Flashcards in Critical Care Kanani V Deck (31):
Define the cardiac output.
The cardiac output is the product of the heart rate and the stroke volume. It is in the order of 5–6 l/min.
Define the cardiac index.
The cardiac index is the cardiac output divided by the body surface area. The minimum acceptable level for adequate tis- sue perfusion is 2.2–2.5 l/min/m2.
What is a pulmonary artery catheter and
what purpose does it serve?
This is a multi-lumen, balloon-tipped, f low-directed catheter that is passed through the right heart and into the pulmonary artery.
In certain instances, it provides a more useful picture of left heart function than CVP measures alone, and can be used to calculate a number of useful parameters of cardiovascular function in the critically ill, thus providing both severity assessment and therapeutic guidance.
By which principle does it reflect left heart function?
When ‘wedged’ in a branch of the pulmonary artery (balloon inf lated in a branch of the PA), there is a continuous column of blood beyond the tip of the catheter that extends to the left atrium. Thus, the pulmonary artery pressure at the ‘wedged’ position is equal to the left atrial pressure.
Give some indications for its use.
The indications for its use are controversial, variable and not absolute:
Concomitant with the use of inotropic support, especially
Post-cardiac surgery in those with poor left ventricular
function and pulmonary hypertension
Those with suspected ARDS and pulmonary oedema
Shock of any cause, e.g. assessing the systemic vascular resistance in those with septic shock
Those with multi-organ failure
Multiple injury with thoracic injury
What physiological parameters does it measure directly?
Mean arterial pressure (MAP)
Mean pulmonary artery pressure (MPAP)
Cardiac output (CO)
Pulmonary artery occlusion pressure (PAOP)
Mixed venous oxygen saturation
What are the derived variables?
Cardiac index (cardiac output/body surface area)
Systemic vascular resistance (SVR)
Pulmonary vascular resistance (PVR)
Indexed SVR and PVR (these values divided by the Body Surface Area)
Oxygen delivery and oxygen consumption
What are the complications of insertion?
Those complications of any central line insertion: see ‘Central line insertion’
Cardiac arrhythmias: most commonly atrial and ventricular ectopics. Ventricular tachycardia and ventricular fibrillation as well as heart block also reported
Cardiac valve injury: leading to incompetence of the tricuspid or pulmonary valves
Pulmonary artery rupture: this presents as shock and haemoptysis. Occurs from injury by the J-wire, or following balloon inf lation
Pulmonary infarction: if the balloon is kept in the wedged position for too long. Also occurs if there is embolisation of a thrombus formed at the catheter tip, or catheter
By which principle is the cardiac output measured?
The cardiac output is measured using the indicator dilution or the thermodilution techniques. Both of these have similar principles.With the indicator dilution technique, indocyanine green is injected into the circulation and samples are taken peripherally from the radial artery. A graph of the concentration of the dye in the peripheral blood over time is plotted. The cardiac output equates to the amount of dye injected divided by the area under the curve.
In the case of the thermodilution method, 10 ml of cold crys- talloid is injected peripherally, and the change of temperature detected by a thermistor at the end of the catheter. A graph is also plotted for the change of temperature of the blood passing the thermistor against time. This graph is used to calculate the cardiac output.
What is the mechanism of action of heparin?
Heparin augments the activity of antithrombin III, an inhibitor of activated factors IX, X, XI and XII, preventing the conversion of fibrinogen to fibrin.
What is the mechanism of action of warfarin?
Warfarin prevents the reduction of vitamin K epoxide to vita- min K, thereby inhibiting the vitamin K-dependant binding of factors II, VII, IX and X to calcium and phospholipid membrane surfaces during the clotting cascade.
What types of continuous renal replacement therapies are there?
There are a number of continuous renal replacement modal- ities, depending on whether they rely on dialysis or filtration, and on the pattern of vascular connection
How does peritoneal dialysis work?
Peritoneal dialysis is a slow form of continuous dialysis that relies on the peritoneum and its capillary network to act as the selectively permeable membrane. As with haemodialy- sis, solute f lows down a diffusion gradient, and f luid f lows by osmosis. The dialysate is introduced into the peri- toneum by way of a Tenckhoff catheter and dwells within the abdomen for several hours before being drained off. This technique has been used in the intensive care setting, but has been superseded by other replacement therapies that are faster and more effective in removing urea and other solutes. However, it still has a place in the haemody- namically unstable patient, and the ambulating patient with chronic renal failure.
What is the classical infective complication? How is this recognised and treated?
The important infective complication is peritonitis that occurs following introduction of exogenous organisms. It may initially be recognised by the presence of a turbid effluent when the dialysate is drained, with
Which of the lung volumes may be measured directly?
Tidal volume: normally 7 ml/kg
Vital capacity: normally 10–15 ml/kg
Inspiratory capacity: inspiratory reserve volume
Define the functional residual capacity (FRC).What factors affect its volume?
The functional residual capacity is the volume of gas remain- ing in the lung at the end of a quiet expiration.
Factors that increase the FRC are
Obstructive pulmonary diseases
PEEP – which increases the intrathoracic pressure
(i.e. makes it less negative) Factors that reduce the FRC are
Increased age and obesity
Factors limiting lung expansion: pleural effusion,
abdominal swelling and incision, thoracic incision, interstitial lung disease
What are the obstructive pulmonary diseases?
How do you differentiate obstructive from restrictive diseases on spirometry?
In cases of obstructive lung disease, there is an increase in the total lung capacity and residual volume due to air trapping. For restrictive diseases, there is a reduction of all of the lung vol- umes. The differences may be seen in the following diagrams:
How may respiratory failure be classified?
Respiratory failure is classified as types I, II and mixed, depending on the CO2.
Type I (hypoxaemic) failure: when PaO2
Give some examples of the different causes of respiratory failure. Type 1
Shunt: intracardiac, e.g. cyanotic congenital heart disease,
Pneumonia (shunting may also occur at some lung
units that are not being ventilated owing to
inf lammatory exudate)
Pulmonary oedema, e.g. cardiac failure, ARDS
Give some examples of the different causes of respiratory failure. Type 2
Cerebral lesion: head injury, brainstem stroke, drug
induced, e.g. barbiturates
Spinal lesion: high cervical trauma, poliomyelitis
Peripheral nerve lesion: motorneurone disease, Guillan–Barre syndrome
Neuromuscular junction lesion: myasthenia gravis
Muscular lesion: exhaustion, e.g. late acute severe asthma
Thoracic cage lesion: flail chest injury with inefficient ventilation
Lung parenchymal lesion: COPD with CO2 retention, obstructive sleep apnoea
What is the systemic inflammatory response syndrome (SIRS)?
The SIRS is the syndrome arising from the body’s reaction to critical illness, such as overwhelming infection or trauma. Its presence is recognised and defined according to a number of clinical criteria
Have you heard of the ‘two-hit’ hypothesis in the development of SIRS?
Yes, this is the observed f inding that those with SIRS who are recovering can have a rapid systemic response to a seemingly trivial second insult, such as a urinary tract or line infection. This may lead to a rapid and terminal deterioration in the patient’s state.
What is pseudohyponatraemia?
This is hyponatraemia that occurs as a peculiarity of the way in which the sodium concentration of the plasma is measured and expressed. In the presence of hyperlipidaemia or hyper- proteiaemia, the sodium concentration may be falsely low if it is expressed as the total volume of plasma, and not just the aqueous phase (which it is normally confined to).
What is the TURP syndrome?
This is a syndrome of cardiovascular and neurological symp- toms that occur following the use of hypotonic glycine- containing irrigation f luid with transurethral resection of the prostate.
The f luid and glycine are absorbed through the injured ves- sels to produce volume overload and hyponatraemia. It leads to bradycardia, blood pressure instability and confusion. In severe cases leads to convulsions and coma.
Which disease processes may trigger the syndrome of inappropriate ADH secretion (SIADH)?
SIADH may be triggered by the following:
Chest infection and lung abscess
Small cell carcinoma of the lung
What are the spinal cord syndromes associated with incomplete injuries?
There are three neurological syndromes associated with incomplete cord injuries:
Central cord syndrome: tends to occur in older individuals following hyperextension of the C-spine and compression of the cord against degenerative discs. Cord damage is centrally located
Anterior spinal cord syndrome: the anterior aspect of the cord is injured, sparing the dorsal columns
Brown–Sequard syndrome: following spinal hemisection
What are the patterns of deficit seen in each of the three syndromes?
Central cord syndrome: motor weakness affects mainly the
upper limbs. Sensory loss is usually less severe
Anterior spinal cord syndrome: there is loss of motor function. There is also loss of pain and temperature sensation, but light touch, proprioception, and vibration sense are unaffected owing to preservation of the dorsal columns
Brown–Sequard Syndrome: There is motor loss below the lesion, with contralateral loss of pain and temperature sensation. There is ipsilateral loss of dorsal column function
What deficits are seen in cases of complete injury?
The following deficits occur:
Motor deficit: initial f laccid paralysis below the level of the
lesion gives way to a spastic paralysis with increased tone
and deep tendon ref lexes due to loss of upper motorneurone input into the cord
Sensory deficit: affecting the anterolateral and posterior columns. These therefore affect the somatic and visceral components to sensation
Autonomic deficit: affecting the sympathetic and parasympathetic outputs of the cord
What is the difference between a Jefferson fracture and a hangman’s fracture?
Jefferson fracture: this is a burst fracture of C1 (atlas), best
seen on the peg view. Seen as widening of the lateral masses and loss of congruity with the axis beneath. This is generally a stable injury
Hangman’s fracture: a fracture of C2 (axis) caused by hyperextension of the neck with the force of the occiput and the atlas bearing down on pedicle of C2