnia and respiratory

alkalosis• Improper setting of sensitivity to trigger the ventilator may result in

“fighting the ventilator” when the sensitivity is set too low• Increased sensitivity may result in hyperventilation; sensitivity is

generally set so that an inspiratory effort of 2 cm H20 / 2 Liters will trigger the ventilation

• Since there is almost no work involved by the respiratory muscles, muscle tone is not well-maintained (data have shown that muscle atrophy starts within 6 hours of AC mode)

Selection of Settings for A/C Mode

• Tidal Volume (VT) : – 8-10 ml/kg of ideal body weight– 6 ml/kg for ALI/ARDS

• Rate (number of tidal breaths delivered/minute):– backup rate is usually set 2 to 4 below the spontaneous rate and then the

effect on the patient of decreasing rate is noted– This can be adjusted depending on the desired PaCO2 or pH ( increased rate

= decreased PaCO2 and increased pH)

• Oxygen Concentration (Fi02): – the initial FiO2 should be 100% unless it is evident that a lower FIO2 will

provide adequate oxygenation

• Inspiratory Flow Rate: – This is the rate air is delivered to the patient to achieve the Tidal Volume set.– The initial flow rate is typically set at 40 to 60 L/minute. – This rate typically needs to be higher in patients with asthma and COPD – An inspiratory flow rate setting LOWER than the patient demand will increase

the work of breathing and is a common cause of patient-ventilator discordance


• Inspiratory Flow Pattern:– This is how flow is distributed throughout the

respiratory cycle– The wave forms usually available are:

1. SINE WAVE - the maximum flow is at mid inspiration and resembles a normal spontaneous tidal breathing

2. SQUARE WAVE - this provides a maximum peak flow throughout the inspiratory period

3. DECELERATING WAVE – the flow is maximal at the start and diminishes as inspiration ends

• PEEP:– “Physiologic PEEP” of about 5 cm H20 should be added

regardless of FiO2 to prevent the alveolar injury due to the shearing effect of opening and closing of the alveoli


• Sensitivity:– Ranges anywhere from –5 to –0.5 cm H20 (pressure

sensitivity) or 1- to 5 liters (flow sensitivity)– The more sensitive (e.g. 0.5 cm or 1 L) , the easier

for patient to trigger the ventilator which may lead to hyperventilation

– The less sensitive (e.g. 5 cm or 5 L), the harder for the patient to trigger the ventilator which can lead to increase of work of breathing and thus can cause patient-ventilator desynchrony

– The usual triggering pressure to start with is –2.0 cm or 2 L


• CMV– Volume Controlled

– CMV• Time or patient

triggered, volume targeted, volume cycled ventilation

• Graphic (VC-CMV)– Time-triggered,

constant flow, volume-targeted ventilation


• CMV– Volume Controlled

– CMV• Time or patient

triggered, volume targeted, volume cycled ventilation

• Graphic (VC-CMV)– Time-triggered,

descending-flow, volume-targeted ventilation


• CMV– Pressure Controlled – CMV

• PC – CMV (AKA – Pressure control ventilation - PCV)

• Time or patient triggered, pressure targeted (limited), time cycled ventilation

• The operator sets the length of inspiration (Ti), the pressure level, and the backup rate of ventilation

• VT is based on the compliance and resistance of the patient’s lungs, patient effort, and the set pressure


• CMV– Pressure

Controlled – CMV• Note inspiratory

pause


• CMV– Pressure

Controlled – CMV• Note shorter Ti



• Airway pressure is limited, which may help guard against barotrauma or volume-associated lung injury– Maximum inspiratory pressure set at 30 – 35 cm H2O– Especially helpful in patients with ALI and ARDS

• Allows application of extended inspiratory time, which may benefit patients with severe oxygenation problems

• Usually reserved for patient who have poor results with a conventional ventilation strategy of volume ventilation



• Occasionally, Ti is set longer than TE during PC-CMV; known as Pressure Control Inverse Ratio Ventilation– Longer Ti provides better oxygenation to some

patients by increasing mean airway pressure– Requires sedation, and in some cases paralysis


• Intermittent Mandatory Ventilation – IMV– Periodic volume or pressure targeted breaths occur at

set interval (time triggering)– Between mandatory breaths, the patient breathes

spontaneously at any desired baseline pressure without receiving a mandatory breath• Patient can breathe either from a continuous flow or gas or

from a demand valve


• Intermittent Mandatory Ventilation – IMV– Indications

• Facilitate transition from full ventilatory support to partial support

– Advantages• Maintains respiratory muscle strength by avoiding muscle

atrophy• Decreases mean airway pressure• Facilitates ventilator discontinuation – “weaning”

– Complications• When used for weaning, may be done too quickly and cause

muscle fatigue• Mechanical rate and spontaneous rate may asynchronous

causing “stacking”– May cause barotrauma or volutrauma


• Synchronized IMV– Operates in the same way as IMV except that

mandatory breaths are normally patient triggered rather than time triggered (operator set the volume or pressure target)

– As in IMV, the patient can breathe spontaneously through the ventilator circuit between mandatory breaths


• Synchronized IMV– At a predetermined interval (respiratory rate), which is

set by the operator, the ventilator waits for the patient’s next inspiratory effort

– When the ventilator senses the effort, the ventilator assists the patient by synchronously delivering a mandatory breath

– If the patient fails to initiate ventilation within a predetermined interval, the ventilator provides a mandatory breath at the end of the time period


• Synchronized IMV– Indications

• Facilitate transition from full ventilatory support to partial support

– Advantages• Maintains respiratory muscle strength by avoiding muscle atrophy• Decreases mean airway pressure• Facilitates ventilator discontinuation – “weaning”

– Complications/Disadvantages• When used for weaning, may be done too quickly and cause

muscle fatigue• With increased WOB resulting in increased oxygen consumption

which is deleterious in patients with myocardial insufficiency/hemodynamic instability

• NOT useful in patients with depressed respiratory drive or impaired neurologic status

Selection of Settings for SIMV Mode

• Rate : – The initial rate set should be close to the

patient’s rate and then the rate decreased noting the effect on patient acceptance.

• Tidal Volume (VT), FIO2, Inspiratory flow rate, Inspiratory flow pattern (Wave form), +/- PEEP are the same as that of A/C.


• Spontaneous Modes– Three basic means of providing support for continuous

spontaneous breathing during mechanical ventilation• Spontaneous breathing• CPAP• PSV – Pressure Support Ventilation


• Spontaneous Breathing– Patients can breathe spontaneously through a

ventilator circuit; – sometimes called T-Piece Method because it

mimics having the patient ET tube connected to a Briggs adapter (T-piece)

– Advantage• Ventilator can monitor the patient’s breathing and

activate an alarm if something undesirable occurs– Disadvantage

• May increase patient’s WOB with older ventilators


• Spontaneous Modes– CPAP

• Ventilators can provide CPAP for spontaneously breathing patients– Helpful for improving oxygenation in patients with refractory hypoxemia and a

low FRC– CPAP setting is adjusted to provide the best oxygenation with the lowest positive

pressure and the lowest FiO2

• Advantages– Ventilator can monitor the patient’s breathing and activate an alarm if

something undesirable occurs– Serves to keep alveoli from collapsing, resulting in better oxygenation

and less WOB.

• Pre-set pressure is present in the circuit and lungs throughout both the inspiratory and expiratory phases of the breath.

• Commonly used as a mode to evaluate the patients readiness for extubation.



• PEEP (Positive End Expiratory Pressure)– “According to its purest definition, the term PEEP is defined as positive

pressure at the end of exhalation during either spontaneous breathing or mechanical ventilation. However, use of the term commonly implies that the patient is also receiving mandatory breaths from a ventilator.” (Pilbeam)

– PEEP becomes the baseline variable during mechanical ventilation

– Helps prevent early airway closure and alveolar collapse and the end of expiration by increasing (and normalizing) the functional residual capacity (FRC) of the lungs

– Facilitates better oxygenation– NOTE: PEEP is intended to improve oxygenation, not to provide

ventilation, which is the movement of air into the lungs followed by exhalation


• Pressure Support Ventilation – PSV– Patient triggered, pressure targeted, flow

cycled mode of ventilation– Requires a patient with a consistent

spontaneous respiratory pattern– The ventilator provides a constant pressure

during inspiration once it senses that the patient has made an inspiratory effort


• PSV


• PSV– Indications

• Spontaneously breathing patients who require additional ventilatory support to help overcome

• WOB, CL, Raw• Respiratory muscle weakness• Weaning (either by itself or in combination with SIMV)


• PSV– Advantages

• Full to partial ventilatory support• Augments the patients spontaneous VT

• Decreases the patient’s spontaneous respiratory rate• Decreases patient WOB by overcoming the resistance of the

artificial airway, vent circuit and demand valves• Allows patient control of TI, VI, f and VT

• Set peak pressure• Prevents respiratory muscle atrophy• Facilitates weaning• Improves patient comfort and reduces need for sedation• May be applied in any mode that allows spontaneous

breathing, e.g., VC-SIMV, PC-SIMV


• PSV– Disadvantages

• Requires consistent spontaneous ventilation• Patients in stand-alone mode should have back-

up ventilation• VT variable and dependant on lung

characteristics and synchrony• Low exhaled VE • Fatigue and tachypnea if PS level is set too low


• Spontaneous Modes– Flow Cycling During PSV

• Flow cycling occurs when the ventilator detects a decreasing flow, which represents the end of inspiration

• This point is a percentage of peak flow measured during inspiration– PB 7200 – 5 L/min– Bear 1000 – 25% of peak flow– Servo 300 – 5% of peak flow

• No single flow-cycle percent is right for all patients


• Spontaneous Modes– Flow Cycling During

PSV• Effect of changes in

termination flow

• A: Low percentage (17%)

• B: High percentage (57%)

• Newer ventilators have an adjustable flow cycle criterion, which can range from 1% - 80%, depending on the ventilator


• Spontaneous Modes– PSV during SIMV

• Spontaneous breaths during SIMV can be supported with PSV (reduces the WOB)

PCV – SIMV with PSV


• Spontaneous Modes– PSV during SIMV

• Spontaneous breaths during SIMV can be supported with PSV

VC – SIMV with PSV


• PSV– NOTE: During pressure support ventilation (PSV), inspiration ends if

the inspiratory time (TI) exceeds a certain value.

• This most often occurs with a leak in the circuit. For example, a deflated cuff causes a large leak. The flow through the circuit might never drop to the flow cycle criterion required by the ventilator.

• Therefore, inspiratory flow, if not stopped would continue indefinitely.

• For this reason, all ventilators that provide pressure support also have a maximum inspiratory time.


• PSV– Setting the Level of Pressure Support

• Goal: To provide ventilatory support– Spontaneous tidal volume is 10 – 12 mL/Kg of ideal

body weight– Maintain spontaneous respiratory rate <25/min

• Goal: To overcome system resistance (ET Tube, circuit, etc.) in the spontaneous or IMV/SIMV mode– Set pressure at (PIP – Pplateau) achieved in a volume

breath or at 5 – 10 cm H2O


• Spontaneous Modes– PSV - The results of your work

35 cm H2O

10 cm H2O

Selection of Settings for PSV

• Inspiratory Pressure: – start with a pressure which gives a desired tidal volume (5 to 7 ml/kg) and

the respiratory muscles have been unloaded– One can start with lower pressures (e.g. 15 to 20 cm H20) and titrate up

until the desired tidal volume is reached.– One good clue that the desired inspiratory pressure is reached is the note of

a decreased frequency in respiratory rate

• FIO2:– set at the inspired oxygen which can give an O2 saturation of 92% or higher

• PEEP:– this can be added on to improve oxygenation or prevent shearing injury to

the alveoli

• TIDAL VOLUME will be dependent on:– Patient’s resistance and compliance– The preset Inspiratory Pressure– The preset Inspiratory Time (which can be adjusted by setting an

Inspiration:Expiration (I:E) time

PSV vs. PCV

PSV• Patient determines RR, VE, inspiratory

time – a purely spontaneous mode• Parameters

• Triggered by pt’s own breath• Limited by pressure• Affects inspiration only

• Uses• Complement volume-cycled modes (i.e.,

SIMV)• Does not augment TV but overcomes

resistance created by ventilator tubing

• PSV alone• Used alone for recovering intubated

pts who are not quite ready for extubation

• Augments inflation volumes during spontaneous breaths

• BiPAP (CPAP plus PS)

PCV• No Patient participation• Parameters

• Triggered by time• Limited by pressure• Affects inspiration only

• Disadvantages• Requires frequent

adjustments to maintain adequate VE

• Pt with noncompliant lungs may require alterations in inspiratory times to achieve adequate TV


• Bilevel Positive Airway Pressure (BiPAP)– An offshoot of PEEP/CPAP therapy– Most often used in NPPV– AKA

• Bilevel CPAP• Bilevel PEEP• Bilevel Pressure Support• Bilevel Pressure Assist• Bilevel Positive Pressure• Bilevel Airway Pressure


• Bilevel Positive Airway Pressure (BiPAP)– Commonly patient triggered but can be time

triggered, pressure targeted, flow or time cycled– The operator sets two pressure levels

• IPAP (Inspiratory Positive Airway Pressure)– IPAP is always set higher than EPAP– Augments VT and improves ventilation

• EPAP (Expiratory Positive Airway Pressure)– Prevents early airway closure and alveolar collapse at the end

of expiration by increasing (and normalizing) the functional residual capacity (FRC) of the lungs

– Facilitates better oxygenation


• Bilevel Positive Airway Pressure (BiPAP)– The operator sets two pressure levels

• IPAP• EPAP

NOTE: The pressure difference between IPAP and EPAP is pressure support

Vent settings to improve <oxygenation>

• FIO2

• Simplest maneuver to quickly increase PaO2

• Long-term toxicity at >60%• Free radical damage

• Inadequate oxygenation despite 100% FiO2 usually due to pulmonary shunting• Collapse – Atelectasis• Pus-filled alveoli – Pneumonia• Water/Protein – ARDS• Water – CHF• Blood - Hemorrhage

PEEP and FiO2 are adjusted in tandem

Vent settings to improve <oxygenation>

• PEEP • Increases FRC

• Prevents progressive atelectasis and intrapulmonary shunting

• Prevents repetitive opening/closing (injury)

• Recruits collapsed alveoli and improves V/Q matching• Resolves intrapulmonary shunting• Improves compliance

• Enables maintenance of adequate PaO2 at a safe FiO2 level

• Disadvantages• Increases intrathoracic pressure (may

require pulmonary a. catheter)• May lead to ARDS• Rupture: PTX, pulmonary edema

PEEP and FiO2 are adjusted in tandem

Oxygen delivery (DO2), not PaO2, should be used to assess optimal PEEP.

Vent settings to improve <ventilation>

• Respiratory rate• Max RR at 35 breaths/min • Efficiency of ventilation decreases

with increasing RR• Decreased time for alveolar

emptying

• TV

• Goal of 10 ml/kg• Risk of volutrauma

• Other means to decrease PaCO2

• Reduce muscular activity/seizures• Minimizing exogenous carb load• Controlling hypermetabolic states

• Permissive hypercapnea• Preferable to dangerously high RR

and TV, as long as pH > 7.15

RR and TV are adjusted to maintain VE and PaCO2

• I:E ratio (IRV)• Increasing inspiration time will

increase TV, but may lead to auto-PEEP

• PIP• Elevated PIP suggests need for

switch from volume-cycled to pressure-cycled mode

• Maintained at <45cm H2O to minimize barotrauma

• Plateau pressures• Pressure measured at the end

of inspiratory phase• Maintained at <30-35cm H2O to

minimize barotrauma

Alternative Modes• I:E inverse ratio ventilation (IRV)• ARDS and severe hypoxemia• Prolonged inspiratory time (3:1) leads to

better gas distribution with lower PIP• Elevated pressure improves alveolar

recruitment• No statistical advantage over PEEP, and

does not prevent repetitive collapse and reinflation

• Prone positioning• Addresses dependent atelectasis• Improved recruitment and FRC, relief of

diaphragmatic pressure from abdominal viscera, improved drainage of secretions

• Logistically difficult• No mortality benefit demonstrated

• ECHMO• Airway Pressure Release (APR)

• High-Frequency Oscillatory Ventilation (HFOV)• High-frequency, low amplitude

ventilation superimposed over elevated Paw

• Avoids repetitive alveolar open and closing that occur with low airway pressures

• Avoids overdistension that occurs at high airway pressures

• Well tolerated, consistent improvements in oxygenation, but unclear mortality benefits

• Disadvantages• Potential hemodynamic

compromise• Pneumothorax• Neuromuscular blocking agents

References

• Mechanical Ventilation for Nursing by M Dearing and C Shelley (ppt)

• Mechanical Ventilation Handout by DM Lieberman and AS Ho (ppt)

• Mechanical Ventilation by Marc Charles Parent (ppt)• Principles of Mechanical Ventilation RET 2284 by

Stultz (ppt)• Sena, MJ et al. Mechanical Ventilation. ACS Surgery:

Principles and Practice 2005; pg. 1-16.• Marino, PL. The ICU Book. 2nd edition. 1998.• Byrd, RP. Mechanical ventilation. Emedicine, 6/6/06.

Health & Medicine

Mv basics lecture