Principles of Mechanical Ventilation RET 2284 Module 6.0 Ventilator Management - Improving...

Principles of Mechanical Ventilation

RET 2284 Module 6.0 Ventilator Management

- Improving Ventilation/Oxygenation

Improving Ventilation / Oxygenation

The first 30 – 60 minutes following initiation of ventilation are generally spent evaluating vital signs, breath sounds, ventilator parameters, lung compliance and resistance, the artificial airway, and documenting patient response to therapy

After that initial phase, the RT is often concerned with improving ventilation and oxygenation and managing the patient-ventilator system


Correcting PaCO2 Abnormalities A change in will often be needed when a

patient is first placed on mechanical ventilation to correct for respiratory alkalosis or acidosis; this is facilitated by making a change in VT or rate (f)


Correcting PaCO2 Abnormalities Methods of Changing Ventilation Based on PaCO2

and pH

If it is appropriate to keep rate (f) constant and change VT, the equations is as follows:

Desired VT = Known PaCO2 x Known VT

Desired PaCO2


Correcting PaCO2 Abnormalities Methods of Changing Ventilation Based on PaCO2

and pH

If it is appropriate to keep VT the same and change rate (f), then the equations is as follows:

Desired f = Known PaCO2 x Known fDesired PaCO2


Correcting PaCO2 Abnormalities Respiratory Acidosis

Volume and Pressure Ventilation Changes When PaCO2 is elevated (>45 mm Hg) and pH is

decreased (<7.35), respiratory acidosis is present and VA is not adequate

Causes PE, Pneumonia Airway disease (e.g., severe asthma attack) Pleural abnormalities (e.g., effusions) Chest wall abnormalities Neuromuscular disease CNS problems

.


Correcting PaCO2 Abnormalities Respiratory Acidosis

Volume and Pressure Ventilation Changes

Guideline: VT to 8 – 12 mL/kg ideal body weight (based on

patient’s pulmonary problem) Maintain plateau pressure <30 cm H2O If VT is already high and/or Pplateau are already high,

then f should be increased

Read example 1, 2 and 3: Respiratory Acidosis, Increasing VT, page 259 – 260 (Pilbeam)


Correcting PaCO2 Abnormalities Respiratory Alkalosis

Volume and Pressure Ventilation Changes When PaCO2 is decreased (<35 mm Hg) and pH

increases (>7.35), then respiratory alkalosis is present and alveolar ventilation is excessive

Causes Hypoxia with compensatory hyperventilation Parenchymal lung disease Medications Mechanical ventilation CNS disorders Anxiety Metabolic disorders


Correcting PaCO2 Abnormalities Respiratory Alkalosis

Volume and Pressure Ventilation Changes

Guideline: Volume ventilation: f, and if necessary, VT

Pressure ventilation: f, and if necessary, pressure

Read example 1 and 2: Respiratory Alkalosis, Decreasing the rate, page 261 (Pilbeam)


Correcting PaCO2 Abnormalities Metabolic Acidosis and Alkalosis

Treatment of metabolic acidosis and alkalosis should focus on identifying those metabolic factors that can cause these acid-base disturbances



Metabolic Acidosis Causes

Ketoacidosis (alcoholism, starvation, diabetes) Uremic acidosis (renal failure to excrete acid) Loss of bicarbonate (diarrhea) Renal loss of base following administration of

carbonic anhydrase inhibitors (e.g., Diamox) Overproduction of acid (lactic acidosis) Toxin ingest that produce acidosis (salicylate,

ethylene glycol [antifreeze], methanol



Metabolic Acidosis Treatment should first deal with the cause of the

acidosis

Secondly, assess the need to reverse the acidemia with some form of alkaline agent



Metabolic Acidosis These patients are often struggling to lower their

PaCO2 to compensate for the metabolic acidemia. As a consequence, these patients are at risk for developing respiratory muscle fatigue

If the patient is losing the struggle to maintain high with spontaneous breathing, assisted ventilation may be necessary to avoid respiratory failure. It is then appropriate to keep the pH (7.35 – 7.45)



Metabolic Alkalosis Causes

Loss of gastric fluid and stomach acids (vomiting, nasogastric suctioning)

Acid loss in the urine (diuretic administration) Acid shift into the cells (potassium deficiency) Lactate, acetate, citrate administration Excessive bicarbonate loads (bicarbonate

administration)



Metabolic Alkalosis Treatment involves correcting the underlying

cause and reversing those factors leading to the alkalosis. In severe cases, carbonic anhydrate inhibitors, acid infusion, and low bicarbonate dialysis my be required

Only in rare circumstances does partial respiratory compensation of metabolic alkalosis occur – PaCO2 will usually not rise higher than 55 mm Hg (Remember that as the CO2 rises, the PaO2 falls)


Correcting PaCO2 Abnormalities Mixed Acid – Base Disturbances

Combined Respiratory Alkalosis and Metabolic Acidosis

Read case studies: Pilbeam, pg. 262 – 263

Combined Respiratory Acidosis and Metabolic Alkalosis

Read case study: Pilbeam, pg. 263


Correcting PaCO2 Abnormalities Increased Physiological Dead Space

If pure respiratory acidosis persists even after alveolar ventilation has been increased, the patient may have a problem with increased dead space

Causes Pulmonary emboli Low cardiac output low pulmonary perfusion High alveolar pressure (PEEP) pulmonary

blood flow Air trapping pulmonary perfusion


Correcting PaCO2 Abnormalities Increased Metabolism and Increased CO2

Production Read case study: Pilbeam, pg. 264 Metabolic rate and VCO2 are increased in the

following patients: Fever Sepsis Burns Multiple trauma and multiple surgical procedures Hyperthyroidism Seizures

.


Correcting PaCO2 Abnormalities Increased Metabolism and Increased CO2

Production In these patients is increased and WOB is

elevated

Treatment Options Increase machine rate to WOB: may cause auto-

peep Add pressure support for spontaneous breaths to

WOB through ET and circuit Switch to PC-CMV, use sedation to WOB


Correcting PaCO2 Abnormalities Intentional Iatrogenic Hyperventilation

Definition Deliberate hyperventilation in patients with acute

head injury and increased intracranial pressure (ICP)

Hyperventilation reduces PaCO2 which causes vasoconstriction of cerebral blood vessels and decreases blood flow to the brain and is believed to lower increased intracranial pressure ICP



Current therapy guideline for head injuries with increased ICP do not recommend prophylactic hyperventilation (PaCO2 <25 mm Hg) during the first 24 hours - may cause cerebral ischemia and cerebral hypoxemia



Hyperventilation may be needed for brief periods when acute neurological deterioration is present and ICP elevated

Mild hyperventilation (PaCO2 30 – 35 mm Hg) may be used for longer periods in a situation in which increased ICP is refractory to standard treatment

The practice of iatrogenic hyperventilation still remains controversial


Correcting PaCO2 Abnormalities Permissive Hypercapnia (PHY)

Definition Deliberate limitation of ventilatory support to avoid

lung overdistention and injury of lung ARDS Status asthmaticus

PaCO2 values are allowed to rise above normal ≥50 – 150 mm Hg

pH values are allowed to fall below normal ≥7.10 – 7.30 Most researchers agree pH ≥7.25 is acceptable



PaCO2 accompanied PaO2 O2 administration must be provided and monitored

closely

PaCO2 stimulates the drive to breath Appropriate to provide sedation to patients in

whom PHY is being employed



Procedures for Managing PHY1. Allow PaCO2 to rise and pH to fall without

changing mandatory rate or volumea. Sedate the patient b. Avoid high ventilating pressuresc. Maintain oxygenation

2. Reduce CO2 productiona. Paralyzeb. Coolc. Restrict glucose



Procedures for Managing PHY3. Keep pH >7.25

a. Sodium bicarbonateb. Tris-hydroxiaminomethane (an amino buffer)c. Carbicarb (mixture of sodium carbonate and

bicarbonate



Contraindications and Effects of PHY Head trauma Intracranial disease Intracranial lesions



Relatively contraindicated in the following Cardiac ischemia Left ventricular compromise Pulmonary hypertension Right heart failure



The use of PHY is restricted to situations in which the target airway pressure is at its maximum and the highest possible rates are being used

The risks of hypercapnia are considered by some to be preferable to the high Pplat required to achieve normal CO2 levels

Read Case Study: Pilbeam, pg. 265 – 266


Oxygenation Using FiO2 and PEEP Adjusting FiO2

Every attempt should be made to maintain the FiO2 <0.40 to 0.50 to prevent the complications of O2 toxicity while keeping the PaO2 between 60 and 90 mm Hg This goal is not always possible and sometimes

a higher FiO2 is required

The SpO2 can be used to titrate FiO2, with the goal of maintaining the SpO2 >90% The SaO2 on an ABG is used to establish the

relationship with the current SpO2

.



ABGs are obtained after mechanical ventilation is initiated and compared with FiO2 being delivered and the SpO2 to establish their relationships

A linear relationship exists between PaO2 and FiO2 as long as VE, CO, Shunt, VD/VT remain fairly constant (cardiopulmonary status)

.



Because of the linear correlation between PaO2 and FiO2 the following equation can be used to select the desired FiO2 to achieve a desired PaO2:

Desired FiO2 = PaO2 (desired) x FiO2 (known)

PaO2 (known)



ExerciseAfter being supported on a ventilator for 30 minutes, a patient’s PaO2 is 40 mm Hg on an FiO2 of 0.50. Acid-base status is normal and all other ventilator parameters are within the acceptable range. What FiO2 is required to achieve a desired PaO2 of 60 mm Hg?



Desired FiO2 = PaO2 (desired) x FiO2 (known) PaO2 (known)

Desired FiO2 = (60 mm Hg) (0.50 FiO2) 40 mm Hg

Desired FiO2 = 0.75


Oxygenation Using FiO2 and PEEP Selection of FiO2 or Adjustment of Paw

Maintaining an FiO2 >60 may lead to: O2 toxicity Absorption atelectasis

Lower limits of target PaO2 is 60 mm Hg Lower limits of target SpO2 is 90%

_

_



When PaO2 remains very low on high FiO2, significant shunting, V/Q abnormalities , and/or diffusion defects are present - other methods to improve oxygenation, besides increasing FiO2, must be considered Paw

PEEP HFOV APRV

_

_


Paw can be used to increase the PaO2

Factors that affect Paw during PPV PIP PEEP Auto-PEEP I:E ratio Respiratory rate Inspiratory flow patterns


_

_

_


Paw is a major determinant of oxygenation in patients with ARDS Mean alveolar pressure oxygenation Alveolar recruitment oxygenation

Typical method to increase Paw PEEP

Other methods to increase Paw HFOV APRV


_

_

_

_


Paw must be monitored closely to prevent: Air trapping Overdistention Barotrauma (e.g. pneumothorax) Venous return CO


_

_

Oxygenation Using FiO2 and PEEP Positive End Expiratory Pressure (PEEP)

Goals of PEEP Enhance tissue oxygenation Maintain a PaO2 above 60 mm Hg, and SpO2

≥90% at an acceptable pH Restore FRC

These goals my be accompanied by the opportunity to reduce the FiO2 to safer levels (<0.50) as PEEP becomes effective Must maintain cardiovascular function and avoid

lung injury



Minimum or Low PEEP PEEP at 3 – 5 cm H2O to help preserve a patient’s

normal FRC

Therapeutic PEEP PEEP >5cm H2O Used in the treatment of refractory hypoxemia

caused by increased intrapulmonary shunting and V/Q mismatching accompanied by a decreased FRC and pulmonary compliance



Optimal PEEP The level of PEEP at which the maximum

beneficial effects of PEEP occur O2 transport FRC Compliance Shunt



Optimal PEEP The level of PEEP is considered optimum because

it is not associated with profound cardiopulmonary side effects

Venous return CO BP Shunting VD/VT Barotrauma Volutrauma

Accompanied by safe levels of FiO2



Indications for PEEP Therapy Bilateral infiltrates on chest radiograph Recurrent atelectasis Reduced CL

PaO2 <60 mm Hg on high FiO2 of >0.5 PaO2/FiO2 ratio <200 for ARDS and <300 for ALI Refractory hypoxemia: PaO2 increases <10 with

FiO2 increase of 0.2



Specific clinical disorders that may benefit from PEEP ALI ARDS Cardiogenic PE Bilateral, diffuse pneumonia



Application of PEEP

Increased in increments of 3 – 5 cm H2O in adults, 2 – 3 cm H2O in infants

Target acceptable PaO2/FiO2 ratio at a safe FiO2 >300 (e.g., PaO2 = 100, with FiO2 = 0.33

(optimal, but not always realistic)



Application of PEEP Patient Appearance

Color, level of consciousness, anxiety – a sudden deterioration may indicate cardiovascular collapse or pneumothorax

Blood Pressure BP of 20 mm Hg systolic drop is significant

Breath Sounds Barotrauma, e.g., pneumothorax




Application of PEEP Ventilator Parameters

VT, Flow, PIP, plateau pressure, VE



Application of PEEP Static Compliance (CS)

As PEEP progressively restores FRC, compliance should increase



Application of PEEP Static Compliance (CS)

Too Much PEEP Overdistention CS

Volume

Pressure

Zone ofOverdistention

“Safe”Window

Zone ofDerecruitment

and Atelectasis

Injury

Injury

Optimized Lung Volume “Safe Window”

Overdistension Edema fluid accumulation Surfactant degradation High oxygen exposure Mechanical disruption

Derecruitment, Atelectasis Repeated closure / re-

expansion Stimulation inflammatory

response Inhibition surfactant Local hypoxemia Compensatory

overexpansion

Application of PEEP



Application of PEEP Arterial PO2, FiO2, and PaO2/FiO2

The usual approach to the management of FiO2 and PEEP is to start with high FiO2 and incrementally decrease it as PEEP improves oxygenation



Application of PEEP Arterial to End-Tidal Carbon Dioxide Tension Gradient

Normal P(a-et)CO2 gradient is 4.5 ± 2.5 (Pilbeam) Is lowest when gas exchange units are maximally

recruited without being overdistended If P(a-et)CO2 gradient increases minimal

acceptable values, it signifies that too much PEEP has been added and is producing a drop in cardiac output and in increase in VD/VT

Application of PEEP



Application of PEEP Arterial-to-Venous Oxygen Difference (C(a-v)O2)

reflects O2 utilization by the tissues Normal value is 5 vol% Increases in C(a-v)O2 with increases in PEEP

may indicate hypovolemia, cardiac malfunction, decreased venous return to the heart, and decreased cardiac output from PEEP

Application of PEEP



Application of PEEP Mixed Venous O2 Tension or Saturation

Normal PvO2 = 35–40 mm Hg (minimal acceptable is 28 mm Hg) Normal SvO2 = 75%

(minimal acceptable is 50%) PEEP usually improves PvO2 and SvO2 When PvO2 and/or SvO2 decrease, with a

increase C(a-v)O2 increase, this indicates a decrease in cardiac output – TOO MUCH PEEP

Application of PEEP



Application of PEEP Cardiac Output

Cardiac output provide key information about the body’s response to PEEP

PEEP improves V/Q Oxygenation CO Too much PEEP Overdistention Venous

return CO

Application of PEEP



Application of PEEP Pulmonary Vascular Pressure Monitoring

When using PEEP >15 cm H2O, it is important to closely evaluate the patient’s hemodyamic status, which may require the placement of a pulmonary artery catheter

If pulmonary artery occluding pressure (PAOP), also known as “wedge pressure,” rises markedly as PEEP is increased, the lungs may be overinflated

On the other hand, when PEEP rises, PAOP may be markedly decreased because of pulmonary blood flow is reduced as a result of decreased venous return to the right side of the heart

Application of PEEP


Data From a Patient with ARDS on MV 24 Hours after Admission

VT: 700 f: 6 VE: 6.6 FiO2: 0.8

PEEP BP HR PCWP CO CS PIPPaO2 PVO2

0 130/65 130 16 4.8 28 50 40 275 120/55 135 13 4.2 31 58 45 3710 135/65 125 18 5.8 33 60 50 3515 130/70 120 19 5.9 36 55 115 3720 110/50 130 25 4.1 27 63 150 29

Can you find the optimal PEEP level?



Weaning From PEEP Patient should demonstrate an acceptable PaO2

on an FiO2 of <0.40 Must be hemodynamically stable and nonseptic Lung conditions should have improved

CS, PaO2/FiO2 ratio Reduce PEEP in 5 cm H2O increments Evaluate SpO2 within 3 minutes to determine

effect – if it falls <20% from previous PEEP level, the patient is ready to tolerate lower PEEP level. If SpO2 drops >20% place PEEP at previous level



Weaning From PEEP Wait between reductions in PEEP and reevaluate

the initial criteria. If the patient is stable, reduce PEEP by another 5 cm H2O. This may take 1 hour or may require as long as 6 hours or more

When the patient is at 5 cm H2O, an additional evaluation is necessary. If reducing the PEEP to zero result is a worsening of the patient, then it may be appropriate to leave the patient at 5 cm H2O until extubation

Date post:	22-Dec-2015
Category:	Documents
Upload:	spencer-fleming
View:	245 times
Download:	8 times

Principles of Mechanical Ventilation RET 2284 Module 6.0 Ventilator Management - Improving...

Documents